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Reduced Interference In the Stroop Effect:

Determining When A Word 1s Read Automatically

Michael David Dodd

A thesis subrnitted in conformity with the requirements

for the degree of Master of Arts

Graduate Department of Psycholog

University of Toronto

G Copyright by Michael David Dodd (2001)

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Reduced Interference in the Stroop Effect:

Determining When a Word is Read Automatically

Michael D. Dodd

Master of Arts, 2000

Graduate Department of Psychology

University of Toronto

Besner, S tolz, and Boutillier ( 1 997) claimed to have eliminated S troop interference by colouring

only a single letter of an inelevant word and requinng participants to narne the colour of that

letter. On this basis, they argued that word reading could not be automatic. Four experiments

dernonstrated that their manipulation reduces but in lact does not eliminate Stroop interference.

It is argued that this reduction in Stroop interference is almost exactly offset by an equivalent

increase in latency due to the added requirement of searching For the coloured letter. This two-

process explanation adequately accounts for the Bcsner et al. ( 1997) results as well as oihrr more

recent results (Macleod, 1999; Mamurek, 1999). Moreover, i t is suggested that other aspects of

the Besner et al. methodolog may have produced the illusion that Stroop interference was

eliminated. Irrelevant words are read even in the single coloured letter variant of the Stroop task.

Acknowledgements

There are a nurnber of individuals who deserve special thanks for this, whether they are aware of it or not.

Fint, my wife, Jessica, for standing by me as I pursue my career goals. This path has been a long one and your love and patience through these times is mily appreciated. You are a large part of the reason 1 have been able to make it this far academically.

To ail of rny fnends who were always eager to listen to (or at least feign interest in) my rants whenever something was troubling me. academically or othenvise.

Thanks to Steve Joordens for providing me with insightful cornrnents and help whenever needed.

Most importantly, my supervisor and mentor, Colin MacLeod. You have provided me with geat insight and are always there when needed (even if only in spin t as you iravel the world looking for adventure a la Indiana Jones). Your hard work and investment into my work and rny future is greatly appreciated and will be remembered always.

Table of Contents

.............................................................................................................................. List of Tables vi .................................................................................................... List of Figures vil

... ................................................................................................................. Appendices Directory vrii

................................................................................................................................ Introduction 1 Reduction or Elimination of Semantic Prirning .................................................................. 5 Reducing or Eliminating the Stroop Effect: Colouring Only ri Single Letter ......................... S A Critique of the Besner, Stolz, and Boutilier ( 1997) Argument ............................................ 19 lnterpreting the Apparently Reduced Interference in the Single-Letter Coloured Condition ....... 27 Why the Reduction and Elimination of Sernantic Prirning May 'Iot Generalize to the Stroop

...................................................................................................................... E ffec t 1s Encoding the First Loner of an Incongruent Colour Word All That is Necessary to Invoke

............................................................................................ a Stroop Effect'? .+ln Extension of the Two-process account: CVill Letter Position Influence Response? .............. The Need for ri More Explicit Test of Whether Distractors are Read ..................................

............................................................................................. Negative Prirning'? Rationale for the Present Study ..............................................................................

................................................................................................... Gencral Method Apparatus and Materials ......................................................................................

...................................................................................................... Procedure Experirnent 1 ................................................................................................................................ 57

Method .......................................................................................................................... 57 ...................................................................................................... Participants 57

. . . . . . . . . . . . . . . . . . . . Procedure ................................ , 58 Results ........................................................................................................................... 58

Reduced Interference in the Single-letter-coloured Condition ........................ 58 ............................................................ Position of the Coloured Letter 61

Discussion .................................................................................................................... 64 Are Colour Nonwords a Suitable Control Condition'? ................................. 65

................................................................. More on PositionaI Effects 66 ................................................................... The Importance of Errors 67

..................................................................................................... Experiment 2a 69 ...................................................................................................................... Zilethod 72

Participants ..................................................................................................... 72

......................................................................................................... Procedure 73 ........................................................................................................ Results and Discussion 75

Colour 'laming Study Phase ....................... .. ..... .. ........................................ 75 .... .............................. Recognition Test Phase .... SZ

........................................................................ Recognition Latency 57 ............................................................................................................................... Experiment 2b 95

Method ........................................................................................................................ 97 ...................................................................................................... Participants 97

Materials ........................ ............. ........................................ 97 Procedure .......... .. ........................................................................................ 97

.............................................................................................. Discussion 102

................................................................................................................................. Expetitnent 3 1 03 ........................................................................................................................ Method 105

................................................................................ Participants ................. .. 105 .................................................................................. ................. Materials ... 105

....................................................................................................... Procedure 105 ..................................................................................................................... Results 106

............................................................................................... Discussion 106 ..................................................................................................................... General Discussion I l l

.................................................................................................... S m r y of the Findings 111 .................................................................. Where Does the Present Study Fit:' 114 .......................................................... Final Words on Besner et ai . ( 1997) ... ....... 11s

............................................................ Why Does Letter Position Not Matter? 122 .................................................. ............................. Suggestions for Future Research ... 125

........................................................................................ The Big Picture 118 .................................................................................................................................. Re ferences 131

List of Tables Page

Table 1: Mean Response Times (in ms) and Percentage Error Rates to Narne Colour as a Function of Condition (AI1 Letten Coloured vs. Single Letter Coloured). From Experiment 2: Besner, Stolz, and BoutiIier (1997). .................................................................... 12

Table 2: Mean Response Times (in ms) to Name Colour as a Function of Condition (Al1 Letters Coloured vs. Single Letter Coloured), Number of Display Colours (One vs. Two) and

.......... Cued Colour (Main vs. Odd) From Besner and Stolz (1999) Experiments 1 and 2.. 14

Table 3: .Mean Response Times (in ms) to Same Cotour as a Function of Number of Display Colours (One vs. Two) and Cued Colour (Main vs. Odd) in the Single Letter Coloured Condition From Besner and Stolz ( 1999) Experiments 3 and 4.. .................................. 16

Table 4: 'ulean Response Times (in ms) to 'iame Colour 3s a Function of Condition (Al1 Letters Coloured vs. Single Lctter Coloured) Xcross Four Groups (Vrirying Response Mode and Controls) From Marmurek (1999). ...................................................................... 23

Table 5 : .Mean Response Times (in rns) to Xame Colour as a Function of Condition (Al1 Letters Coloured vs. Single Letter Coloured) Across Four Groups (Varying Response Mode and Controls) From MacLeod ( 1999). ....................................................................... 26

Table 6: Mean Response Times (in ms) to Same Colow 3s a Function of Response .Mode (Vocal vs. Keypress vs. Typewitten) From Logan and Zbrodoff ( 1998). .................................. 35

Table 7: Mean Response Times (in ms) and Percentagr: Error Rates to Samc CoIour as a Function of Group ( 1, 2. or 3). From Bosner (2000).. .......................................................... 49

Table S: Experiment 1: Mean Response Times (in ms). Standard Deviations, and Error Rates to . Name Colour as a Function of Condition (Ail Letters Coioured vs. Single Letter Coloured).. 59

Table 9: Esperiment 1 : Mean Response Times (in rns). Standard Deviations and Errer Rates to Identify Colour as a Function of Letter Position (Incongruent CoIour Words vs. Control Items) in the Single Letter Colourcd Condition. ...................................................... 62

Table IO: Experiment 23: Mean Responsr Times (in rns), Standard Deviations, and Percentage Enor Rates to Xame Colour as a Function of Condition (.A11 Letters Coloured vs. Single Letter Coloured). .......................................................................................... 76

Table 1 1 : Experiment ?a: Mean Response Times (in ms). Standard Deviations. and Percentage Error Rates to Yame Colour as a Function of Lener Position in Single Letter Coloured Items at Study .............................................................................................. 77

Table 12: Experiment Za: Mean Recognition Rates and Standard Deviations for Hits (FulIy Coloured and Sinzle Letter Coioured Studied Words) and False A l a m ( Words Not Presented During the Stroop Task). .................................................................... 83

Table 13: Experiment Za: Mean Recognition Rates and Standard Deviations for CorrectIy Recognized Words (in the Single Letter Coloured Condition) as a Function of Coloured

.............................................................................................. Letter Position 84

TabIe 14: Experirnent Za: Mean Response Times and Standard Deviations for Words .Appearing During the Recognition Task as a Function of Response Type (Yes or Yo) and Word Type (Fully Coloured Single Letter Coloured, and 'lot Presented). ................................ 89

Table 15: Experirnenr 2a: Mean Response Times and Standard Deviations for Correctly Recognized .............. Words (in the Sinsle Letter CoIoured Condition) as a Function of Letter Position.. 90

Table 16: Experiment 2a: Mean Response Times and Standard Deviations (on the Recognition Task) Collapsed Across Conditions for Words That Xppeared During the Stroop Task

........ (Presented) and Words That Did Not Appear During The Stroop Task (Not Presented). 9 1

Table 17: Experiment 2a: Mean Response Times and Standard Deviations for Correctly .................... Recognized Words (Single Letter Colcwed) ris a Function of Letter Position. 93

Table 13: Experiment Zb: Mean Reading Times (in ms) and Standard Deviations ris a Function .................................. of Conditions (Al1 Letters Coloured vs. Single Letter Coloured). ,100

Table 19: Experiment 2b: Mean Reading Times (in ms) and Standard Deviations as a Function of ..................................................... Letter Position in Single Lener Coloured Words.. 101

Table 20: Experiment 3: Mean Reaction Times (in ms), Standard Deviations. and Error Rates to Name Colours as a Function of Condition (Al1 .Asterisks Coloured vs. Single Asterisk Coloured). .................................................................................................... 107

Table 2 l:_Experiment 3: Mean Reaction Times (in ms), Standard Deviations. and Error Rates to Name Colours ris 3 Function of Coloured .Asterisk Position in the Single Asterisk Colowed Position.. ......................................................................................... 1 OS

vii

List of Figures Page

Figure 1: Possible Patterns of Stroop Interference as a Function of the Position of a Single Coloured Letter (Assuming that the Stimulus Word is Six Letters Long.. .e.g., YELLOW). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Appendices Directory

Appendix A: Word Lists for Experiments 23 and 2b.. ................................................. Appendix B: Participant Consent Form.. .................................................................. Appendix B2: Participant Feedback Sheet.. ..............................................................

....................... Appendix C: Written On-Screen Instructions For Subjects In Experiment 1.. Appendix D: Written On-Screen Instructions For Subjects In Experirnent Za.. ..................... Appendix E: Written On-Screen Instructions For Experiment 2b.. ................................... Appendix F: W ritten On-Screen tnstmc tions For Experiment 3. . ..................................... Appendix G: Summary Table for a 2 x 2 Within Subjects AXOV.4 With Word Type (W)

and Colouring T,vpe (C) as the Independent Variables and Colour 'laming .................................................... Times as the Dependent Variable

Appendix H: Summary Table for a One Way tt'ithin Subjects ANOVA With Letter Position as the Independent Variable and Colour Narning Times as the Dependent Variable (For Control Words Only). ...................................

Appendix I I : Summary Table for a One Way Within Subjects AKOVA With Lener Position as the Independent Variable and Colour 'Jarning Times as the

..................... Dependent Variable (For Incongruent Colour Words Only). Appendix 12: Summriry Table for a 2 x 2 Within Subjects A.1:OV.A With Word Type (W)

and Colouring Type (C) as the Independent Variables and Error Rates as the Dependent Variable.. ...............................................................

Appendix J 1 : Summary Table for a One W3y Within Subjects XX0V.A With Letter Position ris the Independent Variable and Enor Rates as the Dependent Variable (For Incongruent Colour Words Onlyj.. ..................................

Appendix JZ: Summary Table for a One Wriy Within Subjects AXOV.4 With Letter Position as the Independent Variable and Error Rates ris the Drpendent

Variable (For Control iVords Only 1.. ................................................. Appendix J3: Summciry Table for a One Way Within Subjects .AXOVA With Word

Type as the Independent Variable and Error Rates as the Dependent Variable (For Distractors With the First Letter Coloured OnIy).. .................

Appendix 54: S u m r y Table for a One Way Within Subjects AXOVA With Word Type as the Independent Variable and Error Rates as the Dependent Variable (For Distractors With the Second Letter Coloured Only). ..............

Appendix J5: Summary Table for a One Way Within Subjects AXOVA With Word Type as the independent Variable and Error Rates as the Dependent Variable (For Distractors With the Third Letter Coloured Only). ................

Appendix J6: S u m r y Table for ri One Way Within Subjects ANOV.4 With Word Type as the Independent Variable and Error Rates as the Dependent Variable (For Words With the Fourth Lener Coloured Only). ....................

Appendix 57: Sumrn?ry Table for a One Way Within Subjects ;LC'OVA With Word Type as the independent Variable and Error Rates as the Dependent Variable (For Distractors With the Fifth Letter Coloured Only). .................

Appendix J8: Summary Table for a One Way Within Subjects ANOVA CVith Word Type as the independent Variable and Error Rates as the Dependent VariabIe (For Distractors With the Sixth Letter Coloured Only). .................

Appendix K 1: Sumrnary Table for a One Way Within Subjects ASOVA With Word Type as the Independent Variable and Colour 'laming Times as the

.......... Dependent VariabIe (For Words With the First Lener Coloured Only). Appendk KZ: Sunimary Table for a One Way Within Subjects hXOV.4 With Word

Type as the Independent Variable and Colour Naming Times as the ...... Dependent Variable ( For Words W ith the Second Letter Coloured Only).

A p p e n d ~ ~ K3: Summary Table for a One Way Within Subjects ANOVA With Word Type as the Independent VariabIe and Colour Saming Times as the Dependent Variable (For iVords With the Third Letter Coloured Only). .........

Page

137. 138 139 1 JO 141 142 143

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155

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Appendix K4: Summary Table for a One Way Within Subjects ANOVA With Word Type as the Independent Variable and Colour Naming Times as the

......... Dependent Variable (For Words With the Fourth Letter Coloured Only). 159 Appendix KS: Summary Table for a One Way Within Subjects ANOVA With Word

Type as the Independent Variable and Colour Naming Times as the .......... Dependent Variable (For Words With the Fifth Letter Coloured Only). 160

Appendix K6: Summary Table for a One Way Within Subjects ANOVA With Word Type as the Independent Variable and Colour Naming Times as the

......... Dependent Variable (For Words With the S~xth Letter Coloured Only). 16 1 Appendix L: Summary Table for a One Way Within Subjects ANOVA With Colowing

Type (All Letters or One Letter) as the Independent Variable and Colour ........................................... Naming Times as the Dependent Variable.. I62

Appendix .M: S m r y Table for a One Way Within Subjects ASOVA With Letter Position as the Independent Variable and Colour Naming Times as the Dependent Variable.. .................................................................... 163

Appendix N: Summrtv Table for a One Way Within Subjects ANOVA With Colouring Type as the Independent Variable and Error Rates as the Dependent Variable. ................................................................................. 164

Appendix 0: Surnmry Table for a One Way Within Subjects ANOVA With Letter Position as the Independent Variable and Error Rates as the Dependent Variable ...................................................................................... 165

Appendix P: S u m r y Table for a One Way Within Subjects AXOVA With Word Type (Full): Coloured vs. Single-Letter-Coloured vs. Xot Presented) as the

....... Independent Variable and Recognition Rates as the Dependent Variable.. 166 Appendix Q: Summary Table for a One Way Within Subjects ANOVA With Letter

Position as the Independent Variable and Recognition Rates as the Dependent Variable.. ...................................................................... 167

Appendix Rl: Summary Table for a One Way Within Subjects ASOVA With Response Type (Fully Coloured Hits vs. Single-Letter-Coloured Hits vs. Unstudied

False Alarms) as the Independent Variable and Recognition Response Times .............................................................. as the Dependent Variable.. 168

Appendix RZ: Summary Table for a One Way Within Subjects ANOVA With Letter Position as the Independent Variable and Recognition Response Times

................................................................ as the Dependent Variable 169 Appendix S: Summary Table for a One Way Within Subjects ANOV.4 With Letter

Position as the Independent Variable and Recognition Response Times as .................................................................. the Dependent Variable.. 170

Appendix T: Summary Table for a One Way Within Subjects ANOVA With Colouring T>pc as the Independent Variable and Word Reading Times as the Dependent Vanable.. ....................................................................... 17 1

Appendix L': S u m r y Table for a One Way Within Subjects ASOV.4 With Letter Position as the Independent Variable and Word Reading Times as the Dependent Variable.. ....................................................................... 172

Appendix V1: Summslry Table for a One Way Within Subjects ASOVA With Colouring Type as the Independent Variable and Colour Naming Times as

...................................... the Dependent Variable (For Rows of Asterisks). 173 Appendix VZ: S m r y Table for a One Way Within Subjects XXOVA With Letter

Position as the independent Variable and Colour Naming Times as the ............................................ Dependent Variable (For Rows of .4sterisks) 174

Appendix W 1 : Summary Table for a One Way Within Subjects ANOVA With Colouring Type as the Independent Variable and Error Rates as the Dependent Variable (For Rows of Asterisks). ........................................................ 175

Appendix W2: Summary Table for a One Way Within Subjects ANOVA With Letter Position as the independent Variable and Error Rates as the Dependent Variable (For Rows of Asterisks). ....................................................... -176

Reduced Stroop Interference - 1

Since the influentid writings of Cattell(1886). psychological literature has routinely

distinguished between controlled and automatic processes. Any new task is assumed to be

controlled in that. at the outset, it requires a geat deal of attentional resources to perform.

Gradually, however, as an individual becomes more practiced at the task, the resources required

to carry it out decrease. This process was descnbed very well by Schneider. Dumais. and

At first, effort and attention must be devoted to each movement or minor decision, and performance is slow and error prone. Eventually, long sequences of movements or cognitive acts are camed out with Iittle attention, and performance is quite rapid and accurate .... The striking changes that occur with practice have Ied many researchers to propose that qualitative changes

occur in the processing. (P. 1)

As an example, consider an individual first Iearning to drive a car. Initially. a great deal of

attention is paid to the pedals, the gear shifl, mirrors, blindspots, speed, etc. As an individual

gains more driving expenence. however. these things that were initially given a great deal of

attention are now perfonned with great ease and little effort on the pan of the driver. Whereas

the beginning driver has to direct a good deal of attention toward dnving the car, the skilled

driver navigates virtually effortlessly and has resources remaining to engage in other activities

simultaneously if they so wish (e.g., conversing, changing radio stations or, in my fat her's case,

reading) .

It is important to realize that although driving becomes a fairly automatic task. it never

becomes 100% automatic. Attention still must be paid to trafic lights. signs, and directions.

Such is the case with many automatic tasks: Despite the reduction in effort and resources that

cornes with practice, it is still necessary to have control over cenain aspects of every task.

Although the manner in which you drive to work every day may seem "automatic," you would


not want to drive to work automatically on the weekend when your destination was actually the

mall. Thus, the concept of automaticity does not imply that al1 aspects of a task are always

camed out without any effort; it simply suggeas that with practice, less control is required to

perform many tasks.

Friedrich. Henik, and Tzelgov (199 1) outline three main characteristics that constitute

automaticity. First. an automatic process is "effortless in the sense that it does not require the use

of a limited-capacity attentional system" (p. 792). It should be noted, however, that although

such a process requires relatively few resources. it is not always possible to perform numerous

"automatic" processes sirnultaneously. Second. automatic processes develop fiorn increased

practice of these processes. Third, once an automatic process has been activated. it is very

difficult to prevent. This latter point is illustrated nicely by the Stroop effect. often regarded as

the Rosetta stone for automaticity in word reading (Posner & Snyder, 1975).

The Stroop effect was developed by Stroop in 1935, though he was not so pretentious that he

narned it afler himself at the time (MacLeod. 1991). Stroop's discovery was that if an individual

was asked to read a list of colour words when each word was pnnted in a mismatched ink colour

(e.g.. RED in green, Say "red), they would be able to cornplete this task quite easily. If,

however. the task was to name the ink colour rather than the colour word (e.g.. RED in green.

Say "green"), performance was considerably slowed, despite explicit instructions not to read the

word. Thus, the word is thought to be read automatically and consequently the individual c m o t

avoid this interference. Indeed, the Stroop effect has often been considered a key piece of

evidence for the idea of automaticity in reading words (e.g., Posner & Snyder, 1975).

The Stroop effect. however, is just one of many exarnples of automatic processing. Another

often cited automatic effect involves sernantic priming in word pairs. The sernantic priming


effect refers to the phenomenon whereby the processing of a word is facilitated when preceded

by a semantically related context (Neely, 1977). As an example, the word "nurse" is responded

to more quickly in a lexical decision task, where the task is to decide if a string of letters foms a

word. when preceded by the word "doctor" as opposed to the word "bicycle."

Friedrich et al. (1991) built on the work of Neely (1977) and Posner and Snyder (1975), and

pointed to three processes which they assumed to be integral for semantic priming: automatic.

attentional, and postlexical. The automatic process is related to a rapid spread of activation in

memory from the stimulus or concept that is activated to a11 related stimuli or concepts, which

are quickly primed. As an exampie. if an individual were presented with the letter "a," it is

thought that "the lexical representation of a11 words containing that letter will also receive sorne

degree of activation" (Friedrich et al., 199 1. p. 792). Thus. they consider human memory to be a

network of interconnected representations. be they visual. semantic. or phonological. The

representations are thought to be interrelated so that the activation of a visual cue may also

activate a corresponding semantic or phonological cue (cf. Seidenberg & McClelland. 1989).

This activation is not under the control of the individual. Simple presentation of a stimulus

"automatically" activates related concepts at some level (Friedrich et al.. 1 99 1 ).

The second aspect of semantic pnming involves attentional processes. The attentional

mechanism is thought to generate a series of words andor concepts that are related to the prime

word that has been presented (Friedrich et al., 199 1). As an example, if the word "doctor" was

presented to an individual as a prime, the individual would then generate a list of words that

could follow, ranging fiom highly likely (e.g., nune) to less likely (e.g., stethoscope). The idea

of "likelihood of response" is what sets apart attentional processes from automatic processes.

Whereas automatic processes spread through mernory activating a11 related concepts. attentional


processes are more selective, priming some words more than others. In the previous example.

the word "nurse" would be more highly pnmed by the word "doctor" than by the word

"stethoscope." Friedrich et al. (1991) refer to this as the relatedness proportion effect.

Fnednch et al. (199 1) also speak of a third priming mechanism in semantic prirning, a

mechanism which occurs at the postlexical level. The authors provide this example of a

semantic-matching process:

... after lexical access of the target word has occurred, the presence of a relationship between the prime and target words can be used to speed the word-nonword decision process. (Friedrich et al., 199 1, p. 793)

Collectively, these three processes are thouçht to combine to create semantic priming. Semantic

priming effects are found under a variety of situations ranging fiom verbalizing the prime word

(e.g.. Henik et al.. 1983) to making semantic judgments about a prime (e-g.. Parkin. 1979: for an

extensive Iist, see Friednch et al.. 199 1).

Aithough both the Stroop effect and the semantic priming effect are key examples of

automaticity, there is an interesting finding that separates these two effects: A nimber of studies

that have documented procedures that can eliminate the semantic priming effect by changing the

manner in which the participant orients to the prime (e.g., Henik et al.. 1983; Smith. Theodor. &

Franklin, 1983). Despiie a number of documented ways to reduce Stroop interference (e.g.,

manual response; see MacLeod, 199 1. for a review). no methodology had yet led to an

elirnination of the Stroop effect. That was until 1997 when Besner, Stolz, and Boutillier claimed

that they had elirninated Stroop interference by colouring only a single letter of an irrelevant

word and requiring participants to name the colour of that letter. Their interpretation of their

results has come under scrutiny from a number of researchers (e.g. Marmurek. 1999; MacLeod.

1999), as have several aspects of their methodology.


This thesis will explore the concept of automaticity in the context of the Stroop effect and the

elirnination of semantic priming. It was the reduction and elimination of oemantic priming in

past studies that led Besner et al. (1997) to determine whether they could eliminate the Stroop

effect by a sirnilar manipulation. The original Besner et al. (1997) study and its successors will

be reviewed as will the unpublished work by Mamurek (1999) and MacLeod (1999) which

question the Besner et al. ( 1997) methodologies and conclusions. Altemate explanations and

interpretations for the results of these studies will be presented. Finally. new research will be

laid out as an attempt to add further insight to the processes that could account for Besner et al.3

( 1997) results

Reduction or Elimination of Semantic Priming

Numerous studies have been conducted which have shown that the effects of semantic

priming can be altered (e.g.. reduced or eliminated) if certain procedural modifications are made.

Most notably. researchers have been able to elirninate priming effects by changing the manner in

which the panicipant orients to the prime (e.ç., Henik, Friedrich. & Kellogg, 1983; Smith.

Theodor. & Franklin. 1983). Henik et al. (1983) had participants perform one of two tasks on a

prime (naming or letter search) and one of two tasks on a target (lexical decision or colour

naming). Interestingly. when the prime was named. lexical decision was facilitated whereas

colour naming was impeded. Both of these effects were eliminated. however. when a letter

search task was performed on the prime.

Other researchers have employed procedural modifications such as varying the stimulus onset

asychrony (e-g., Besner & Stolz, 1996: Neely, 1977) or having participants scan letters of a target

rather rhan responding to that target as a word (e.g.. Henik. Friedrich, & Kellogg, 1983). One of

the best examples of this latter approach is the study conducted by Friedrich et al. (199 1) to


examine the influence of semantic primes presented in the context of a letter search task. Similar

to Henik et al. (1983). these researchers compared differences in semantic priming magnitude

when participants were asked to name a prime word as opposed to searching through that prime

word for a target letter. The idea was to determine whether semantic primes would still be

encoded at a lexical level when searching for a target letter. even though this was not necessary

for the task. To investigate this. they presented participants with three different types of primes:

Semantic. cross-case identity. and unrelated. Identity priming is essentially repetition prirning,

with the prime word also being presented as the target word. Cross-case identity priming refers

to repetition priming in which the prime appears in lowercase letters whereas the target appears

in uppercase letters (or vice versa). This method of presentation ensures that priming is due to

the word itself rather than just to a lingenng iconic representation of the features or shape of the

word. When participants were asked instead to search for a probe letter in the prime. semantic

priming was eliminated. Identity priming remained, however. thus supporting the idea that the

lexical propenies of the prime word are encoded even in the letter search task. which apparently

only prevents the individual from making semantic associations.

The results of two subsequent experiments by Friedrich et al. (1991) indicated that lexical

priming was undected by switching to the letter search task. Thus, even when a word is

scanned letter by letter, the participant detemiines a lexical identity for the word that then primes

the word in identity pnming. This is an important finding as it demonstrates that letter by letter

processing does not necessarily lead to a disruption in word reading. Consequently, identity

priming was attributed to lexical priming rather than to letter by letter priming. A founh

expenrnent indicated that semantic priming retumed if the word was also spoken aloud by the

expenmenter, even when the participant was instructed to scan for target letters.


Previous studies had demonstrated that responding to a prime word in a nonlexical marner

reduced or eliminated the facilitation from semantic associates (e.g., Henik et al., 1983). The

Friedrich et al. (1 99 1) study demonstrated differential effects of priming for identity vs. semantic

targets when participants were required to visually scan the prime word for a probe letter. The

reduction of semantic priming did not seem attributable to nonlexical priming. Indeed, it appears

as though a lexical entry for prime words is encoded. even when the prime word is s c a ~ e d for a

probe letter. "These data instead suggest that either activation of semantic features of the prime

word or activation of semantic associates is disnipted" (Friedrich et al., 199 1. p. 802). Semantic

priming was reinstated. however, when that prime word was auditorily presented to the

participant. even when the prime word was being scanned for a probe Ietter. Presumably, this

occurs because searching a word for a probe letter may make the word easier to ignore. or less

wordlike because it is broken down into its individual letter components. Presenting the word

uiidiroriiy makes it more difficult to ignore. however. even if the individual has visiially parsed

the word into individual letter cornponents (it would be an interesting but difficult task to try and

attempt to have participants break down words into their individual sound components. and

determine whether this would also reduce semantic priming by making the word less wordlike).

Thus, it appears as though simply presenting a prime is not sufficient for priming effects to

occur. Rather. the individual must process the prime word in a certain way to a certain level.

Henik et ai. (1983) offer an intriguing explanation for these effects, saying that:

... activation of semantic associates rnay require some attentional resources and that the letter-search task focuses resources at the Ietter level, thus preventing spreading activation to related concepts.

(Friedrich et al., 199 1. pp. 793-794)

.Athough the prime word may be processed at some level, that processing may not be enough to


activate related concepts. Friedrich et al. (1 99 1) oEer a different explanation. When the

semantic prime is broken down into letter components by having participants search for a target

letter, prime words may not be encoded lexically, eliminating the beneficial effect of the prime.

This point is important as it provided the grounds on which Besner et al. (1997) posited that they

could eliminate Stroop interference by colouring a single letter of an irrelevant word.

Similar findings to those of Friedrich et al. ( 199 1 ) have ansen from the direct and indirect

remembering literature. It appears as though simply presenting a word to an individual is not

suficient for that word to be consciously remembered (MacDonald & MacLeod, 1998). Some

level of attentional resources is necessary for a word to be encoded. coinciding with the Friedrich

et al. (199 1) suggestion that there may be a partial attentional component to automatic processes.

If these attentional resources are disrupted, then the normal priming effects associated with

presenting words may be reduced or eliminated. Results from these and similar studies have

been used to cal1 into question the "automatic" word processing that was assumed to be present

in semantic priming.

Reducing or Eliminating the Stroop Effect: Colouring Only a Single Letter

The purpose of the present thesis is to investigate the possibility that the Stroop effect can be

eliminated in like manner, a claim made by Besner et al. (1997). Stroop interference on a single

incongruent item has been demonstrated to be as large as 180 ms (MacLeod. 199 1). Besner et al.

(1997), however, claim that the results of their second experiment show that the Stroop effect

can be completely eiiminated by colouring only a single letter rather than the entire word.

Besner et al. (1997) were cunous to determine whether the reduction and perhaps elimination

of semantic priming effects in the lexical decision task would generalize to a reduction or

elimination of semantic priming effects in the Stroop task. To investigate this, they performed


two experiments. They predicted that if only one letter of a word were coloured in a Stroop task,

this mi@ negatively affect the processing of the word as a word and consequently reduce andor

eliminate the S troop effect. The logic of this research is thus sirnilar in nature to the studies of

Friedrich et al. (1991) and others with regard to semantic priming. Although the Besner et al.

(1997) study contained two experiments, it is the second of these that is most relevant to the

present proposa1 and as such will receive the most attention. This is not to Say. however. that the

first experiment is not important, as it has helped to lay the theoretical groundwork for the

present t hesis.

In their first experiment, Besner et al. (1997) used a colour word condition (e.g.. BLUE in

red) and a pseudohomophone condition (e.g., BLOO in red) "to assess the idea that the Stroop

effect is nonnally larger for words than for the pseudohomophones" (p. 222). Stimuli appeared

in either a congruent colour (e-g., RED in red) or an incongment colour (e.g., RED in green) with

either the entire word coloured or a single letter of varying position coloured. Sixty-four

participants each perfonned 288 test trials and 72 practice trials in which al1 eight conditions

(Le., word type X congmency condition X amount coloured) were intermixed. balanced. and

randomly presented. Participants were to indicate the colour of the entire word or of the single

coloured letter by pressing one of four buttons on the keyboard corresponding to the four colours

used (red, yellow. green. and blue).

Analyses indicated that congruent trials were responded to faster than incongment trials. This

finding is consistent with a great deal of past research on the Stroop effect (MacLeod, 199 1). As

well. fully coloured words were responded to more quickly than words containing a single

coloured letter. There was no effect of word type (colour word vs. pseudohomophone). but there

were two significant interactions. First, there was an interaction between the congniency


condition and the word type, with a larger Stroop effect occurring for colour words relative to

pseudohomophones. This would be expected on the basis of many studies investigating the

impact of word type (e-g., Klein, 1964). Most critically, though, there was also an interaction

between the congruency condition and the method of colouring, with larger interference

occurring for fully coloured words relative to single-letter coloured words. Besner et al. (1997)

took these results to suggest that the notion that words are read automatically may be too strong:

If words are always read autornatically, there should not be a difference in the magnitude of the

Stroop effect for fûlly coloured words vs, words with only a single letter coloured. It is worth

noting that a similar argument was made against strong automaticity and in favour of weak

automaticity by Kahneman and Chajczyk (1983) in their "Stroop dilution" studies. Besner et al.

(1997) also argued that the reduced Stroop effect for pseudohomophones relative to words is

evidence that there is more to semantic comprehension than simply phonemics. Theii second

experirnent extended this argument.

Experiment 2 was similar in procedure but featured two key differences: Congruent t d s

were eiiminated (because they encourage the participant to read the word) and

pseudohomophones were replaced with a control condition referred to as "congnient nonwords"

( E T . YENILE. GREM), and BLAT). The use of the word "congnient" applied to nonwords

serving as controls in the context of Stroop research is somewhat confusing, so it may be better

to refer to their controls as "colour nonwords." Besner et al. (1997) chose these control items

rather than baseline X's because, to thern, these items appeared more word-like. Also, the

"controls were chosen to match the incongruent stimuli on Iength and on the first two letters.

given the repeated worry that a Stroop effect cm result simply from the first two letters spelling

a color word" (p. 223). This is an intriguing argument, and one :hat will be examined in more

Reduced Stroop Interference - 1 1

depth &er their Experiment 2 has been reviewed.

In Experiment 2. 64 participants perfonned 36 practice trials followed by 144 experimental

trials. The response tirnes and error rates are presented in Table 1. Again, single letter coloured

stimuli took longer to respond to than did M y coloured words. AIso, incongruent triais took an

overall longer time to respond to than control trials. Finally, there was a critical interaction

between the congruency condition and colouring type. with no Stroop interference apparent

when a single letter was coloured relative to the entire word. It is this last finding that provides

evidence that the Stroop effect has been eliminated. In their words "simply coloring a single

letter instead of the whole word eliminated the Stroop effect" (Besner et al.. 1997. p. 22 1). As

Table 1 shows. when only a single letter appeared in colour, there was no difference between

incongment and control condition response times relative to a 30 ms difference when words

appeared fully coloured. Besner et al. (1997) clairned that these results were proof that the

Stroop effect could be elirninated and that. therefore. automaticity accounts of word reading are

too strong.

This line of work has continued in a series of subsequent articles. Besner and Stolz ( 1999a)

hypothesized that the elimination of the Stroop effect in their 1997 study could have been due to

a narrowing of spatial attention that prevented lexical activation of target words. Thus. they

designed four expenments to test the level of Stroop interference under a variety of different

spatial and colour conditions. In their first two expenments. they used the incongment and

congruent conditions (e.g., colour words appeanng in either congruent or incongment colours).

There were two experimental manipulations. The first was a cuing manipulation that consisted

of a single arrow or string of arrows appearing above and below either the entire colour word, a

single coloured letter which displayed the colour to be named. or a single coloured letter which


Table 1

Mean Response Times (in ms) and Percentage Enor Rates to Name Colour 'as

3 Function of Condition (Al1 Letters Coloured vs. Single Letter Coloured). Froin

Emerirnent 2: Besner. Stolz, and Boutilier ( 1997).

NI Letters Colored Single Letter Coloured

Incongnient 777 2.9

Control 713 2.4

Difference 34 0.5

Note: RS = response Times; %E = percentage of errors


displayed a colour other than that which was to be named. The second was a colour

manipulation in which colour words appeared entirely in one colour (e.g. RED in blue) or in two

colours with one letter in the colour to be named and the other letters in one of the other three

colours used (e.g., RED with R in green and ED in yellow). The only difference between

Experiments 1 and 2 was that colour words in Experiment 2 were displayed with a blank

character space between successive letters "to investigate whether larger spaces between adjacent

letters would lead to irnproved spatial selection" (p. 100). In all, there were 256 trials that

consisted of 8 blocks of 32 trials for each experimental manipulation. As in the Besner et al.

(1997) snidy, al1 responses were made manually.

The results of the first two experiments are presented in Table 2. The key finding was that

cuing a single letter when only one consistent display colour was used reduced Stroop

interference relative to cuing al1 letters. When al1 letters were cued, 126 ms of interference was

observed whereas only 88 ms of interference was observed when a single letter was coloured.

When two colours were used on each Stroop display. the results indicated that cuing the to-be-

responded-to congruent colour decreased interference relative to cuing the dominant incongruent

display colour. When the dominant incongruent display colour was cued, 33 ms of interference

was observed whereas ody 2 1 ms of interference was observed when the to-be-responded-to

congruent colour was cued. The pattem of results in Experiment 2 was comparable to that of

Experiment 1. with the sole exception that responses were made more quickly in every condition.

The pattem of interference, however, did not seem to be significantly reduced by displaying a

blank character space between successive letters. Although the manipulations in the first two

experiments led to a reduction in interference, the researchers were certain that this interference

could be eliminated, so they designed two further experiments to test this notion.

In Experiments 3 and 4. both single colour displays and congrnent trials were elirninated. In


Table 2

Mcan Remonse Times (in ms) to Name Colour as a Function of Condition (Al1 Letters

Coloured vs. Single Letter Coloured). Number of DimIav Coloun (One vs. Two) and

Cued Colour (Main vs. Oddl From Besner and Stolz (1999) Esmriments 1 and 2.

All Letters Colored Single Lctter Coloured

Colour Condition Incongrucnt Conruent Difference Incongnicnt Congruent Diffcrcncc

One Colour 897 77 1 126 866 788 78

Two Coloun %tain Colour Cued N/A N/ A N/A 952 9 19 3 3 Odd Colour Cued NIA NIA NIA 905 884 2 1

One CoIour 860 73 2 128 8 13 75 1 62

Two Colours Main Colour Cued N/A N/ A N/ A 880 829 5 1 Odd Colour Cued N/ A NIA N/ A 830 805 2 5


place of congruent triais, the researchers used the colour nonwords from Besner et al. (1997) as a

control condition "to elirninate any benefit from reading the word" (p. 102). Due to the

elimination of single letter coloured trials, the cue manipulation now consisted of oniy single-

letter cues, rather than whole-word cues. The only difference between Experiments 3 and 4 was

that the to-be-responded-to colour was cued on half of the trials (the incorrect colour was cued

on the other half of the trials) in Experiment 3 whereas the to-be-responded-to colour was cued

on every trial in Experirnent 4. Thus, Experiment 4 encouraged "spatial selectivity" as the

desired response was always precued whereas attending to the cued position in Expenment 3 was

not necessarily helpful. and in fact, potentially hindenng (on 50 % of the triais).

The results of Experiments 3 and 4 are presented in Table 3 . These expenments provided

further evidence that the Stroop effect could be eliminated. When the dominant colour (not the

to-be-responded-to colour) was cued in Experiment 3. there was still 18 ms of interference based

on the difference between incongruent trials and neutral trials; however. when the to-be-

responded-to colour was cued, this difference was reduced to 1 ms. Similarly, in Experiment 4.

there was a small but nonsignificant difference of 9 rns between incongment and neutral trials.

These findings are in accord with Besner et al. (1997) and consequently. were interpreted by

Besner and Stolz (1999a) as further evidence that Stroop interference can be eliminated. and that

therefore word reading is not automatic.

In a further study. Besner and Stolz (1999b) investigated the relation between spatial attention

and the Stroop effect by rnanipulating the position of a coloured letter in colour words, shape

words. and geometric objects. In their first two expenrnents, either colour words (red. blue.

green, yellow), colour-neutral words (these words were al1 five Ietters in length but no

description was explicitly given). or shape words (circle, triangle, square) appeared within a


Table 3

Mean Response Times (in ms) to Name Colour as a Function of Nurnber of DimIav

CoIours (One vs. Two) and Cued Colour (Main vs. Odd) in the Single Letter Coloured

Condition Frorn Besner and Stolz (1909) Eswriments 3 and 4.

Colour Condition Incongrnent Congrnent Difference

Expcriment 3

Two Colours

Main Colour Cued

Odd Colour Cued

Eweriment 4

Two Colours

Odd Cotour Cued 820


bounding box. These words appeared with either a single letter coloured or with every letter

coloured. When a single letter was coloured. its position was randomly determined. Unlike the

previous studies, however. the coloured letter could not appear in every position: Only the

beginning (first), middle (third), and end (fifth) positions were used. The display colour to be

responded to was either congruent or incongruent with respect to the colour words. In the third

experiment, distractor colour words appeared within geometric shapes (circle. triangle. square)

whose borders were either fully coloured or partially coloured. As with the two previous studies.

participants responded manually to the colour display.

The results of these experiments again indicated that Stroop interference was a fbnction of

spatial attention. When targets appeared fully coloured, there was approximately 33 ms of

Stroop interference based on the difference between incongruent and congruent trials in every

experiment. When only a single element of a target was coloured. however. interference was

eliminated in both Experiments 1 and 2. This effect was most pronounced in the second

experiment using shape words. In Experiment 3. however. colouring a single element still

resulted in 20 ms of interference. Besner and Stolz (1999b) provided the following interpretation

of their results:

When the color camer stimulus and the spatially distinct color word belonged to the same domain (Le., when they were both words), the single-colored-element manipulation largely eliminated the Stroop effect (Expenments 1-2). Ln contrast. when the color camer stimulus and the spatially distinct color word belonged to difFerent domains. the single-colored-element manipulation produced a Stroop effect that was as large as when the color camer stimulus was al1 colored.

(P. 453)

Collectively, Besner et al. (1997; Besner & Stolz 1999ah) concluded that the Stroop effect can

be eliminated when participants are made to perform a letter search task on distractor words.


They concluded that this was caused by blocking the flow of activation to semantics:

Explicit processing of a letter's form or color in the context of a word is associated with letter and lexical level activation that is blocked from flowing strongly to the semantic level. Instead, lexical level activation feeds back to the letter level to enhance processing there.

(Besner et al.. 1997, p. 224)

Besner et al. (1997) offer little additional suppon for this argument. For this idea to be

plausible. one would have to provide direct evidence that letter search tasks divert attention From

semantics while simultaneously enhancing letter-level processing. In fact. evidence to the

contrary was presented in an earlier study by Besner himself. Besner. Smith. and MacLeod

(1990) investigated the effects of semantic relatedness when pairs of related words. unrelated

words. or nonwords were searched for a common letter and found that "even though participants

were set to look for individual letters in the search task. semantic level processing occurred" (p.

866). Indeed. words that were semantically related were processed more slowly than words that

were semantically unrelated in a letter search condition even though semantically related words

pairs were processed much more quickly than unrelated pairs in a lexical decision task.

Stolz and Besner (1996) also demonstrated semantic priming effects in a letter search task

when there was a short stimulus onset asychrony between the prime trial and the probe trial.

Thus. switching to a letter search paradigm did not prevent a flow of activation to the semantic

Ievel in either of these studies, though it did result in differential effects of semantic relatedness.

This point will again be important when we consider the work of Besner et al. (1997) who

argued that colouring a single letter of a word prevents activation of that word at the semantic

level and as a consequence. eliminates Stroop interference. Their explanation for this

elimination of the Stroop effect was the same one that they had previously offered to explain the

elimination of semantic priming when a letter search task is used. Their explanation relies on the


concept of "blocking" which they use to identie the prevention of a normally well established

automatic process (word reading) due to an increased focus on the nonlexical aspects of an

irrelevant word (in this case. the coloured letter). Besner et al. (1997) argue that. in essence,

colouring a single letter directs attention towards individual letters, thereby, preventing the

irrelevant word From being viewed as a whote. This argument is dependent upoo the observed

equivalence in response times between incongnient colour words and colour nonwords when a

single letter is coloured. According to these researchers. these response tirne data provide stronç

evidence that "at least some aspects of word recognition are not automatic" (p. 224)

A Critique of the Besner, Stolz, and Boutilier (1997) Argument

Despite their elimination of the difference between a classic incongruent condition and a

novel control condition by the single letter coloured manipulation. interpreting the Besner et al.

( 1997) results is not as straightforward as it might initially appear. Rather than eliminating the

Stroop effect, it appears as though their particular procedural choices decreased the Stroop effect

substantially (by using manual response and unusual controls). arriving at about 30 ms of

interference. Their single letter coloured manipulation thus only had to remove this remaining

30 ms to produce apparent elimination of the Stroop effect 1.

Manual responding has been shown to remove up to 75 % of Stroop interference relative to

vocal responding (MacLeod. 199 1). A number of studies have demonstrated a reduction in

Stroop interference when manual response is used (e.g., Nielson, 1975: Redding & Ge jets,

lThere is one actuai flaw with the Besner et al. (1997) nudy that appean minor. though its magnitude is nor readily apparent. The authors note that they conmcted their control items using the first two letters of each cotour word. flic control nonword "grend." however. uses the first three letters of the colour word "green." Due to the fact that the present study intends to replicate and estend the Besner et al. (1997) findings, the nonword "grend" wi11 also be used in the present study. For consistency ilth the stated method and the other three control items. an item such as "grond" or "grund" wouid actually have been &ter. 1977). This simple methodological variation can reduce normally large (e.g., 180 ms) Stroop


interference down to 45 ms. Certain control conditions may also appear to reduce the Stroop

effect (e.g.. colour nonwords such as BLAT) given that the Stroop effect has been s h o w to

occur even when only the first letter of a conflicting colour word is presented (Regan, 1978;

Logan & Zbrodoff, 1998). Recall that Besner et al. (1997) constructed their control stimuli

( E T . GREM). YEMLE. and BLAT) "to match the incongruent stimuli on length and on the

first two letters, given the repeated worry that a Stroop effect can result simply from the first two

letters spelling a color word" (p. 223). It is quite unclear how this constitutes a corifrd condition

in that it seems likely, even by the authors' own admission. that interference could also occur for

these new nonword stimuli (Le.. there could actually be interference in the nominal control

condition). Thus, it would be difficult to determine whether response times to stimuli are similar

because interference has been eliminated or because the Stroop effect is occumng for both sets

of stimuli.

In essence, incongrnent colour words and control nonwords may be effectively equivalent if

the first two letters of colour words are the source of the majority of Stroop interference. This

possibility will be considered in more depth when the work of Regan ( 1 978) and Logan and

Zbrodoff (1998) is reviewed. The fact that Besner and Stolz (1999ah) replicated Besner et al.

(1997) does not vitiate this argument given that the same response mode and control conditions

were used. Thus, by using colour nonwords (e.g.. RET and YENILE). and by having

participants respond manually, Besner et al. ( 1997) only had to eliminate a 30-rns Stroop effect.

which is very small when compared to the more typical size of the S troop effect ( 1 80 ms). Were

different controls and vocal responding used, it is quite possible that up to a 150-ms Stroop effect

would still be observed between colour words and nonwords, even if the Besner et al. (1997)

manipulation reduced inteièrence by 30 ms.


The critical concem with the Besner et al. (1997) study. however, is their bold theoretical

claim that they have eliminated the Stroop effect. A careful examination of the obtained results

suggests a rather different interpretation. Their reduction in the Stroop effect is derived not from

a decrease in response time for the incongruent Stroop tnals so much as fiorn an increase in

response time for their novel control Stroop trials. This seems countenntuitive because one

would expect that a decrease in Stroop interference would be a function of incongruent response

time decreasing to meet control response tirne. Besner et al. ( 1 997). however, have shown an

increase in the response tirne to the control condition, leaving the incongruent condition

unaffected. This does not necessarily mean that Stroop interference has not been reduced or

eliminated, but it is an unexpected pattern of results and one that certainiy deserves further

scrutiny. It is dificult to view Stroop interference as having been eliminated when response

times for incongruent tnals (the condition responsible for Stroop interference) in both the whole-

word-coloured and single letter coloured conditions are vimially identical.

Collectively, these problems have made several researchers dispute the claims of Besner et al.

( 1997). Studies by Marmurek ( 1999) and by MacLeod ( 1999) have replicated Besner et a l . 3

( 1997) basic methodology but have also included additional conditions, such as vocal responding

and different, more common controls. Marmurek's control stimuli consisted of the animal names

cat. lion. mouse, and rabbit. These controls have the advantage of being words that are identical

in length to the colour names but that decrease the chance of invoking a Stroop effect (although

rabbit does begin with the same letter as one of the colour words). In addition, the words belong

to a single category (animais) as do colour words (colours). Besner et a1.k control stimuli and

button pressing procedure were also used for cornparison purposes. Thus. using a 2 X 2 design,

Marmurek compared the performance of four groups of participants. One group responded


manually (button pressing) to Besner et al.'s (1997) stimuli (colour words and nonwords), a

direct replication of the Besner et al. (1997) study. A second group also used Besner et a1.k

(1997) stimuli, though participants responded vocally to the to-be-identified colour. A third

group responded manually but used the aforernentioned animal word controls. The final group

also used the animal control stimuli but responded orally to the colour.

The results of the Marmurek (1999) study showed that Besner et a1.k ( 1997) apparent

elimination of the Stroop effect was dependent on the modality of response (see Table 4). When

rnanual responding was used, Besner et al.'s ( 1997) reduction of interference in the single letter

coloured condition was replicated. though interference was not completely eliminated. For fully

coloured items. conflict trials took longer to respond to than did control tnals. When only a

single letter was coloured. the difference betwcen conflict and control trials was reduced. This

reduction was açain due to an increase in response time for control trials whereas inconçment

tnals sbowed little increase. Clearly. colouring a single letter led to a decreased amount of

interference between conflict and control trials. It is not clear, however, that the decrease in the

difference between contlict and control trials is due to a reduction of Stroop interference nor that

it always constitutes elirnination. Marmurek's ( 1999) results suggest that the difference between

conflict and control trials in the Besner et al. ( 1 997) study could be attributed to the use of button

pressing. When the entire word was coloured, there was a 42 rns response time difference

between conflict and control trials, with conflict trials taking longer. When only one letter was

coloured. both contlict and control tnals decreased in response time. though there remained a 42

ms difference; far fiom an elimination, if even a reduction. of Stroop interference.

When Marmurek ( 1999) switched to his own controls (animal names). a similar if not more

stnking pattern of results occurred. With manual responding. there was a 38 rns difference in


Table 4

Mean Response Times lin ms) to Name Colour as a Function of Condition (AI1

Letters Coloured vs. Single Letter Coloured) Across Four Grou~s (Vanine

Remonse Mode and Controls) From Mannurek ( 1999).

Ai1 Letters Colored Single Lctter Colourcd

Triai Type Incongruent ControI Difference Incongruent Control D ifference

Manual response. nonword controls (Besner et al. replication):

RT 882 83 9

Manual remonse, animal word controls:

RT 789 75 1

Orai response. nonword controls:

RT 758 711

Oral reswnse. animal word controls:

RT 823 709

Note: RT = response tirnes


response tirne between conflict and control trials, with conflict trials taking longer. This

difference decreased to 25 ms when only a single letter was coloured. al1 attributable to increased

response time in the control condition. In the oral response situation, the difference between

conflict and control trials was 1 14 ms when the entire word was coloured. When only a single

letter was coloured, conflict trials decreased in response time by 3 1 ms. though there was still a

substantial difference of 77 rns between contlict and control trials. These results certaidy do not

indicate rlimitiatiori of the Stroop effect. Once again, the diminished difference reflects slower

responding in the control trials in the single letter coloured condition.

Besner et al. (1997) seem to be correct that the difEerence diminishes by 30 ms in moving

from fully coloured to single-letter coloured items. and this seems to be fairly consistent across

response mode and control type. The question rernains. though. as to how this reduction relates

to interference. More critically, their strong daim of eliminating Stroop interference is not

supponed. Besner et al. ( 1997) had stated that their "results are consistent with the claim that it

is possible to prevent the computation of semantics." thereby leading to an elimination of Stroop

interference (p. 224). The Marmurek (1999) results, however. are contrary to this argument

because the colouring manipulation did not prevent sernantic cornputation, nor did it lead to

elimination of' t he S troop effect .

MacLeod (1999) has also replicated the Besner et al. ( 1997) study, again exarnining different

controls and modes of response. Although independent. the MacLeod (1999) study was also a 2

X 2 design crossing mode of respondinç with type of control item. Here. however, different

controls were used: Repeated letter swings (www, m. sssss, mrnmmmm) as opposed to animal

words or colour nonwords. In a sense. whereas Marmurek ( 1999) moved "up" fiom Besner et

a h (1997) colour nonwords to real noncolour words. MacLeod (1999) moved "down" to real


nonwords.

There is a wide-ranging debate in the Stroop literature as to what constitutes an appropnate

control (cf. MacLeod. 199 1). It has been argued that such letter stnngs are less word-like and

therefore. not a domain-appropriate control for comparison with words (Besner et al.. 1997). On

the other hand. many forms of words and nonwords have been shown to produce Stroop

interference (e.g., Dalrymple-Alford, 1972. Klein. 1964). making it difficult to distinguish a

reduction in intei-ference on a confiict trial fiom an increase in interference on a control trial (see

Besner et al.. 1997). For this reason. there is no universally agreed upon control type. though by

including the letter stnngs condition. MacLeod (1999) contributes to an overall clearer picture of

the effect of colouring a single letter of a control relative to colouring the entire item.

Theoretically. any difference between the results of MacLeod ( 1999). Marmurek ( 1999). and

Besner et al. (1997) can be attributed to differences in the control stimuli (letter stnngs vs.

animal words vs. nonwords) because al1 other features of the methodology (button pressing).

colour words, and presentation order (randomized) were held constant. Clearly this will not

solve the argument as to the appropriate type of control stimuli to use in Stroop studies. though it

does ailow for comparison as to the different levels of interference each control condition might

produce.

The MacLeod (1999) results are presented in Table 5. It is not necessary to explicitly outline

the results of al1 four conditions. because they are comparable to Marmurek's (1999) four

conditions. It should simply be noted. however, that MacLeod (1999). despite obtaining a

perfect replication of the Besner et al. (1997) results in the comparable ce11 of his 2 X 2 design.

clearly did not eliminate interference when vocal responding and different controls were used.

Despite a decrease in response time in codict trials as well as an increase in response time in


Table 5

mea an Res~onse Times (in ms) to Name Colour as a Function of Condition

(ALI Letten Coloured vs. Sin& Letter Coloured) Across Four Groups

f Vaning Remonse Mode and Controls) From MacLeod (1990).

Al1 ~etk&olored

Trial Type Incongruent Control Difference

Single Letter Coloured -

Incongruent Control D ifference

Manual response. nonword controls (Bcsner et al. replication):

RT 73 3 700 33

Manunl response. animal word controls:

RT 7 43 696

Oral remonse. nonword controls:

RT 778 728

O n l remonse. animal word controls:

Note: RT = response times


control trials, there was still a substantial difference between the two conditions in al1 but the

Besner et al. (1997) ce11 of his design. indicating reliable Stroop interference. even in the single

letter coloured condition.

Collectively, the works of Besner et al. (1997). Marmurek (1999), and MacLeod ( 1999)

indicate that the Stroop effect is not eliminated by colouring only a single letter. contrary to the

claim of Besner et al. (1997). This holds true for the nonwords condition used by Besner et al.

(1997), the animal words used by Marmurek (1999). and the letter strings condition used by

MacLeod ( 1999). .Mthough these researchers successfully replicated Besner et al.'s ( 1997)

pattern of results under precisely the conditions they used. these newer data have in no way

provided evidence that the Stroop effect was eliminated. In fact. when responding is made vocal

rather than manual, there is a substantial Stroop eRect observed in the single-letter coloured

condition. In al1 three studies. however. the consistent finding has been that the control condition

rises by about 20-30 ms when only one letter is coloured. Unquestionably. this is an interestinç

result just not necessady the elimination of Stroop interference that Besner et al. (1 997)

interpreted it to be. Thus. the pnmary purpose of the present research was to better understand

the manner in which Stroop interference is reduced in Besner et al.3 ( 1997) sinçle letter

coloured condition. In addition. the present research will try to determine why a 30 ms increase

in response time is observed in every condition except the classic incongruent condition.

Interpreting the Apparently Reduced Interference in the Single-Letter Coloured Condition

There are a number of possible explanations for this apparent decrease in Stroop interference.

though there is currently no evidence to uniquely confirm any one of these explanations. One

possibility is that response times to colour words in the single-letter coloured condition do not

increase. perhaps due to a ceiling effect in the fùlly coloured condition, not to a reduction in the


Stroop effect. If response times for colour words in the incongrnent condition were at a ceiling

level when hlly coloured, then the letter search manipulation could not lead to an increase in

response time above that level. This would explain why the letter search task can consistently

lead to an increase in response time for control stimuli but no such response increase for colour

word trials (which always demonstrate the highest response times when al1 letters are coloured).

It seems unlikely, however. that a ceiling effect would explain the results of al1 three expenments

reviewed thus far (Besner et al.. 1997, MacLeod, 1999: Marmurek. 1999) in that response times

of 750-800 ms are not particularly high when one considers individual differences (response

tirnes ranging fiom 1000-1200 ms are not unheard of). Response times would need to be more

inflated before it could be concluded that response time was at a ceiling levei.

A second possibility could be that the contrast between the coloured letter and the rest of the

letters in the word is critical, a type of pop-out effect (see. e.g., Treisman & Gelade. 1980). For

example. the colours red and blue are fairly discemible €rom the colour white when being viewed

on a black background whereas the colours yellow and green do not stand out as much. Thus.

when the coloured letter is red or blue it might stand out more from the irrelevant word making

the irrelevant word easier to ignore (and perhaps, less wordlike). Yellow and green letters may

blend into the white type. however, making it difficult for the participant not only to identiQ the

display colour. but also to ignore the irrelevant word as its integnty is fairly well maintained.

Thus, it might be possible to alter performance on the Stroop task by varying the background

colour or the brightness and hue of the colour displays.

Unfortunately, there is nothing in the literature to suggest that either of these two factors is the

key variable. Besner et a1.k (1997) "semantic blocking" explanation for these eKects is

interesting, though there are some difficulties associated witn generalizing semantic piiming


explanations to the Stroop effect, as outlined in the following section. Nonetheless, I would like

to propose an alternative account for these results that does not necessarily involve the

"blocking" of irrelevant words: Rather. 1 would argue that these results likely point to the

influence of two processes. In taking this stance. it is necessary to believe that Besner et a1.k

(1997) account is partially correct and that it is not merely the control trials that are slowing

down when a single letter is coloured: rather, everything slows down and the response time data

indicate a true reduction in Stroop interference masked by overall slowing of equivalent

magnitude.

There is a distinct possibility that when an individual processes a word with a single letter

coloured (and al1 other letters appearing in white pnnt), they do so differently than if the same

word appeared entirely in a single ink colour. This is not to say that the individual does not read

the word: rather. colouring a single letter may make a word appear unique or less wordlike. If

this is the case, such differential processing rnay lead to a decrease in Stroop interference of

about 20-30 ms. This 20-30 ms reduction in Stroop interference is then likely offset by an

increase in the time that it takes to identiS, the coloured letter because the individual is forced to

search for the coloured letter which varies in position randomly from trial to trial. When the

entire word is coloured, however, it does not matter where in the word an individual Iooks

because every letter position will provide the desired colour response information.

For example, if the first word a participant encounten is a four-letter word with the founh

letter coloured, the participant would have to search for that coloured letter. Once the colour

letter has been identified, the participant would either remain fixated on that position on the

screen or return to the fixation position. In either case, if the next word that appeared was a fully

coloured word, no search time would be necessary and a response could be made irnmediately.


If. however, the next word that appeared was a five letter word with the second letter coloured,

then the participant would again have to engage in another search, thus increasing the time

necessary to make a response. The letter search might in tum iead the participant to process the

distractor word differently, thus leading to a decrease in Stroop interference. This explanation

does an adequate job of accounting for the finding that Stroop interference was decreasing due to

an increase in control response times as opposed to a decrease in response times in the

incongrnent condition. Essentially. incongruent response times stay the same when a single

letter is coloured because the extra time to search for the coloured letter is offset by a decrease in

Stroop interference, which makes it seem as though the times do not change in switching from

fûlly coloured words to single letter coloured words. In the control condition. there should

(ideally) be no Stroop interference and therefore. only the increase in search time affects the

colour naming response. Henceforth. this explanation will be referred to as the two-process

account.

The two-process account implies that a reduction in Stroop interference occun when a single

letter is coloured. It is dificult to accept Besner et al.'s interference elimination conclusion,

however, based solely on similar response times in the incongruent and control conditions of

their study. To argue that the Stroop effect has been eliminated. it would be necessary to show

that the distractor words in Stroop trials are not being read. Besner et al. (1997) have suggested

that colouring a single letter tums the Stroop task into a lerter search task, thus disrupting

semantic processing of the distractor word. Their inference is that this letter search task has

prevented participants from reading the distractor words and, thus, from processing the semantic

properties associated with these words. In a sense then, they are suggesting that colouring a

single letter causes the individual to treat the word as though it is not actually a word. It is


difficult to make this argument relying solely on response time data, however. as response time

is an indicator of how quickly participants performed the colour naming task and provides no

information as to whether distractors were read. That distractors are not being read has been

inferred due to equivalence in response times. but no explicit demonstration has been made to

show that colouring a single Ietter causes participants to break words down into their individual

letter components. Thus hrther evidence is needed to corroborate Besner et a1.k (1997)

response time data.

Why the Reduction and Elimination of Semantic Priming May Not Generalize to the Stroop Effect

As was suggested earlier, the evidence is not strong and at best indirect for Besner et a1.k

( 1 997) claim that colouring a single letter blocks semantic processing while simultaneously

enhancing processing at the letter level due to feedback from the lexical level. Without such

evidence, the appropnateness of this rather complex expianation is difficult to judge. Moreover.

as in Friedrich et al. (199 l), semantic primes are usually presented to the participant pior to the

target stimuli. In Stroop research, however, and particularly in the Besner et al. (1997) study, the

distractor item (the colour word) is presented simultaneously with the stimulus that requires a

response. Presenting distractors simultaneously rather than in advance is likely to affect the

rnanner in which a panicipant orients to a task. As an example. if the distractor word in a Stroop

task were presented prior to the presentation of a display colour, participants could employ a

strategy that would enable them to ignore the distractor prior to display colour presentation. At

the very least, presentation of a distractor p ior to a target may provide participants the time

necessary to process and then block the distractor. When distractor and display colour are

presented at the same time, however, simultaneous demands are placed on the participant.

making it harder to ignore the distractor. Thus, a different type of strategy may need to be


adopted rather than ignonng (or processing and then blocking) the first stimulus presented.

Indeed, a nurnber of studies have been reported in which the stimulus onset asynchronies (SOA)

of distractor word and display colours were vaned in the context of a Stroop task. Although

Stroop interference is always observed, different patterns of interference are observed (generally,

a decrease in interference when irrelevant colour words are presented pnor to the colour display)

as a function of different SOAs (e-g. Dyer, 1974; Glaser & Glaser. 1982; Koch & Brown, 1994;

MacLeod & Hodder, 1998: Sugg & McDonaId, 1994; see MacLeod. 199 1, for a review).

Aiso. certain words are more Iikely to be facilitated by a prime stimulus than are other words.

In the earlier example, the word "nurse" is facilitated to a greater degree by the word "doctor"

than by the word "stethoscope." This is due to the higher degree of association between the

words "nurse" and "doctor" relative to the words "nurse" and "stethoscope." In Stroop studies.

however. the colour words used are al1 very highly associated with each other and to the concept

of colour. For this reason. even if a reduction in semantic pnming were observed for a letter

scan task, it is doubtful that the semantic properties of the colour words could be ignored on

every trial. Although various researchers (e.g., Fnedich et al.. L 99 1 ) have s h o w that the effects

of semantic primes can be reduced and in some cases eliminated. no researcher has shown that

the pnming effects of such highly related words can be eliminated al1 of the time, particularly

with such a tiny stimulus/response set as is the case in the standard Stroop task. Due to the

strong semantic relation among colour words, as well 3s to the fact that each word is presented

numerous times. it is unlikely that the semantic relation between these words could be absoluteiy

ignored. This is evidenced by the fact that when vocal responding is used in the Besner et al.

( 1997) task a Stroop effect does occur (Macleod, 1999; Marmurek, 1999). indicatinç that the

semantics of the colour words have been computed. Finally, as has been previously noted. there


are a number of documented ways to reduce the Stroop effect (e.g., manual responding), which

are not necessanly relevant to semantic priming (see Sharma & McKema, 1998). Besner et al.

(1997) have employed two of these methods in their study: manual responding, and items that

produce interference in the "control" condition.

When letter search was examined by Marmurek (1999) and MacLeod (1999) with vocal

response and different controls. a substantial Stroop effect was still observed despite the

reduction of Stroop interference by 30 ms. Clearly, the colour word is being read in the

Marmurek (1999) and MacLeod (1999) studies. showing that semantic pnming occurs in at least

some, if not most, situations even when a Ietter scan task is used. It should be noted. however,

that Besner et al. (1997) are correct in stating that words are not always automatically read.

CIeariy there are many situations in which word reading is not necessary or would even be

detnmental. As an example. consider an individual who scans a dictionary to find a definition

for a cenain word. On a single page of a dictionary there are numerous words and definitions for

these words. The individual does not. however. automatically read every single word on the

page as this would be a great waste of time. Rather. they quickly scan until they find the word

for which they are seeking a definition. perhaps reading (automatically or otherwise) only a few

other words during the search.

Automatic effects must be contemually bound, with the previous example providinç a

situation in which al1 words are not automatically read. Aithough words are not always

automatically read, however, Besner et al. ( 1997) provide insufficient proof to support the daim

that colour words are not being read in their letter scan condition. Simple response time data are

not enough to prove that a word has not been read and that the semantics of the word have not

been encoded and understood. Response time data solely indicate how long participants take to


colour name when a single letter is coloured, they do not provide an unarnbiguous rneasure of

whether distractors are read. It is appropriate to use response time in studies of semantic priming

as there is an expected response benefit of having previously seen a related word. An absence of

this benefit would indicate that the prime word had not been read. In Stroop studies. however.

one cannot argue that an equivalence in response time between incongment colour words and

colourlike nonwords is indicative of distractors having not been read. Other possibilities exist.

An equivalence in response time rnay be the result of both distractor types being read and

producing approximately equal interference. Such an equivalence could also be caused by more

than one process contributing to response tirne in the Stroop task. This is the basis for the two-

process account that I have outlined.

It has already been suggested that Besner et ah's (1997) original Stroop effect was quite

small. and likely due to a number of other methodologicai choices they made (e-g.. manual

response. novelcontrol condition). Thus. rather than trying to account for a complete elirnination

of the Stroop effect, it seems more reasonable to atternpt to account for the approximatcly 20-30

rns of decrease in Stroop interference that is observed when only a single letter is coloured. The

two-process account explains this decrease nicely. and will receive further scmtiny in the present

thesis.

Thus far, however. there has been a key omission in the discussion of factors that may have

infiuenced the results of Besner et al. (1997): the possible influence of the position of the

coloured letter when irrelevant letters appear with only a single letter coloured. 1 will now

review the relevant literature that suggests that the first letter of an incongment colour word may.

in and of itself, produce Stroop interference. Following this, the NO-process account will be

extended to allow for an investigation of the position of the coloured letter under this Framework.


Is Encoding the First Letter of an Incongruent Colour Word Al1 That is Necessary to Invoke a Stroop Effect?

The influence of the position of the single coloured letter has yet to be investigated but.

according to the preceding logic, this may be crucial. Experiments by Singer, Lappin. and

Moore ( 1975) and Regan (1 W8), as well as recent research by Logan and Zbrodoff ( 1998). have

suggested that the first letter of an incongment colour word may be al1 that is necessary to invoke

a full-blown Stroop effect. This idea was first put forth over 25 years ago by Singer et al. (1975)

who examined the potential difference in Stroop intederence when the to-be-named colour

appeared in the first. middle. or last two letters relative to the entire word being coloured.

Aithough the greatest Stroop interference was observed when the entire distractor word was

coloured. there was also an effect whereby the first letters of an incongment colour word

interfered more than did the letrers in the rniddle or at the end of that word. This finding was

confirmed by Regan (1978). who demonstrated a delay in colour naming when only the first

letter of an incompatible colour word was presented as a distractor.

Regan (1978) designed a study to investigate the efFect of presenting only the first letter of an

incongment colour word afier previous researchers had demonstrated no Stroop interference for

colour words relative to colour-word anagrams (e-g.. BELU). which were used as control stimuli

(Hintzrnan et al., 1972). She argued that the colour-word anagrams may have been encoded in a

meaningful manner by panicipants despite their apparent lack of meaning. Thus, Regan sought

to investigate whether these effects would generalize to single letters:

Single letters per se are irrelevant to color naming. If al1 the stimuli are single letters and the task is to name the color of the ink in which a letter is pnnted. participants should be able to ignore the letters. However, if these familiar stimuli are processed automatically and if those letters that are color-name initials interfere with or facilitate color naming, it would be even stronger evidence in suppon of the


involuntary nature of the processing of familiar items ... .If such an effect occurs, it would further indicate that ... the way such an item is processed may (be determined) by the way its processing fits into the overall computation required by the task. (P. 134)

Regan discovered that the first letter of incongruent and congruent colour words led to a pattern

of Stroop interference and facilitation similar to other studies in which entire colour words were

presented: "Aithough letters alone may not elicit color names as primary associates. given the

color-naming task they are processed in relationship to color names" (p. 135). This point is also

highly relevant to my previous critique of Besner et al.'s (1997) controls as inappropnate: Given

the colour-narning task, colour nonwords may have been encoded in a meaningful rnanner and.

consequently may have elicited Stroop interference. Though Regan's ( 1978) work is very

important to the present thesis. little attention was given to this line of research until Logan and

Zbrodoff ( 1 998) recently reponed a very related finding.

Logan and Zbrodoff(i998) examined the size of the Stroop effect with different modalities of

response: vocal. button pressinç. and typewritten responses. In the latter case. participants had to

actually type out the word corresponding to the colour of ink in which colour words appeared.

Only the first and last of their experiments are relevant here. Their first expenment was a simple

Stroop study with three groups, corresponding to the three response modes just mentioned.

Logan and Zbrodoff s reason for including the typewritten response section was simple: By using

button pressing as a modality, researchers leave open the possibility that the decrease in

interference is due to transfer inappropriate processing rather than to a decrease in the effect of

automaticity. It is difficult, therefore, to determine whether button pressing leads to a decrease in

interference due to a dissociation of response (an arbitrary bunon press representing a colour

rather than a colour word response) or to an increased response time due to a legitimate decrease


in the effect of automaticity (e.g., such as the notion that semantic priming can be disrupted

when a letter search task is applied to a prime word). The typewritten response. on the other

hand. has the benefit of being similar in response modality to button pressing and similar in

automaticity to vocal response. In addition, for experienced typists (which Logan and Zbrodoff

used). typewritten responses should be approximately as automatic as vocal responses. Thus,

their incorporation of typewritten response provides a good test of whether response differences

are due to response modality rather than to automaticity:

If the difference was due to response modality. then the typewritten Stroop effect should be no larger than the arbitrary-keypress Stroop effect. If the difference was due to automaticity. then the typewntten S troop effect should be as large as the vocal Stroop effect, and both should be larger than the arbitrary keypress. (P. 98 1 )

Logan and Zbrodoff(1998) were also interested in the response time of typewritten responses as

a function of congruity. In other words. would typewritten responses be more rapid when Stroop

trials were congruent relative to incongment trials. where the incompatible colour word could

irnpede typing speed?

The results of their first experiment demonstrated a larger Stroop effect for typewritten

responses relative to vocal and keypress responses (see Table 6). There was a 62 ms difference in

response time between typewritten responses and the next longest condition, arbitrary

keypresses. It is of interest, however. to note that the longer response times in the typewritten

response condition were not attributable to the length of time that it took to type an entire

response. but rather to the length of time it took to make the first keypress of the response. Thus.

if the ink colour to be typed was "red." the word "red" was typed at an equal rate in al1 three

conditions. In typewritten responses to the incongment condition, however, participants

dispiayed a greater delay in onset response time before typing the letter "r."


Table 6

Mean Remonse Times (in ms) to Name Colour as 3 Function of Remonse Mode

(Vocal vs. Kehpress vs. T\.pewritten) From Logan and Zbrodoff ( 1998).

Response Mode

Keyp ress

Esperiment One (3 response modes. incongrnent colour words)

Colour Word RT Y 12 96 1 1023

Esperiment Ttvo (T>pe~vtitten response. incongruent colour words vs. incongrnent single letters)

Colour Word RT NIA NIA 1032

Single Letter RT NIA NIA 1 0 16

Note: RT = response times


This typewritten response pattern led Logan and Zbrodoff (1998) to investigate the idea that

an incongment single letter was al1 that would be necessary to produce a Stroop effect. In their

third experiment, they exarnined the difference in Stroop interference for typewitten responses

of an entire colour name relative to just the first letters of colour names. The resuits of the

experiment demonstrated that there was an approximately equal amount of Stroop interference

for both incongruent colour names (RED in green) and incongment single letters (R in green).

There was only a 16 ms difference between typing times for the two conditions (10 16 ms for

single letters and 1032 ms for colour words). which was not a significant difference. Thus.

Logan and Zbrodoff (1 998) suggested that the observed equivalence in Stroop interference

implies that al1 letters after the first letter of an incongment colour word have iittle. if any. effect

on the magnitude of Stroop interference.

Thus far, the concepts of automaticity. the Stroop effect. and semantic priming have been

considered. Studies have been reviewed which demonstrated a reduction in and/or an

elimination of semantic priming effects. These studies served as the basis for Besner et al.'s

(1997) experiments in which they claim to have eliminated the Stroop effect. It has been argued

that Besner et a1.k ( 1997) explanation is not without cornpetitors and that their data do not

necessanly demonstrate the elirnination of the Stroop effect. A number of the elements of the

Besner et al. ( 1997) study have been outlined which may explain their apparent elirnination of

Stroop interference in ways other than the way they espouse. A two-process explanation was

also introduced to account for the results of Besner et al. (1997). Finally. research was reviewed

that suggests the imponance of the first letter of an incongment colour word in producing Stroop

interference. To this end. there is another possible explanation for the Besner et al. (1997) data.

which is a closely related to the two-process account. This should not be viewed as a separate


explanation for the Besner et al. (1997) results so much as an extension of the already outlined

two-process account.

An Extension of the Two-process account: WiIl Letter Position Influence Response?

The design of Besner et al.'s (1997) second experiment was within participants and,

therefore, participants were exposed to every condition (single ietter word. single letter nonword.

fully coloured word, fully coloured nonword) in a mixed fashion. Had participants received ody

hlly coloured words. it would be possible to constantly fixate on any part of a word or nonword

because each position would provide the desired response. This is not possible. however, in the

single-letter coloured condition because fixating on any given point will not provide the relevant

information al1 of the time. To compensate for this. the participant has to find the easiest rnethod

to identify the colour of the stimuli across al1 trials.

It is likely that scanninç each word or nonword from left to right would be the rnost "natural"

method to arrive at a response. Under this scenario, if the entire word is coloured. then the

participant can respond to the colour afier scanning to only the first letter. If a single letter is

coloured, then the participant must scan from lefi to right until the coloured letter is observed.

Thus, faster response times would be observed for a single letter coloured at the beginning of a

word than for a single Ietter coloured toward the end of a word. In addition. response times for a

single letter coloured at the beginning of a word relative to words which appear entirely in colour

should be approximately equal due to being able to respond after scanning across to the first

letter in either case.

The results of Besner et a1.k (1 997) second expenment can be interpreted in the following

manner. assurning that participants are scanning left to right so as to locate the colour to be

named. In the fully coloured incongrnent condition. participants are able to respond to the colour


after they have processed the first letter. When the colour to be named can be processed

immediately, this does not necessitate breaking the word down into individual letters. Indeed, a

response can be made before individual letters have been processed and rejected, leaving the

entire word intact. This increased the likelihood that the word would be viewed as a whole.

Therefore, when the stimuli are colour words, the word interferes with colour naming, creating

Stroop interference. When the stimuli are not colour words. however. Stroop interference

ostensibly does not occur because nonwords are less likely to provide interference relative to

colour words. Thus, colour naming is achieved 30 ms more rapidly when a colour nonword

(Besner et al.'s RET-type control) is presented relative to when the coloured stimuli are actual

colour words.

Consider, however, the single-letter coioured condition. Here, the word is broken down into

its individual letter components and the individual scans left to right until the coloured letter is

encountered. If the coloured letter were the first letter encountered, then equal Stroop

interference should occur in both the coIour nonword control and the colour word, consistent

with the findings of Logan and Zbrodoff (1998). If the coloured letter is not the first letter. then

the expected pattern of Stroop interference is less clear-cut. As words are broken down into their

single-letter components, this increases the chance that the words will be processed differently.

This logic is similar to that of Besner et ai. (1997). though there is currently no need to suggest

that this differential processing necessitates that the distractor is not read. There should be an

increase in response time, however, due to the extra time it takes to scan across the tarset

stimulus until the coloured letter is encountered. Thus. the 30 ms of response time that would be

added due to scanning for a single letter is counteracted by a reduction in Stroop interference

when the coloured letter is not the first lener. This does not suggest an elirnination of the effect:


Stroop interference rnay still be highly prevalent when the first letter encountered is the coloured

letter (in accordance with the prediction of Logan & Zbrodoff. 1998) and perhaps to a lesser

degree when subsequent letters are coloured. This strategy of scaming left to right until the

coloured letter is encountered seerns reasonable as it would lead to a response afier o b s e ~ n g the

Srst letter on more than 50 % of the t d s (e.g.. every trial in the fully coloured condition as well

as any time that the first letter is coloured in the single-letter coloured c o n d i t i ~ n ) ~ .

Under this explanation, there are three possible pattems of Stroop interference that could anse

for coloured letters occumng afier the first letter. These pattems are outlined in Figure 1.

assuming that the stimulus is the 6-letter word YELLOW printed in red. Similar functions could

be displayed for shoner words. One possibility is that as long as the coloured letter is not the

first letter, no Stroop interference should occur (LA) . This would suggest that the moment a

word is broken down into its individual letter components, it is no longer viewed as a meanineful

whole. thus preventing Stroop interference. This logic is panially in line with Besner et al.'s

(1997), though they did not explicitly take into account the importance of the position of the

coloured letter in that study or in any of their subsequent studies.

A second possibility is that more Stroop interference would be observed for coloured letters

appearing eariier in a word relative to coloured letters occumng later in the word ( 1 -B).

The third possibility is that when the first letter is not the coloured letter. greater Stroop

ULogan is correct in assuming tlint the first letter of a confhcting colour word is al1 that is needed to invoke a Stroop effect, then it might be eqxcted tliat there would be equai Stroop interference betwecn the conflict and control trials when the entire word is coloured. This is not thc case. hotvever: Besner et al. ( 1997). Marmurek (1999) and MacLeod (1999) al1 observe an approximate 30 ms Merence between confiict and control trials when Besner et a1.k methodolog-. is replicated. There is no clear espimation for this difference in pattern in Logan's research. There is. hotvever. the possibility that colouring a single letter makes a word appear less wordlike and as such forces the individual to deal with the stimuli on a letter by letter basis. Unfortunatel-. there is no esisting evidence for tiiis in the litenme.

Amount of interference (in ms)

Amount of interference (In ms)

Amount of interference (in ms)


Figure 1: Possible patterns of Stroop interference as a funcüon of the position of a single coloured Ietter (assuming that stimulus word is six letters long ... e.g., YELLOW)

Position of the single coloured lettet

1 2 3 4 5 6

Position of the single coloured letter


interference would occur for coloured letters O C C U ~ ~ ~ later in a colour word as opposed to

earlier (1-C). Even though the word itself has been broken d o m into its individual letter

components, larger Stroop interference may be registered when more of the word has to be

scamed to find the coloured letter, analogous to a letter-by-letter reading strategy.

There is no evidence in the literature to support any of these daims; the influence of the

position of the coloured letter remains an empirical question. The concept of dealing with letters

irdividrraliy is important because this is what is thought to decrease or eliminate the Stroop effect

as it did in semantic pnrning expenments (e.g.. Friedrich et al.. 1991). The importance of the

position of the coloured letter, however, has not been given any explicit pnor research attention.

Consequently. the proposed research will focus initially on the position of the coloured lerter and

its corresponding response. [t seems certain that the first letter of a conflicting colour word will

produce interference. but the influence of the position of the colour on the remaining letters is

not as clear-cut. As just outlined. there could be no difference in interfierence between the

remaining letters (al! showing relatively little) or there could be an increase or decrease in

interference for letters which occur later in a word relative to letters which occur earlier in a

word. As one goal. the present study will for the first time examine the pattern of response times

dependent upon the position of the coloured letter.

The Need for a More Explicit Test of Whether Distractors are Read

To argue that Stroop interference has been eliminated, it would be necessary to show that

the word in Stroop trials has not been read, given that the Stroop effect rests on the notion that

the irrelevant word is automatically read and interferes with the colour naming response. Besner

et al. (1 997) argued that they had eliminated the Stroop effect but did not directly show that the

word had not been read, instead relying on the equivalence in response times to support their


argument indirectly. Critically. they did not remark on the apparently increased control

condition times. 1 have already argued that simple response time data are not necessarily

indicative of whether irrelevant words have been read, as there are numerous factors that can

influence participant response. 1 have also outlined previous research that suggests that

irrelevant words are likely still being read when a single letter is coloured. even if this colour

manipulation leads participants to process distractors differently than they would if they

appeared fùlly coloured. To further emphasize this point. 1 will now briefiy review a few

relevant findings.

Friedrich et ai. ( 199 1 ) have already shown that identity priming remains when a single letter

is searched for. even though semantic priming is disrupted. kueger and Weiss (1976) have also

shown letter search effects that indicate that nontarget letters are still being attended to. Across

three experiments, t hey observed faster search times for fixed position target letters in words

relative to nonwords. This finding also held when participants searched for the presence vs.

absence of a target letter in words vs. nonwords. The inability to disrupt the word superiority

effect - the effect whereby "participants are reliably faster and more accurate in letter

identification if the letter appears within a word" (Reisberg. 1997. p. 47) - on any level suggests

that words and nontarget letters are still being perceived. despite their irrelevance to the letter

search task (Knieger & Weiss. 1976). Thus. it is highly likely that colour words are being read

in the Besner series of studies, even if the possibility exists that they are being processed in a

different manner than they would be had al1 letters appeared in colour. In addition, the limited

set of words used in Stroop studies means that these words are presented to participants on

multiple trials and are even primed by the corresponding to-be-named colours. It is likely.

therefore, that enhanced repetition priming woiild occur relative to Friedrich et a1.k (1 99 1) smdy


in which participants ody saw each target word. Clearly. there is ample evidence to suggest that

distractors are likely read, even when a single letter is coloured. Nonetheless, we are unable to

speak of the fate of the Stroop effect without a more explicit test of whether distractors are being

read when a single letter is coloured. Thus, the main focus of the present thesis will be to

provide an explicit test of the manner in which irrelevant words are processed when they appear

on a Stroop trial with a single letter coloured.

Negative Priming?

One way to examine whether a word has been read would be to examine negative priming

trials in the Besner et al. (1997) and Besner and Stob (1999db) studies. Negative priming refers

to slowed processing on a second trial when the required response is that which was suppressed

on a first trial. This effect was first observed within the context of a Stroop study (Dalrymple-

Aford & Budayr. 1966). For example. if a participant receives the word GREEN in the colour

blue (say "bluet') on one trial and then receives any other colour word in the colour green (say

"green") on the following trial, they will be slower to respond than if the colour to be named on

the second trial was anything other than that which was suppressed initially (green). Negative

priming occurs because the individual read the word GREEN on the first triai and understood not

to respond to it. Thus. when required to respond to the colour "green" on the subsequent trial.

their suppression of the word on the previous trial interfered with their response to the colour.

thus indicating that a semantic connection was made (see Fox. 1995 for an in depth review of the

negative priming literature).

If participants in the Besner et al. (1 997) study were slow to respond to negative priming trials

on which a single letter was coloured, it would demonstrate that distractors are being read.

Undoubtedly there would be a number of negative priming trials in the Besner et al. (1997) data


due to the small stimulus set and large number of trials. Negative priming trials could also be

used to investigate whether the Besner et al. (1997) colour nonwords elicited Stroop interference.

If they did. then slowed responding would be expected if. for example, participants saw RET in

green (say "green") followed by BLLJE in red (say "red) relative to BLUE in yellow (say

"yellow"). The slowed response would occur because suppression of the word RET might be

functionally sirnilar to suppression of the word RED. Unfortunately. we do not have access to

the Besner et al. (1997) data to search for negative priming trials.

As it happens, Besner (2000) himself has recently perfiormed a negative priming study using

his single letter coloured manipulation:

the elimination of a Stroop effect does not speak to the ultimate fate of the irrelevant word. One way of determining whether the irrelevant word ever gets processed despite the absence of a Stroop effect is by examining whether there is a negative priming effect.

(Besner. 2000. p. 4)

Besner used a number of different procedures in an attempt tu prevent panicipants from reading

irrelevant words ( e g spatial cueing. colounng a single letter) and then tested whether negative

priming would occur. a certain indicator of whether words are being read. In a single

expenment. he employed three groups of participants, each of whom completed a slightly

different Stroop task. The first group identified the colour displays of irrelevant colour words in

which al1 words appeared fully coloured (50% congruent trials, 50% incongment trials) and with

al1 letters spatially cued by a small arrow (similar to the cueing procedure from Besner & Stolz,

1999db). 125 ms pnor to the appearance of the stimulus. This group served as the baseline

condition. The second group identified colour displays of irrelevant colour words (50%

congruent trials. 50% incongment triais) in which a single letter appeared in colour and dl other

letters appeared in white. On every trial. the coloured letter was cued, again using a small arrow.


Finally, a third group completed the same task as the second group. with the sole exception being

that the proportion of congnient to incongment trials was altered in an atternpt to discourage

participants fiom reading colour words (as previously explained. congruent trials may encourage

participants to read distractors as they provide the desired response). Thus, only 20% of trials

were congruent whereas 80% were incongment. As in previous experiments. there were four

display colours (red, blue, yellow. and green) and their corresponding colour words served as

distractors. The response buttons were also the same as in al1 previous experiments in the Besner

et al. series. Al1 participants cornpleted 40 practice trials followed by 120 experimental trials.

The results of the experiment are presented in Table 7 and again it appears as though the

single letter coloured manipulation led to an elimination of Stroop interference. There were 93

rns of Stroop interference in Group 1 but only 1 rns of interference rernained in Group 33 .

It is encouraging to the argument of Besner et al. (1997) that this experiment provided another

replication of the effect of the single coloured letter manipulation on Stroop interference. When

Besner (2000) cornpared response times on negative priming trials to non-negative pnming

trials. however. non-negative priming trials were responded to 52 rns more quickly than negative

priming trials. This would appear to be quite strong evidence that words are being read

autornatically dunng the Stroop trial and that the irrelevant response is suppressed. thus ieading

to a difficulty in responding to the previously suppressed response on the trial that follows. The

existence of negative priming here would therefore appear to be inconsistent with the Besner et

al. account.

Intnguingly. Besner (2000) offers a different interpretation of these results, suggesting that

%t should be noted that it may not be entirely appropriate to use Gmup 1 as a badine cornparison for Croup 3. A more appropnate cornparison would be a fourth group where the entire irrelmant word appeared firlly coloured and 20% of trials were congruent whereas 80% were incongment. The focus of the present review. howmer. is negative priming and. therefore, this point will be given no hirther scrutin!.


Table 7

~Mean Response Times (in ms) and Percentage Error Rates to Name Colour ,as

a Function of Grouo r 1 , 2. or 3). From Besner (2000).

Group 1 Group 2 Gro~ip 3

Incongruent 753 3.4 723 3 .O 759 2.3

Control 660 2.0 68 J 2.9 758 2.9

Difference 9 3 1.4 39 0.1 1 0.6

Note: RT = response Times; %E = percentage of errors


irrelevant words are being processed subsequent to colour naming (a sort of second-look

hypothesis following response). This was a necessary position for him to assume, given his

rejection of automatic word reading in this condition. Unfortunately, this is a post hoc argument

and is not consistent with the prevalent accounts of negative priming. My position is that the

presence of negative priming in the single letter coloured condition is expected if the word is

read automatically, and that this evidence is therefore actually inconsistent with Besner's overall

claim. Negative priming in the single letter coloured condition shows that the words in that

condition were being read, and quite strongly implies that they were being read automatically.

Had words been read subsequent to colour naming, there would be no reason to suppress them as

they would no longer interfere with the colour naming process already cornpleted for that trial.

Rationale for the Present Study

That negative priming is found when a single letter is coloured is the firsr piece of explicit

evidence that supports the notion that distractors are being read when a single letter is coloured.

Nonetheless. to further determine whet her distractors are being read, and thus whether the Stroop

effect can be elirninated. additional evidence is necessary. Consequently. 1 have chosen to use an

altemate method of detennining whether distractor words are being read when a single letter is

coloured in the Stroop task relative to the entire word being fully coloured.

The main focus of the present study will be an examination of memory for words that are

presented during Stroop t d s by providing participants with a number of words that are not the

colour words usually used. Following a Stroop-like task - using ordinary noncolour words as

distractors - participants will be presented with a surprise recognition test for these words. with

half of the items having appeared during the Stroop trials and half not having appeared during


the Stroop trials. If participants are able to identiQ words which were presented to them with a

single letter coloured during Stroop trials at a level greater than chance, it will indicate the words

have in fact been read and understood. If performance on single letter coloured words is low (at

or around chance levels), however. the critical cornpanson will be between words that appeared

with a single letter coloured and words that appeared fully coloured. If recognition mernory for

fully coloured words is also low. then it would suggest that either the recognition task is not an

appropriate measure of whether distractor words are being read, or that there is no appreciable

difference in the manner in which participants process distractors appearing hlly coloured and

distractors appearing with a single letter coloured. If. however, recognition is high for hlly

coloured words but low for single letter coloured words, this would definitely be consistent with

the Besner et al. (1997) argument that colouring a single letter of a distractor word blocks

semantic processing by making the processing of these words different from that of hlly

coloured words.

Although the major focus of the present thesis is to determine whether irrelevant words are

read when a single letter is coloured dunng the Stroop task. there are additional issues that will

also be investigated. First, the possible influence of the position of the coloured letter has

already been outlined and will receive ample attention in the present research. Al1 experiments

have been designed in a manner such that useful comparisons can be made with regard to

response time and recognition rates as a function of coloured letter position. Second. regardless

of whether distractors are being read during the Stroop task. an additional issue has arisen: that

being the manner in which and the extent to which irrelevant words are processed when a single

letter is coloured relative to when al1 letters are coloured. Even if irrelevant words are always

read during the Stroop task, independent of colouring type, the single Ietter coloured


manipulation may lead to differential processing of irrelevant words relative to that engaged by

fully coloured words. For example, a two-process explanation has been introduced to account

for the results of Besner et al. (1997) in which response times would rise due to an increase in

search time but coincidentally drop due to a decrease in Stroop interference, perhaps because the

irrelevant word becomes less word-like. Although it seems reasonable to speculate that two

processes may offset each other to influence response tirne when a single letter is coloured. it is

difficult to argue for this unless these processes are parsed apan and s h o w to influence

responding individually in the predicted manner. Thus. a number of aspects of the present

expenments will attempt to separate these mechanisms so that the manner in which single letter

coloured words are processed relative to fully coloured words can be determined.

The present study consists of four experiments. The fkst expenment is a replication of the

Besner et al. (1997) Expenment 2 with an iocreased emphasis on the position of the coloured

letter. It is assumed that Besner et al.'s (1997) basic pattern of results will replicate. given that

these results have already been independently replicated twice. The influence of the position of

the coloured letter is less readily predictable and remains an empincal question. This replication

will provide no information, however. as to the rnanner in which distractors are processed and

whether rhey are being read. Thus. Expenment 2a is the first explicit test in the thesis of whether

distractors are being read when a single letter is coloured during the Stroop task. A task similar

in nature to the Stroop task in the present Experiment 1 will be used, but noncolour words will

serve as the distractor items. Though these words are not supposed to be read. a recognition task

will test participant memory for the presented words. Particular interest will be given to those

words that appeared with a single letter coloured relative to words that appeared fully in coiour.

If single Ietter coloured words are recognized at a level equal to fully coloured words (and at a


rate greater than chance). it would suggest that irrelevant words are read when a single letter is

coloured, contrary to the claims of Besner et al. (1997) but consistent with my interpretation of

the Besner (2000) negative priming data. It is predicted that single letter coloured words will be

remembered at a rate greater than chance and at a rate equal to fully coloured words. The

influence of the position of the coloured letter on both response time and recognition rate will

also be investigated in this experirnent. Again, no a priori predictions were made regarding any

expeaed pattern of positional results. only that letter position may be infiuential.

Regardless of the outcome of Experirnent 2a. there is still a need to investigate the manner in

which irrelevant words are processed when a singie letter is coloured relative to al1 letters.

Specifically, a two-process expianation has been introduced to account for the results of Besner

et al. ( 1 997). Thus. Experiments 2b and 3 served two functions: ( 1) to supplement the results of

the first two expenments in determining whether irrelevant words with a single letter coloured

are processed differently then irrelevant words with al1 letters coloured. and (2) to parse apan the

two processes that 1 have posited to account for the results of Besner et al. ( 1 997). In particular.

these experiments test the notion that colouring a single letter of an irrelevant word increases

participant response time relative to colouring al1 letters of an irrelevant words while

sirnultaneously reducing response time due to a decrease in interference associated with the

irrelevant word, resulting in a net offset between the two processes.

Experiment 2b was a simple word reading task in which some words appeared hlly coloured

whereas others appeared with a singie random ietter coloured. The expectation, in accordance

with the two-process account. was that words with a single letter coloured would take longer to

read than would words that appeared hilly coloured.

Finally. Experiment 3 was a Stroop-like task using rows of asterisks rather than irrelevant


words as distractor items. Again it was expected that it would take participants longer to colour

name when a single asterisk was coloured relative to al1 asterisks being coloured. If this rise in

response time for single letter coloured displays is prevalent in both Expenments 2b and 3. it

would provide considerable support for the two-process account. It is difficult to determine

whether colounng a single letter reduces Stroop interference without experiencing this

corresponding increase in response time. Nonetheless, colouring a single letter has led to a nse

in response in al1 conditions except the colour word condition across three experiments (Besner

et al.3 colour nonwords; Marmurek's animal words. and MacLeod's rows of' X's) and it

therefore seems reasonable to posit that a decrease in colour word interference in the colour word

condition is responsible.

General ~Method

Apparatus and Materials

Each expenment in the present study was wntten in QuickBASIC and was adapted from the

same program that was used for MacLeod's (1999) independent replication of the Besner et al.

(1997) Experiment 2. An IBM-compatible 486 cornputer was used to display the stimuli on a

Ma_enavox 15" colour monitor and participants responded using either the keyboard

(Experiments 1 .2a and 3) or a microphone (Expenment 2b). dependent upon the experimental

requirements. When the experimental task was a colour-identification task (Experiments 1. 2a.

and 3)+ participants responded using one of four response keys (z, x, >, or ?) representing the

four colours (red. blue. yellow. and green. respectively). The response rnethod for the

recognition test in Experiment 2a and for the word reading task in Experiment 2b will be

outlined in the Apparatus and Materials sections of those experiments.


A practice task was also incorporated into each experimental session that consisted of

displays of asterisks with either a single astensk or al1 astensks coloured. Participants indicated

display colours using the same four keys as previously outlined.

The stimuli used in Experiment 1 were the same colour words (red, blue. green. yellow) and

corresponding colour nonwords (ret, blat. grend. yenile) used by Besner et al. (1997). Ail colour

words and colour nonwords appeared in incongruent colours and lowercase type. to be consistent

with Besner et al. ( 1997), Marmurek (1999), and MacLeod ( 1999).

The stimulus set for Experiments 2a and 2b consisted of 160 noncolour English words (see

Appendix A). The selection procedure for these words will be fùrther detailed in the materials

section of those experiments. For Experiment 3. the stimulus set consisted of rows of asterisks.

between 3 and 6 asterisks in length. Ail stimuli in Expenments 2 and 3 appeared with either a

single character or with al1 characters coloured. In al1 experiments. there was an equal number of

fully coloured and single letter coloured trials. When only a single letter was coloured. each

letter position was represented in as equal a rnanner as possible.

Procedure

This section will detail the procedure for every colour naming task (Expenments 1. ?a. and

3 ) The procedures for the recognition task in Experiment 2a and for the word reading task in

Experiment 2b will be detailed in their respective Method sections. Al1 other experiments are

replications and extensions of Besner et a1.k (1997) Experiment 2.

Prior to testing, participants indicated their willingness to panicipate by filling out a consent

form that bnefly detailed the nature of the study (see Appendix B 1). Participants then sat

approximately 30 cm fiom the monitor. with a keyboard in front of them. To farnilianze

participants with the button pressing procedure that was to be used during the expenment, they


initialy perfomed 72 tnak in which a row of asterisks at the center of the screen appeared either

fully in colour or with a single, randomly varying, asterisk coloured. Participants were instructed

to indicate the display colour of the row of astensks or the single asterisk by pressing the

corresponding button on the keyboard, as just outlined. This task allowed participants to Ieam

the button pressing procedure in the absence of distracting colour words and equivalent

nonwords. Following the familiarization task, participants took part in the actual expenment.

Participants were initially presented with wntten. on-screen instructions (see Appendices C

through F) which were supplemented by oral instructions pnor to the experiment. On each trial,

a single word, colour nonword. or row of asterisks was presented in the middle of the computer

screen. Each stimulus appeared either entirely in colour or with a single randorn character

coloured. Participants were instructed to ignore the irrelevant word or nonword and to indicate

the display colour of the entire item or single letter by pressing the conesponding button on the

keyboard as quickly and as accurately as possible. Immediately following a manual response.

the stimulus disappeared fiom the screen. Afier a penod of 500 ms. a row of asterisks appeared

on the screen to alert participants that another item would follow rnomentarily. The nea target

stimulus appeared after 500 ms. This warning was used in al1 experiments except Expeirment 3.

in whkh a row of dashes were used because the distractor items were rows of asterisks.

Each panicipant completed a set of practice trials followed by the actual experimental trials.

Al1 trials were randornized. with half of the trials being incongruent trials (colour words) and half

being neutral trials (colour nonwords). In addition. half of the trials had al1 letters coloured

whereas half had only a single Ietter coloured. In this latter case, the position of the single

coloured letter varied randomly, with each position being equally likely to be the critical

coloured letter. Upon completion of al1 experimental trials, participants were debnefed as to the


purpose of the experiment and @ven their credit slip (see Appendix B2).

Experiment 1

This is a replication and extension of the Besner et al. (1997) Experiment 2. with the

additional goal of determining the effect of the position of a coloured letter relative to a fully

coloured word. The basic procedure for the experiment was exactly the same as in Besner et al.

(1997). and it was expected that the principal phenornenon they observed - the

decline/elimination in interference in the single letter coloured condition - would replicate. given

prior replications by Marmurek (1999) and MacLeod (1999). The new emphasis would be on

additional analyses camed out to determine the effect of the position of the coloured letter.

The three different potential patterns of results already outlined for coloured letter position are

shown in Figure 1. Basically. Stroop interference could increase or decrease as the participant

scans lefi to right. A decrease would be expected if Logan and Zbrodoff (19%) are correct in

positing that the first letter of an incongnient colour word is mostly responsible for Stroop

interference. On the other hand, an increase would be expected if interference increases as a

hnction of how much of the irrelevant word is scanned (e-g., if the individual is scanning lefi to

right. they will have seen more of the irrelevant word when the last letter is coloured relevant to

when the first letter is coloured). In accord with the findings of Regan (1978) and Logan and

Zbrodoff (1998), it was anticipated that colouring the first letter would lead to equal Stroop

interference in both the incongnient and control conditions, but the effect for the other letter

positions is less readily apparent.

Method

Participants. Thirty-five University of Toronto at Scarborough undergraduates, both male

and female, participated for course credit. Each participant had normal or corrected to normal


vision as determined by self-report. Testing time for each participant was approximately 15

rninu tes.

Materials. The stimulus set used in the present expenment was the same colour words (red.

blue, green, yellow) and corresponding colour nonwords (ret. blat. grend, yenile) used by Besner

et al. (1997), together with the same four pint colours..

Procedure. The present expenment replicated, as closely as possible. Besner et al.3 ( 1997)

Experiment 2 and the basic procedure was outlined in the General Method section. Following

the initial practice session, participants compieted 192 colour-identification trials.

Results

Al1 analyses in the present experiment and in the following two expenments were performed

using SPSS. Response times were excluded from analysis for trials on which colour naming

errors were made. or when naming times were less than 300 ms or greater than 2000 ms. These

latter exclusions were also considered errors. but occurred very rarely.

Reduced Interference in the Single-letter-coloured Condition

One purpose of the present expenment was simply to replicate the pattem of results that was

reported in Besner et al. (1997). To examine the success of this goal. mean colour naming times

for incongrnent colour words and control colour nonwords. appearing both fully coloured or with

a single letter coloured. were obtained for each participant. Mean naming times. standard

deviations, and percentage errors are show for each of these conditions in Table 8. Besner et

al.'s (1 997) basic pattem of results was the reduction/elirnination of Stroop interference in the

single letter coloured condition. The present results replicated the reliable reduction but not the

elirnination in their pattem of data. When distractor words appeared fully coloured, it took

participants longer to colour name on incongruent trials relative to control trials, a difference of


Table 8

Esperiment 1: Mean Response Times (in ms), Standrird Detiations. and Enor Ehtcs to

N m e Colour as a Function of Condition (AI1 Letters Coloured tvs. Single Letter Coloured).

Al1 Létters Coloured Single Lctter CoIourcd

Condition RT SD %E RT SD %E

Incongruent 729

ControI 684

D ifference 45

112 3.7 742 102 4.9

1 O4 4.0 720 1 Or) 4.2

8 0.3 22 2 0.7

Note: RT = response times; SD = standard deviation: Y& = perccntagc of mors


16 ms. When distractor words appeared with a single letter coloured, however, this difference

reduced to 22 rns. Again, participants took longer to colour name when distractor words were

incongruent as opposed to control. This reduction in naming (or. more properly. identification)

response was mostly attributable to a rise in naming times in the control condition, also

consistent with Besner et al. 's ( 1997) pattern of results.

Although their basic pattern of results replicated. the present study did not obtain an

elimination of the Stroop effect: There were 22 ms of interference remaining when a single letter

was coloured relative to the entire distractor being coloured. A 2 X 2 within-participants

anaiysis of variance (ANOVA) was performed on the mean colour naming response times with

colour type (fully coloured or single letter coloured) and distractor type (incongruent colour

word or colour nonword) as the independent variables. There was a significant main effect of

colour type, (1, 34) = 14.68, MSe = 1367.03, g < .O0 1. and a significant main effect of

distractor type. F (1. 34) = 33.53. MSe = 1 184.83. g < .O 1. In addition. there was a significant

interaction between colour type and distractor type, F ( 1. 34) = 4.24, MSe = 1 1 16.12, EI < .05. A

summary table for the ANOVA appears as Appendix G.

Two planned comparisons firther corroborated that the present pattern of results was sirnilar

to the pattern obtained by Besner et al. ( 1997). There was no significant difference between

colour naming tirnes for incongruent colour words appearing fully coloured (730 ms) and

incongruent words appearing with a single-letter coloured (742 ms), ( 1. 34) = 1.84. MSe =

1367.03, p > . O 5 There was, however. a significant difference between colour naming times for

control items appeanng fully coloured (684 ms) and control items appearing with a single letter

coloured (720 ms), F (1. 34) = 16.59. MSe = 1367.03. g < .O 1. Two additional paired sarnple t-

tests demonstrated that there was a significant difference between the interference observed


when al1 letters were coloured (46 ms) relative to a single letter (22 ms), [ (34) = 2.06, p < -05

and a significant difference between the interference observed when a single letter is coloured

and zero interference, 1 (34) = 3.52, g < .O 1. This finding is key as it demonstrates that Stroop

interference was still highiy prevalent when a single letter was coloured. contrary to the findings

of Besner et al. (1 997) and Besner and Stolz (1 999ah).

Mean error rate deviations as a function of distractor type and colour type can also be found

in Table 8 and mean error rates as a function of the position of the coloüred letter can be found in

Table 9. A 2 X 2 within participants ANOVA was performed on mean error percentages, again

with colour type and distractor type as the independent variables. There was no main effect of

distractor type nor was there a significant interaction of colour type and distractor type, both Es <

1. There was. however. a marginally significant main effect of colour type. F ( 1, 34) = 3.4 1,

MSe = 0.0005, e = .07, with more errors occurring for words with a single letter coloured than

for fully coloured words (see Appendix H). This demonstrates the absence of any speed-

accuracy tradeoff between the critical colour type conditions.

To further examine the significant effect of colour type, two planned cornparisons were

conducted. The first demonstrated no overall effect of colour type on control items. F < 1. In

contrast, the second demonstrated a significant effect of colour type on incongruent colour

words, (1 , 34) = 10.08, MSe = 0.0005, g < .O 1, with more errors occumng when a single letter

was coloured relative to al1 leners being coloured.

Position of the Coloured Letter

Due to the present interest in the position of the coloured letter (namely. is interference a

function of letter position with more interference occumng for coloured letters appearing earlier

in words versus coloured later?), several analyses were camed out to examine this effect in


Table 9:

E~mriment 1 : Mean Response Times (in ms). Standard Deviations and Error Rates

to Identifv Colour as a Function of Letter Position (Incongruent Colour Words vs.

Convol Items) in the Single Letter Coloured Condition.

incongruent Words Control Items

Coloured Letter Position RT SD %E RT SD %E - . .-

1 745 97 6.4 743 179 6.7

2 742 135 4.2 73 1 120 4.9

3 744 115 5.8 692 83 1.8

4 751 146 3 . 3 719 140 4.1

5 732 170 4.6 723 145 3.4

6 709 188 1.4 710 164 0.0

Note: RT = response Urnes: SD = standrird de~iation; %E = percentage of mors


greater detail. The critical analyses concerned the response time data and the outcome was

absolutely clear.

Two one-way within-participants ANOVAs demonstrated no main effect of the position of a

coloured letter on response times for control items. E (5. 170) = 1-36. MSe = 8062.73. p = 24. or

for incongrnent items, 4 (5, 170) = 0.71, MSe = 1 1050.72, p = -62 (see Appendices 11 and 12).

Error rates were also examined, and ANOVA tables for these analyses can be found in

Appendices J 1 through J8. Due to the large number of compansons (most of them

nonsignificant), they will not be reported in depth. There are, however, a few interesting

compansons worth noting.

Two one-way within participants ANOVAs were performed to examine the effect of the

position of the coloured letter on error rates for incongrnent colour words and colour nonwords.

These analyses demonstrated no effect of the position of the coloured letter for incongruent

colour words. F (5, 170) = 1.58. MSe = 0.01. g = .30 (see Appendix J 1). but a significant effect

for colour nonwords, E (5. 170) = 5.09. MSe = 0.004. e < .O1 (see Appendix J2). It should be

noted that the highest error rates were recorded when the first letter was coloured and the next

highest error rates were recorded when the second letter was coloured (see Table 9).

There were no significant differences in error rates between incongruent colour words and

colour nonwords when either the first letter or the second letter were coloured, both Es < 1.

There were, however, significant differences between incongruent colour words and colour

nonwords when the third, founh, or fifih letters were coloured (see Appendices J3 through J8),

with a greater number of errors occumng for words. This is to be expected given that the third

letter of each colour nonword is the position where these words are first confirmed not to be

colour words. This difference did not hold, however, when the sixth letter of colour words and


control items appeared in colour (i.e.. the word YELLOW or the nonword YENILE). This could

simply be due to the low proportion of trials with the sixth letter coloured in the present

experiment (less than 5% of experimental trials).

Due to the significant effect of the position of the coloured letter on colour nonwords. a

Tukey HSD was performed to compare the difference in error rates as a function of the position

of the coloured Ietter in control items. Of particular interest to the present experiment was the

finding that there was only a slight difference in error rates between controls with the first letter

coloured and those with the second letter coloured (with more errors occumng for items with the

first letter coloured). There were. however, substantially more errors when the tirst letter was

coloured relative to al1 other coloured positions.

Discussion

The results of Experiment 1 replicated one of the key elements of the basic pattern of results

from Besner et al. (1997). with reduced interference for single-letter-coloured as opposed to fùlly

coloured words. Unlike in Besner et al. (1997). however. in this experirnent interference was not

eliminated when only a single letter was presented in colour. Although 24 ms of Stroop

interference was removed when participants responded to single letter coloured words relative to

M y coloured words (which is almost the exact average of Stroop interference eliminated across

the Besner et al.. 1997. MacLeod. 1999. and Marmurek. 1999. studies), a reliable 22 rns of

Stroop interference remained. So even in the Besner et al. (1 997) set of conditions - manual

responding and colour nonword controls - the outcome is not always elimination of interference

in the single letter coloured condition (see also Marmurek, 1999).

The safest conclusion is that colouring only a single letter reduces interference by 20-30 ms.

an interesting result certainly deserving explanation. Again, however. the majority of the


reduction in Stroop interference can be attnbuted to a nse in control response times rather than to

a decrease in incongruent response times. This was also the case in the Marmurek (1999) and

MacLeod (1999) replications and extensions of the Besner et al. (1997) study. as indeed it was in

the Besner et al. series of studies. Thus. the present results corrorborate the results of Marrnurek

(1999) and MacLeod (1999) in demonstrating that the Stroop effea is reduced but not elirninated

by colouring a single letter of a distractor word. Rather it appears that a number of the

methodological choices that were made by Besner et al. (1997) sometimes lead to apparent

elirnination of Stroop interference.

Are Colour Nonwords a Suitable Control Condition?

A secondary senes of analyses provide sorne suppon for the argument that the Besner et al.

(1997) controls may be inappropriate. Earlier. 1 argued that using the colour nonwords (e.g..

RET) as controls rnay be questionable given that Stroop interference has been known to occur

for the first letter or letters of an incongruent colour word. A series of one-way within-

participants ANOVAs was conducted to compare response times for letter positions in control

items relative to incongruent colour words. There was no significant difference when the first or

second letter was coloured. This pattern changed, however, when either the third or fourth letter

was coloured. When the third letter was coloured. naming times were faster for control

distractors by 52 ms. M e n the founh letter was coloured, narning times were again faster in the

control condition, though this difference only approached. and did not achieve. conventional

levels of signîficance (p = .07; see Appendices K I through K6 for ANOVA summary tables and

Table 9 for means and standard deviations).

This is the pattern of results that would be expected if the first two letters of control items

(and thus, the control condition itself) elicit Stroop interference. No significant difference.


however, was recorded between narning times for control and colour words when the fifth and

sixth letters were coloured, thus making this pattern of results difficult to interpret. It could be

that there are an insufficient number of trials where the fifih or sixth letter is coloured (only in

GREEN, GREND, YELLOW, and YENLE) to make any valid inferences regarding these

positions. The design of the present experiment led to a low proportion of such trials given that

each position was coloured equally ofien for each word length, and that five and six letter words

with a single letter coloured accounted for a mere 25% of the experimental data across al1 letter

conditions (and therefore, about 5% across al1 tnals).

More on Positional Effects

There was little efFect of letter position in the present study. None of the three foreseeable

patterns of results were even remotely evident. This may suggest that participants either fail to

form a strategy when performing the Stroop task, or that they form a strategy that is less intuitive

than those previously outlined. One such possibility would be a twist on the predicted pattern of

results that was outlined in the two-process explanation for the Besner et al. ( 1997) results.

Here. 1 suggested that panicipants rnight scan fiom left to right when each new word is

encountered such that the first letter of distractor words would provide the desired response on

more than 50 per cent of tnals (Le., al1 fully coloured trials plus some single letter coloured

trials). The response time results for individual letter positions in Experiment I do not support

this possibility.

Alternatively, participants may remain fixated on the position of the screen where the last

single coloured letter appeared and not engage in additional search until another single letter

coloured item appears (with the coloured letter likeiy to be in a diferent position than that in the

previous single letter coloured item). This seems reasonable, as the participant no longer decides


to retum to the lefi side of the screen to orient to the next target, rather they remain fixated on the

position where they ended up on the previous trial. For example, if the participant encountered a

four-letter word with the third letter coloured, they would search for that coloured letter and

respond. If the following word appeared fully in colour. or was another four letter word (or, to a

slightly lesser extent, a three or five letter word) with the third letter coloured. then the

participant would be able to respond without engaging in any additional search for the coloured

letter. This response strategy is probably easy for the participant to engage in. but unfonunately

does not lend itself to a predictable pattern of results, given that the position of the coloured letter

and the order of presentation of trials is cornpletely randomized4. To preface the following three

expenments, positional analyses were also conducted. but in no case was any interpretable

pattern observed. I will, therefore. limit the discussion of positional analyses hencefonh.

The Importance of Errors

1 have argued that the Besner et al. (1 997) controls may be inappropriate because Stroop

interference may be occumng for these items. due panicularly to the first two letters. This

result was supponed by the previously reviewed response time data and is fùrther corroborated

by the present error rate data. First, there was no overall difference in error rates between colour

words and colour nonwords. Assuming that errors are the result of improperly responding to the

distractor. one should expect fewer errors in the control condition if the controls are free of

Stroop interference. This is not the case here, suggesting that the colour nonword controls do

elicit Stroop interference.

4~nother way of thinking of the dinerence behveen hilly colowed words and words tkiiih a single lener coloured is that the latter contain a greater number of characten which m e elicit a response. For esample. relative to a sis letter word nith one letter coloured a fully coloured sis letter word has six times as man? chancten which the individual can respond to (if the word is king btoken dowvn into individuai letter components). A stimulus sampfing theory (e-g. Estes. Bjork. & Skaar. 1974; Estes. Mlrneyer. & Reder. 1976) might prove usefbi in thinking about how these letters are processed,


This interpretation was further corroborated by the positional analyses that were camed out

on error rates. When either the first or second letter of a control item appeared in colour, there

was no difference in error rates between incongruent colour words and colour nonwords. When

the third letter was coloured. however, substantially more errors were made when the distractor

was a word rather than a control. According to the Regan (1978) and Logan and Zbrodoff

(1998) positional theories, no difference should be observed between the first two letters of

colour words and the Besner et al. (1997) colour nonwords. This was indeed the case.

Interestingly, for both incongment colour words and incongment colour nonwords. most errors

were made when either the first or second letter was coloured. consistent with the argument that

the first one or two letters of incongrnent colour words may provide the bulk of Stroop

interference. A sirniiar pattern of results emerged in the positional comparisons. There was a

significant (or marginally significant) diference in error rate when the first letter was coloured

relative to when any letter other than the second letter was coloured. I î is also of interest to note

that significantly more errors were made by participants when inconpent colour words

appeared with one letter coloured relative to fùlly coloured. This pattem of errors is highly

inconsistent with the Besner et al. (1997) argument that colouring a single letter prevents the

semantic processing of an irrelevant word. If this were the case, then one would expect fewer

errors when a single letter is coloured relative to the entire word being coloured. assuming ihat

errors are the result of an inappropnate (or uncontrolled) response to the irrelevant word?

Despite the fact that I have argued against Besner et al.3 (1997) explanation for their results.

it is unquestionable that these researchers have still obtained an interesting pattem that deserves

%bis argument seems teasonable b u s e there is no dinerence in enor rates as a fùnction of colouring hpe in the e~periment that follows. which consists of a merent English language word on eveq trial. These words are l e s likely to cause interference relative to incongruent colow words.

further attention. In particular, the reduction of Stroop interference. and the corresponding rise


in control condition response times rather than decline in incongment condition response times.

both seem to be highly replicable. The following experiments further investigate the manner in

which Stroop interference is reduced and the differential processing that may occur for single

letter coloured items relative to fully coloured items. Experiment 2a will investigate recognition

memory for words that appeared during a Stroop task versus words that did not appear

Experiment t a

The results of Experiment 1 partially replicated the basic pattern of results from Besner et al.

(1997). Expenment 2. Looking solely at response time data colouring a single letter reduced the

Stroop effect by 23 rns - a result which matches the average reduction in Stroop size across

conditions in the MacLeod (1999) study. This reduction did not. however, result in elimination

of the Stroop effect. Thus. including the present Experiment 1. numerous studies have now been

conducted which have demonstrated a reduction in Stroop interference as a function of colouring

a single letter (Marmurek, 1999; MacLeod. 1999). but not an entire elimination of this

interference. This would suggest that rnerely colounng a single leiter does not fully eliminate

Stroop interference, contrary to the claim of Besner et al. ( 1997; Besner & Stolz 1999ah). As

has been previously stated. however, these studies al1 must rely on response time data with no

explicit evidence to suggest that colouring a single letter prevents participants from reading a

distractor word or, at the very Ieast, that distractor words are processed differently. Thus.

Experiment 2a was designed as a more explicit test of whether distractor words were being read

dunng Stroop trials.

The first phase of Expenment 2a consisted of a Stroop-like colour naming task in which each

distractor was a different noncolour word. Naming the colours of these noncolour words was

treated as a kind of study phase. so that memory for them could be tested following the Stroop


task with a surprise recognition test. It would not have been wonhwhile to test memory for

target words in the first study because the same eight items (colour words and colour nonwords)

were presented numerous times and. therefore, would be very easy to identiQ. By presenting

each target word only once, the recognition task here provides a more powerfil test of whether

words are being read in the single letter coloured condition, contrary to the explicit instructions

not to read them. If something about colouring a single lerter causes a word to be processed in a

less wordlike manner. it would be expected on a levels of processing basis (cf Craik & Lockhart.

1972) that fully coloured words should be recognized more quickly and more accurately than

single letter coloured words. This would also seem to be what Besner et al. ( 1 997) would

predict, given their argument that words are not automatically read and that semantic analysis is

blocked in the single letter coloured condition. If. however. single letter coloured words are in

fact encoded as words to the same extent as hlly coloured words. then a different recognition

pattern is expected: Single- letter-coloured words should stiil be recognized at a rate greater than

baseline (Le., the false alarm rate of incorrectly recognizing words that did not appear during the

Stroop task), and indeed should be recognized just as well as fully coloured words.

Both Marmurek ( 1999) and MacLeod (1999) obtained a Stroop effect for words appearing

with a single letter coloured in their studies pattemed after Besner et al. (1997) when they used

vocal as opposed to manual responses. The present Expenment 1 also obtained Stroop

interference. albeit reduced. when a single letter of an irrelevant word was coloured. Clearly, in

these cases. the target word is being read despite explicit instructions not to read it. It is

therefore predicted that the recognition memory test will indicate that words appearing with a

single letter coloured are in fact being read. If so. this would indicate that automatic reading is

not elirninated simply by colouring a single letter of an irrelevant word and that a diKerent


explanation is needed for the results of Besner et a1.k (1997) response latency data. Two

possible explanations regarding Besner et a h (1997) methodology that rnay have influenced the

data have already been outlined (manual response, unusual control stimuli) but surely other

viable explanations exist. If, however, recognition for words appearing with a single coloured

lener is poor relative to that for words appearing fblly coloured, this would be consistent with

Besner et a1.k (1997) daim that the Stroop effect can be eliminated in certain situations by

colouring a single letter, which defeats "automatic" reading and thereby leads to differential

encoding in the whole word and single letter coloured conditions.

Nonword control stimuli were eliminated from this expenment because the object was to

determine whether participants have read and understood the words that they were to ignore.

Even if nonwords were read, the panicipant would be unlikely to attach any meaning to these

stimuli or to be able to rernember them and. therefore. they are not appropriate for study. In

addition to accuracy. response latency was measured on the recognition test to compare times for

words appearing with a single letter coloured relative to words appearing fully coloured. As with

the first experirnent, the position of the single coloured letter vaned.

There is a possibility that recognition may be dependent upon the position of the coloured

letter. though there is no existing evidence to suggest this. If. for exarnple, colounng a single

letter causes a word to be processed in a less wordlike manner, then slower recognition times

might be observed for words that appeared with a single letter coloured relative to words that

appeared fully coloured. The influence of the position of a coloured letter on recognition

response cimes remains an empincal question. Thus, recognition memory as a function of lener

position was also investigated in the present experiment. Though the focus of interest in the

present expenment was recognition as a function of colour type, recognition as a function of


letter position and response time was also considered.

Method

Participants. Thirty-eight University of Toronto at Scarborough undergraduates. both male

and fernale, participated for course credit. Al1 participants had normal or corrected to normal

vision (as determined by self report) and none of the participants had participated in the first

experiment. Testing time for each participant was approximately 15 minutes.

Materials. The materials used for the experiment were 160 noncolour common English

words. These noncolour words. ranging from three to six letters in length, were matched with

regard to word length and frequency using the Thorndike and Lorge (1944) noms. Of these 160

words. 40 were generated for each individual colour-word length (Le.. 40 three letter words. 40

four Ietter words, etc.). None of the 160 words began with the letters 7." "b." "y," or "g" to

avoid the potential problern that Besner et al. ( 1997) may have experienced with their controis.

There is a chance that a minimum of Stroop interference could occur with these noncolour words

seeing as Stroop interference has been reported for other English language words and not just the

colour words which are traditionally used in Stroop studies (Klein. 1964; see MacLeod, 199 1 ).

Such interference, however, is inconsequential to the present study as the purpose will simply be

to determine whether distractors are being read under letter search conditions. Nonetheless,

response time data will be analyzed to determine whether the obtained pattern of results mirrors

the pattern obtained by Besner et al. ( 1997).

These 160 words were divided into two equal lists of 80 words for use in the present

expenment, and in Expenment 2b. Rather than creating random lists of 80 words for each

participant. two permanent list randomizations were created and used randornly for Experiments

2a and 2b. The two lists were created because of concems emanating from pilot testing in which


it was detennined that a number of words fiom the list of 160 shared some similanties (e.g..

iconic representation, sirnilar phonetics). Thus, there was cause for concem that some words

might provide complications for the participant during the recognition phase of the present

expenment. For example, if the participant saw the word "fog" during the Stroop-like colour

naming study task they could mistake it for "dog" or "log" during the recognition task. Thus,

the two lists of 80 were constnicted to ensure that such confusions would be kept to a minimum

(e.g.. the best attempt was made to keep rhyming words or words with very similar iconic

representations on opposite lists). The order of presentation of the two lists was a function of

participant number, with odd nurnbered participants seeing List 1 in Experiment 2a and List 2 in

Experiment 2b. whereas even numbered participants saw the sarne lists in the opposite order.

This was done to ensure that the obtained results were not confounded by the order of

presentation of lists.

Unlike the first expenment, words could appear in any of the four expenmental colours. This

was deemed unproblematic given that colour words and colour nonwords were not used in the

present experirnents and that there should be no inherent connection between stimulus length and

colour word length. This was also the procedure used for Experiments 2b and 3. in which al1

distractors could appear in any colour.

Procedure. Though the stimuli were different. the basic procedure of the study phase of this

expenment was similar to Besner et a1.k (1997) Experiment 2 and to the present Experiment 1 .

Although there were nearly 192 experimental trials in the first experiment (in accordance with

Besner et al., Mannurek, and MacLeod), the number of trials in this experiment wase decreased.

Because a greater number of different words were used here (a different noncolour word on each

triai). it would be unreasonabie to present participants with nearly 150 words and expect them to

Reduced S troop Interference - 74

remember many of them. In fact, pilot data suggested that presenting participants with even 64

words to remember was too great a load: Recognition performance did not exceed chance for

either single letter coloured or fully coloured words. Thus after the initial practice phase.

participants received 32 expenmental trials. Due to the small number of trials, it was impossible

to ensure that al1 letter positions would be equally represented on trials in which a single random

letter was coloured. This was not deemed problematic given that the results of the first

experiment, coupled with the pilot data for the present experiment, demonstrated that there were

no interesting effects of the position of the coloured letter.

Every participant had an experimental set of 64 words randomly drawn from the original list

of 80 words, with 32 of these words acting as colour naming study trials and recognition test

targets, and 32 acting as distractors on the recognition test that followed. Upon completion of al1

32 colour naming study trials, participants took pan in a surprise recognition test consisting of 64

triais. Half of the trials were made up of the words that appeared during the Stroop task (16 fully

coloured. 16 single letter coloured) and the other 32 were distractors. selected randomly fiom the

original list of 80 words. Each word appeared in white type on a black background. The order

of presentation of studied and distractor words was again randomized. Panicipants were

instructed that they were now to identifi the words that they had seen during the Stroop task and

to reject the words that they had not seen, taking as much time as necessary to complete this task.

On each trial, a single word appeared in the middle of the computer screen. Participants

responded to each word by pressing either the "P' button if they believed that the word appeared

during the Stroop task or the "z" button if they believed that the word was not presented during

the Stroop task. Hits, errors, and response latencies were recorded and analyzed for the

recognition test.


Resuits and Discussion

Of the 38 participants that were tested, the data of four participants were excluded from final

analysis. Three of these participants were disqualified for failing to follow instructions during

the recognition test. Each of these three failed even to examine the words presented to them and

indicated that they did not think they had seen over 90% of the words. responding at a very rapid

speed. Dunng debriefing, two of the three participants admitted that they had completely

guessed during the recognition test and did not think there was any chance they had read the

words during the Stroop task. Thus. they did not even try during the recognition task. One

additional panicipant rnisunderstood the directions during the recognition task and was under the

mistaken impression that every word that was presented during this task had also appeared on the

Stroop task. This led her to indicate that she had seen nearly every word on the Stroop task

resulting in high hit rates for fully coloured and single letter coloured words. but a

correspondingly high false alam rate.

Because a large nurnber of analyses were conducted. the results will be divided into a number

of subsections. In addition, individual discussion sections will be presented following each

result section to avoid losing sight of important findings. The first set of analyses was perfiormed

on the initial phase of the present experiment. which consisted of the 32-item colour naming task

with a different English word presented on every trial.

Colour Naming Study Phase

Mean response times and error rates were calculated for each participant. Response time.

error rate, and standard deviation data for words as a function of colour type and position of the

coloured letter can be found in Tables 10 and 1 1.

A one-way within participants ANOVA was performed with colour type (fully coloured or


Esperiment Za: Mean Remonse Times (in nis), Stnndxd Dc\iations. and Perccntaae Error Rates

to Name Colour as a Function of Condition (AH Letters Coloured vs. Single Letter Coloured).

RT SD %E

Fully Coloured

Single Letter Coloured

DifTerence

Note: RT = response times: SD = standard deviation: %E = pcrcentage of errors


Table 1 1

Emrirnent 2a: Mean Remonse Times (in ms). Standxd Deviations, and Percentage Error Rates

to Name Colour as a Function of Letter Position in Single Letter Coloured Items at Studv.

Coloured Lener Position RT SD O/oE

1 733 137 7.1

7 756 157 4. J

3 695 190 1.5

4 754 180 2.9

5 800 257 1.5

6 693 197 0.0

Note: RT = response times: SD = standard de\riation; %E = percentage of crrors


single lener coloured) as the independent variable and colour narning times as the dependent

variable. There was a main effect of colour type, F (1, 33) = 9.49. MSe = 2958.30, g < .01, with

participants taking longer to colour name words that appeared with a single letter coloured (743

rns) relative to words in which al1 Ietters were coloured (703 ms) (see Appendix L). A second

one-way ANOVA demonstrated a marginally significant effect of the position of the coloured

letter, E (1, 34) = 2.17. MSe = 25988.84, g = .O6 (see Appendix M). As in Experiment 1. there

was no meaningfii pattern of response time as a fùnction of coloured letter position and,

therefore. no further analyses were camed out in this regard.

As in Experirnent 1, a nurnber of planned analyses were camed out on error rates. A one-way

within participants ANOVA was performed with colour type as the independent variable and

error rate as the dependent variable. The analysis yielded no effect of colour type, < 1 (see

Appendix N). A second one-way within participants ANOVA. however, yielded a significant

effect of the position of the coloured letter on error rate. F (5. 165) = 2.37. MSe = 0.01. p = .O4

(see Appendix O). To investigate further this difference in error rates, a Tukey HSD was

performed to investigate al1 pairwise comparisons. As in the first experiment, there was only a

slight difference in error rates between controls with the first letter coloured and those with the

second letter coloured (with more errors O C C U ~ ~ ~ for items with the first letter coloured).

Participants, however, made significantly more errors when the first letter was coloured relative

to letters coloured later in the word. This is surprising because none of the distractor words in

the present experiment shared a first letter with the first Ietter of the incongruent colour words

that were used. Thus, this digerence could again be attributable to the higher proportion of trials

with the first letter coloured relative to trials with other lener positions coloured. The low

proportion of other Ietter trials makes it difficult to develop a cohesive explanation for these


results.

Though the initial Stroop task was mostly used as the study phase for the surprise recognition

task to follow, the response time data add support to the two-process account. Recall that the

two-process account posits that colouring a single letter may reduce Stroop interference while

simultaneously increasing response time. In the present experiment, there should be little to no

interference attributable to distractors because colour words were not used. Even if the

distractors elicit a small amount of interference, however (as has been show to happen with

English language words; see Klein. 1964). this should have little systematic effect on response

tirne data because each trial contains a different random word with each word consequently

having an equal chance of eliciting Stroop interference. It is, therefore, safe to assume that the

present response time data Vary solely as a function of colouring type (single letter vs. whole

word). To that end. the response time data frorn the Stroop task support the notion that colouring

a single letter slows the colour naming response relative to words appearing fully coloured. Note

that this difference is in the 40 ms range, as is the difference in response time between colour

nonwords appearing fully coloured and colour nonwords appearing with a single letter coloured

in Besner et al. (1997). Experiment 2 (see Table 1). Thus, only incongruent colour words fail to

demonstrate an increase in response time when fully coloured words are compared to single

letter coloured words.

There is no reason to think that incongruent colour words are in any way immune to colouring

a single letter; thus. it is probable that an increase in response time occurs there as well. but is

offset by a coincident reduction in Stroop interference. This logic is consistent with the two-

process account proposed. Unfortunately. there is a fundamental problem in interpreting a

change in amount of interference (a difference score) when there is also a change in the control


condition (against which the difference score is computed). In the Besner et al. case. they

emphasize the decrease in inteference fiom 30 ms to O ms as condition changes for fùlly

coloured to single letter

coloured. Yet at the same time. the control condition increased 30 ms From fully coloured to

single letter coloured. So there was essentially a perfect tradeoff Did interference decrease in

the incongment condition or did it increase in the control condition or was there just some

overall slowing in the single-letter coloured condition. perhaps due to the necessity to search for

the colour infomration? This complicates interpretation yet also forms the basis for the two-

process

explanation set out here. Further insight into this explanation will be gained when Experiments

7b and 3 are considered because they explicitly attempt to separate the two posited processes.

As in Experiment 1. there was no effect of colouring type on error rates. Error rates were

equal regardless of whether the entire word was coloured or a single letter of the word was

coloured. This result is not surprising if noncolour English words elicit hardly any Stroop

interference. It has been shown, however, that noncolour English words can elicit Stroop

interference, though not as much as incongment colour words (e.g. Klein, 1964: Dalrymple-

Alford, 1972). If distractors in the present experiment do elicit Stroop interference. then the

absence of an effect of colour type on error rates funher discredits Besner et a1.k (1997)

semantic disruption hypothesis. Under the semantic disruption hypothesis, error rates when a

single ietter is coloured should be reduced, if not eliminated, relative to error rates when the

entire word appears coloured. If this is the case. it would further corroborate the identical

finding from Experiment 1. where colounng a single letter of an incongruent colour word did not

reduce error rates relative to fùlly colouring incon_muent colour words. It is difficult to


determine whether Stroop interference is present in the distractor words in Experiment 2% and

therefore this explanation is speculative.

As in the first experirnent. the data on the effect of the position of the coloured letter were

unirformative. Though there was a marginally significant effect of the position of the coloured

letter, none of the anticipated patterns of results were observed. Again, the altemate explanation

outlined in the discussion section of Experirnent 1 would seem to apply here: Participants are

trying to invent an efficient response strategy but it is certainly not strictly based on coloured

letter position.

There was, however, an unanticipated effect of the position of the coloured letter on error

rates, with higher error rates occumng for words with the first letter coloured relative to words

with later letters coloured. This result is surprising given that none of the distractors began with

the first letter of an incongruent colour word and. thus, cannot be accounted for by the ietter

position account derived from Regan (1978). Again. it may be that this result is an artifact of the

higher proportion of t d s with the first letter coloured relative to words with other letters

coloured. That aside, the important contribution of the Stroop task is in showing that colour

naming is slower for words containing a single coloured letter relative to words containing al1

letters coloured. This is entirely consistent with the two-process account because the Besner et

al. ( 1997) single letter coloured manipulation has led to an increase in naming response when

any distractor is used other than colour words (e.g. rows of X's, noncoIour words, colour

nonwords, animal names). Again. there is no reason to think that colour words are in some way

immune to this response time increase and therefore, it seems reasonabie to assume that a second

process is Ieading to a corresponding decrease in response time. The two-process account is

prornising in this regard.


Recognition Test Phase

The key point of interest in Experiment 2a was the recognition rates for words appearing

during the initial colour narning study phase. If Besner et al. ( 1997) were correct in theorking

that colouring a single letter disrupts semantics, then words appearing fully coloured during the

Stroop task should be better recognized than words appeanng with a single letter coloured during

the Stroop task. IE however, colouring a single letter does not lead to differential processing of

the irrelevant words dunng colour naming relative to colouring the entire word, then there should

be no difference in recognition rates between fùlly coloured and single letter coloured words.

Mean hit rates and false alam rates were recorded for each participant. The false alarm rates

were used as the baseline recognition measure. No trials were eliminated due to response times

that were deemed too fast or too slow because participants were told to take as much time as they

needed for recognition. Mean recognition rates for each word type, expressed as the proponion

of "yes" responses, can be found in Table 12. Mean recognition rates as a function of the

position of the coloured letter can be found in Table 13.

A one-way within participants ANOVA was perfonned with word type (fully coloured, single

letter coloured, or new) as the independent variable and the probability of saying "yes" as the

dependent variable. There was a significant effect of word type, F (2. 66) = 53.68. MSe = 0.01.

p < .O 1 (see Appendix Q). To fùnher quali@ this main effect, three plamed comparisons were

conducted. These analyses demonstrated that the probability of a "yes" response to Mly

coloured words and single letter coloured words was significantly greater than to words that did

not appear dunng the Stroop task, F (1, 33) = 90.05, MSe = 0.0 1. p < .O 1 and F (1. 33) = 71 -79,

MSe = 0.0 1, p < .O 1, for fully coloured words and single letter coloured words, respectively. -

There was no difference. however. between hit rates for hlly coloured and single letter coloured


Table 12

Esperiment Za: Mean Recognition Rates and Standard Deviritions for Hits (FiiIlv Coloured and Single

Letter Coloured Studied Words) and False Alarms (Words Not Presented During the Stroop Task).

PR("Yes") SD

Fully Colourcd 0.53 O . 17

Single Letter Coloured O. 50 0. 18

Not Presented 0.25 O. 13

Note: PR("Yes") = Probability of a "Yes" Response (Hit for a Studied Word: False Alarms for an Unstudied Word): SD = Standard Dwiation


Table 13

Eweriment 2a: Mean Recognition Mes and Standard Deviations for Comctlv Rccomized

Words (in the Single Lettcr Coloured Condition) as a Function of Coloured Letter Position

Coloured Letter Position PR("Yes") SD

Note: PR("YesV') = Probability of a "Yes" Response (Hit for a Studied Word: False Alarms for an Unstudied Word): SD = Standard Deviation


words, (1, 33) = 1.10, MSe = 0.01, g = .3 1.

As with the previous experiments, positional analyses were conducted in the present

experiment. This was accomplished by examining recognition rates as a function of coloured

lener position for words that appeared during the Stroop task. As in the two previous

experiments. there was no interesting effect of the position of the coloured letter, nor was there a

significant difference in recognition rate as a function of letter position, F (5, 165) = 1.04.

MSe = 0.12, = .40 (see Appendix R2). Tentatively, these results seem to discount the Besner et

al. (1997) argument that colouring a single letter leads to a disruption of semantic processing (or

even different ial processing).

The major point of interest in the present study was recognition rates for previously presented

distractor words. Recall that Besner et al. ( 1 997) proposed that colouring a single letter of a

distractor word tumed the colour naming task into a letter search task thus disrupting semantic

processing of the irrelevant colour word. In essence, Besner et al. (1997) argued that colouring a

single letter of a word causes that word to be processed in a less wordlike manner. The basis of

this argument was formed solely on response time data, however, in the absence of any esplicit

proof that the irrelevant words were not beinç read. Thus, the present recognition test was

designed to provide a more explicit test of how distractors are processed.

The results of the recognition test are inconsistent with the daim that colouring a single letter

disnipts semantics. There was no difference in hit rates between words that had appeared fully

coloured during the Stroop task and words that had appeared with a single letter coloured dunng

the Stroop task (53% and 50% for fully coloured and single letter coloured words. respectively).

Both of these word types, however. were correctly recognized at a much greater rate than the

false alarm rate for unstudied distractor words (25%). Had colouhg a single letter disrupted


semantic processing. then it would be expected that recognition rates for such words would be

closer to the 25% false alarm baseline than to the 53% hit rate for studied targets. Clearly. this is

not the case.

Of course. it could be argued that recognition processes are not solely based on semantic

processing, and this is quite correct. Thus, the 25% baseline I am using may be a bit low. The

key issue in this experiment was whether single letter coloured words would be recognized at a

rate equivalent to fully coloured words, and this was indeed the case. Besner et a1.k (1997)

semantic blocking argument applies only to single letter word. not fully coloured words. That

recognition rates are equal for these two word types is indicative of some level of processing

above what has been predicted by Besner et al. (1997). even if it is possible that other factors

influence recognition rates. Though it could also be argued that recognition rates are still

relatively low for words that appeared during the Stroop task. they are being correctly recognized

considerably better than distractors are being incorrectly recognized. The low recognition rates

rnay simply be a function of the size of the stimulus set and their limited indirect. which may

make it difficult for participants to remember a greater proportion of words. Nonetheless. the

fact that fully coloured words were not systematically recognized at a rate greater than single

letter coloured words is encouraging for the present argument.

The relatively low recognition rate for words that appeared dunng the Stroop task is actually

not surpnsing in the face of past research. Szyrnanski and MacLeod (1 996) compared

recognition memory for words that were explicitly read to that for words that appeared as

distractors during a colour naming task and found that explicitly read words were recognized at a

greater rate than implicitly processed distractor words. When an implicit repetition priming

lexical decision task was presented to participants, however, explicitly read and implicitly read


words showed equal levels of priming. Thus it could be the implicit nature of the present Stroop

study task that led to recognition rates of only 50%, but these words undoubtedly are being read.

One additional way to test this would be to give participants an initial Stroop task (identical in

nature to the Stroop task in the present experirnent) and follow it up with an implicit task such as

the lexical decision one used by Szymanski and MacLeod (1996) or the reading one used by

MacDonald and MacLeod (1 998). Identical performance for hlly coloured and single letter

coloured words on an implicit test would Further corroborate the present results in demonstrating

that colounng a single letter does not necessarily lead to a disruption in automatic processing.

Once again, the effect of the position of the coloured letter was investigated in the present

analyses and no interesting effect of letter position was found on recognition rates. Panicipants

were equally likely to recognize single letter coloured distractor words independent of the letter

that had appeared coloured.

Recognition Latency

Though no explicit predictions were made concerning recognition latency, a number of

analyses were performed in this regard. This was done for several reasons. First, in normal

recognition. it is generally expected that participants will respond to words that they have seen

faster than to words that they have not seen (Reisberg, 1997). In the present experirnent.

however, words were presented in a more irnplicit manner and, therefore, it seemed worthwhile

to determine whether this pattern would hold. Second. it might be expected that participants who

are slower on the initial Stroop task would have better recognition for distractor words because

they are exposed to these distractor words for longer than participants who are faster on the

initial Stroop task. Finally. these analyses were perforrned to investigate the pattern of

recognition response tirnes as a function of the position of the coloured letter during the Stroop


phase. Mean response times and standard deviations for each word type and as a function of

coloured letter position are presented in Tables 14 and 15. respectively. Collapsed mean

response times for words that appeared during the Stroop task and words that did not appear

during the Stroop task, independent of correctness, can be found in Table 16.

To investigate whether participants would respond more quickly to words they had seen

during the Stroop task relative to words that they did not see during the Stroop task, a number of

ANOVAs were conducted that paralleled the recognition accuracy analyses from the preceding

section. Thus, a one way within participants ANOVA was performed to compare response times

for fully coloured hits (correct "yes" responses), single letter coloured hits, and unstudied false

alarms (incorrect "yes" responses). This analysis yielded no effect of previous exposure to

words on response time, F (2. 66) = 1.45. MSe = 1404 12.3, p = .24 (see Appendix S 1 ). A fùnher

analysis performed on recognition response times as a function of coloured letter position also

yielded no effect of letter position, F ( 5 , 165) = 1.2 1, MSe = 274557.7. p = 0.3 1 (see Appendix

S2). The expected pattern of response was observed. however. in the sense that participants were

faster to correctly accept words that had appeared dunng the colour naming study task (answer

"Yes") than to incorrectly accept nonpresented distractor words. Panicipants were also slower to

incorrectly reject words that appeared dunng the colour naming study task than to correctly

reject words that did not appear during study (see Table 14). The failure to reach statistical

significance may simply be due to the implicit nature of the present task. When participants

explicitly study words for later recognitioa they are thought to practice them in the interim and

therefore. have more experience with these words when they are to be recognized. In the present

experiment. however, participants were oniy implicitly exposed to words by unintentional

reading of distractors and it is extremely uniikely that these words were rehearsed in the interirn


given that participants were

Table 14:

Esueriment 2a: Mean Response Times and Standard Dcviations for Words Auuearinq

Durine. the Recognition Task as a Function of Response Tvpc (Yes or No) and Word Tvpe

jFu1lv Coloured, Single Letter Coloured. and Not Presented).

Response Type

Ycs No

Full! Coloured 1189 493 1397 669 Single Letter Coloured 1214 572 1370 65 1

Not Presented 1342 556 1273 500

Note: RT = response times; SD = standard detiation


Table 15

EsPenment 2a: Mean Remonse Times and Standard Deviations for Conectlv Recognized

Words (in the Single Letter Coioured Condition) as a Function of Letter Position.

Coloured Letter Position RT SD

6 1130 413

Note: RT = response cimes; SD = standard daiation


Table 16:

E~wriment Za: Mean Responsc Times and Standard Dcviations (on the Recognition Taskl

Collapsed Across Conditions for Words That Appeared Durine: the Stroop Task (Prcsented)

and Words That Did Not Appear During The Stroop Task (Not Presented).

RT SD

Presented

Not Presented

DLBerence

Note: RT = Response Times: SD = Standard Deviation


unaware that recognition test would follow. As in previous analyses, there was no effect of the

position of the coloured letter at study on recognition response times.

To investigate the possibility that participants who were relatively slow during colour naming

would have greater recognition memory for distractors due to increased exposure time - a kind

of "total time" of study hypothesis (cf'. Cooper & Pantle, 1967) - participants were divided into

two groups of 17 via a median split on speed of response. Recognition rates were then çenerated

for participants who were fast vs. slow overall, fast vs. slow on fully coloured words. and fast vs.

slow on single-letter coloured words. Three one-way independent samples t-tests indicated that

there was no difference in recognition rates between participants who were fast or slow in colour

narning for fully coloured words. single letter coloured words, or overall responses. al1 1s < 1 . In

retrospect. thiç is not a shocking result as recognition is generally dependent upon the manner in

which words are processed rather than on their duration of exposure (cf Craik & Lockhart,

1972). Slower participants did not necessanly engage in any extra processing of irrelevant

words relative to faster participants and, therefore. the observed equivalence in response rates is

not surprising

Finally, the present experiment investigated recognition response time for correct responses

as a function of the position of the coloured Ietter dut-ing the Stroop study phase. Mean response

times and standard deviations are presented in Table 17. It should be noted that not every

participant is represented at every letter position because some participants failed to provide any

correct responses for certain Ietter positions. In these cases. the mean of al1 participants for that

letter position was used. A one-way within participants ANOVA demonstrated no significant

difference in response time as a function of letter position, F (5, 165) = 1.2 1 MSe = 332588.7,


Table 17:

Esperiment 2a: Mean Res~onse Times and Standard Deviations for Correctlv Recomizcd

Words (Single Letter CoIoured) as a Function of Letter Position.


1 1185 40 1

2 1205 977

3 13 82 50 4

4 125 4 544

5 1108 428

6 1130 4 13

Note: RT = response times; SD = standard de~iation


e = -3 1 (see Appendix T). At this point. it seems unnecessary to perform any positional analyses

in later experiments. Nonetheless, 1 will continue with these analyses as they have been planned.

in case a meaningfùl pattern of results should anse in Iater experiments. This is a possibility

given that Experiments 2b and 3 are of a slightly different nature than the present experiment and

Expenment 1.

Due to the relatively undenvhelming nature of the response time results in Experiment Za, it

would be undesirable to terminate the present discussion without returning to the most important

findings from this experiment. First. there was no difference in recognition rates between words

that appeared filly coloured during an initiai Stroop task and words that appeared with a single

letter coloured during the same task. Both of these word types. however, were recognized at a

rate far greater than baseline. This finding casts serious doubt on the Besner et al. (1997) daim

that coiouring a single letter of an irrelevant word dismpts processing of that word and that it is

this that is responsible for the elimination of the Stroop effect. Clearly. single letter coloured

words are being read by participants. and apparently to the same extent as fully coloured words.

despite explicit instructions to the contrary. This is at least consistent with the idea that even in

the single letter coloured condition words are read automatically. Consequently, an altemate

expianation is needed for the results of Besner et al. (1997).

Data from the present colour naming task support the previously outlined two-process

explanation, with participants taking 40 ms longer to colour name when a single letter of a word

is coloured relative to al1 letters being coloured. This is another key finding as it funher

confinns the results of previous experiments in which there is a response time cost associated

with the colounng of a single letter for al1 irrelevant stimuli other than incongnient colour words.

This is now the fourth recorded set of stimuli (along with colour nonwords, animal words, and


rows of X's) that experience a response time increase when a single letter is coloured relative to

al1 letters. colour words being the sole expection. It appears increasingly likely that this increase

in response time is offset by a decrease in Stroop interference in the incongruent colour word

condition and that this is what accounts for the results of Besner et al. (1997). 1 have already

offered considerable evidence (past theory and research) to suggest that the use of questionable

control items and the use of button pressing have contnbuted to Besner et al.'s (1997) finding of

an elimination of Stroop interference and their resulting account. When these procedures are

replaced by the more traditional vocal response and more standard controls (e.g., rows of X's).

then reliable Stroop interference is observed. even when a single letter is coloured (Mamurek.

1999; MacLeod, 1999). Thus, it seems more appropriate to view colounng a single letter as a

way to reduce, but not to eliminate Stroop interference. This reduction is what then must be

explained, which is what the present two-process account attempts to do.

Expenments 1 and 2a appear to offer considerable support for the two-process account. even

though this account was not the central focus of these experiments. I now tum to two

experirnents which further investigate the two-process account; namely the differential manner in

which stimuli are processed when a single character is coloured, relative to the entire stimulus

appearing in colour.

Experiment 2b

Though Experirnent 2a provides a more explicit test of whether single Ietter coloured words

are being read during the Stroop task. another possible explanation needs to be further explored.

The possibility that the 20-30 ms reduction in Stroop interference is due to two processes was

previously reviewed and will be given further attention in the next two experiments.

It is nonetheless interesting to consider the manner in which the 30 ms of Stroop interference


was reduced in the Besner et al. (1997) and Besner and Stolz (1999ab) studies, given that their

pattern seems counterintuitive. Recall that response times in the incongment condition were not

reduced to match response times in the control condition when a single letter was coloured;

rather, response tirnes in the control condition rose to match those in the incongruent condition.

The two- process explanation for these results is that colouring a single letter leads the individual

to process distractor words in a different manner that. in tum, eliminates approximately 30 ms of

Stroop interference. For example. one possibility is that colouring a single letter of an irrelevant

colour word causes that word to be processed in a less wordlike manner. thus leading to a

reduction in interference. This reduction. however, rnay be offset by an increase in colour

naming in the single letter coloured case that coincidentally totals about 20-40 ms. This would

occur because colouring a single letter forces the participant to search for the letter. whereas

colouring the entire word allows the participant to remain fixated on one point. This would

explain why control response times rise (seeing as they should be relatively unaffected by Stroop

interference) when a single letter is coloured. whereas incongment trials rise in response time

due to search time but subsequently decrease due to a decrease in interference. This leads to the

illusion that the response times are unaffected by a shift to single letter colouring. were presented

with a single, randornly selected. letter coloured. Participants should take longer to read words

with a single letter coloured than words entirely in colour if the presence of a single coloured

letter disrupts processing. Slower reading times for single lener coloured words would not only

suggest that colouring a single letter slows processing relative to colouring an entire word. but

would also suggest that colouring a single letter may cause a word to be processed in a less

wordlike manner. This does not necessarily mean that words are not being read so much as they

may be being processed differently.


To test this explanation. and the manner in which individuals process words with a single

letter coloured, Experiment 2b was a simple word reading task. The design was similar to the

Stroop task used in Experiment 2a. except that participants were now instructed to read the

words and ignore their ink colours, as quickly and as accurately as possible. Half of the words

were presented entirely in one of four colours (red, blue, yellow. green); the remaining words

Method

Participants. Participants in the present experiment were the same 38 University of Toronto

at Scarborough undergraduates, both male and female. participated for course credit in

Experiment 2a. Though every participant performed satisfactorily in the present experirnent. for

consistency, the 4 participants whose data were excluded fiorn the previous experiment were also

excluded here. Testing time for each participant was approximately 15 minutes.

Materials. The materials in this experiment were 80 noncolour English words. These 80

words comprised the second list that was created from the original 160 words in Experiment 2a.

Unlike the previous expenments, a microphone and voice key was used to record participant

response due to the nature of the task.

Procedure. The word reading study was presented to participants upon completion of the

recognition test fiom Expenment 2a. Alhough many of the aspects of Experiments 2a and 2b

were counterbalanced (e-g.. number of trials. order of lists). Experiment 2b was always

performed following Experiment 2a. This was done to ensure that the stimulus set from

Experiment 2b did not interfere with the recognition test from Experiment 2a. and because

Expenment 2a was considered to be the more crucial of the two experiments.

For each participant, 64 words were randomly selected From the original list of 80. The

experimenter had a printout that listed the order of word presentation so that participants could


be monitored for errors. The experiment consisted of 64 trials in which participants were

instructed to read aloud the words presented to them as quickly and accurately as possible.

ignoring the display colour.

On some tnals, the microphone did not pick up the participant's response and on some trials

the participants rnisread the target word. Ail such incidents were recorded but only the latter

were considered true errors.

Results

True errors (misread words) were made by panicipants on less than 1.5% of trials overall.

This was not deemed problematic because the task in the present expenment was word reading.

not coloiir naming, and there was therefore no reason to expect that error rates should

systematically Vary as a function of whether words appeared fully coloured or with a single letter

coloured. A one-way ANOVA confirmed that there was no difference between the nurnber of

errors made for words appearing fully coloured relative to words appearing with a single letter

coloured, F (1, 33) = 1.9 1. MSe = 1.73, e = 0.18. Indeed. the very existence of the standard

Stroop effect indicates that it is easier for participants to ignore print colour than actual words

and therefore the position of coloured letters should exert little influence. Response time

differences had been expected if colounng a single letter had indeed caused a disruption in

semantic processing, not due to any type of search as in the previous experiments. In addition.

errors did not systematically differ as a function of colouring type: There was an approximately

equal number of errors for fully coloured and single letter coloured words. Aside from tme

errors, response times were excluded fkom analysis when the microphone did not pick up the

initial response, or when narning times were less than 300 ms or greater than 2000 ms. These

constituted less than 1% of the data and did not differ between the fully and singly coloured


conditions.

The purpose of the present experiment was to examine the manner in which individuals

process words with a single letter coloured relative to words appearing entirely in colour. To

examine this, mean word reading times for words appearing fully coloured vs with a single letter

coloured were obtained from each participant. These mean reading times and standard

deviations are shown in Table 18. As expected, participants were siightly faster reading words

appearing fùlly coloured (564 ms) relative to words appearing with a single random letter

coloured (571 ms). This result was nearly. though not entirely. confimed by a one-way within

participants ANOVq with colour type (fülly coloured or single letter coloured) as the

independent variable and colour naming times as the dependent variable. The main effect of

colour type was not significant, F (1. 33) = 2.77. MSe = 422.59, g = - 1 I . though it approached

marginal levels of statistical significance (see Appendix ü).

-4s in Experiments 1 and îa, a second one-way within panicipants ANOVA was conducted to

examine the effect of the position of the coloured letter. Mean reading times and standard

deviations are shown in Table 19. Although there is virtually no difference in reading times

when any of the first four letters are coloured, it took participants quite a bit longer to read words

that appeared with either the fifth or sixth letter position coloured. This difference was

confirmed by a one-way within participants ANOVA, (5, 165) = 3.10. MSe = 1337.36, Q = .O 1.

with the position of the coloured letter serving as the independent variable and reading times as

the dependent variable (see Appendix V). It should be noted that this difference was due solely

to the difference in reading times between the first four letter positions and the last two letter

positions, as there was little response time variance within these two groups of positions (see

Table 19).


Table 18

Esperiment 2b: Mean Reading Times (in ms) and Standxd Dcviations ,as

a Function of Conditions (Al1 Letters Coloured vs. Sinek Letter Coloiircd).

Fully Coloured

Single Letter Coloured

Difference

Note: RT = reading cimes; SD = standard deviation; %E = petccntage of errors

Reduced Stroop Interference - 1 O 1

Esperiment Zb: Mean Reading Times (in ms) and Standard Deviations

ris a Function of Letter Position in Sin& Letter Coioured Words.


1 57 1 96

2 568 87

3 567 9 1

4 567 89

5 5 88 103

6 589 102

Note: RT = v o n s e cimes: SD = standard deviation; %E = percentage of erron


Discussion

The results of the present study add some support to the notion that it takes longer to process

stimuli appeanng with a single letter coloured relative to stimuli appeanng fully coloured. This

difference, however. was smaller than might be expected (only 7 ms). Perhaps coloured letters

are more difficult to read off of a black background than are white letters. Thus, any difficulty

that may be associated with reading a word with a single letter coloured could be offset by a

relative ease in deciphenng the Ietters appeanng in white. Another possibility that could account

for these results is practice effects canying over From the first expenment. Though the two tasks

differed, each experiment involved differential colouring of stimuli and the exposure to such

colouring in Experiment 2a may have led to a decreased sensitivity to colouring type in the

present expenment. The built-in experiment order confound does not permit determining

whether this is the case.

An altemate, and perhaps more plausible explanation is simply that words are generally easy

to read itticntiotialfy, regardless of whether they appear fully coloured rather than with a single

letter coloured. In Expenrnents 1 and 2% the task was to identify the display colour and not to

read the word that is presented on the screen. Thus. whereas colouring a single letter rnay

substantially disrupt the automatic and unintentional reading of a distractor word, it may not

disrupt the intentional reading of a target word. The point of the present experiment, however.

was simply to examine whether words appearing with a single letter coloured are processed

differently than words appearing fully coloured, and the present results do provide slight suppon

for this possibility, despite the mal1 effect size. Experiment 3 also provides a test of whether

single letter coloured words and fully coloured words are processed differently in a manner that

Reduced Stroop Interference - 1 O3

is more similar to Expenments 1 and 2a..

Experirnent 2b also exarnined word reading times as a function of the position of the coloured

letter. Unlike the first two experiments, there was a significant main effect of the position of the

coloured letter. with participants taking longer to read words that appeared with the fifth or sixth

letter coloured relative to words that appeared with one of the first four letters coloured.

Although statistically significant, this result is of little practical importance to the present study

given the interest in how colouring a single letter affected the colour naming process and the

unintentional reading of distractor words, not the intentional reading of target words (where the

position of the coloured letter should not actually affect response time). Again, this could be an

artifact of an insufficient number of trials where the fifth or sixth letter is coloured.

Aithough the present experirnent was designed to test the possibility that colouring a single

letter of a word leads to different processing relative to fùlly colouring a word. it is difficult to

apply the present results to those of the first two studies given that a different task was used

(word reading as opposed to colour naming). The present results somewhat support the notion

that colounng a single letter of a word may slow processing relative to fully colounng a word but

this may be a function of the task that was used rather than the colouring of the words

themselves. Thus, to further test how participants process stimuli with a single character

coloured. one further experiment was conducted under colour naming conditions.

Experiment 3

1 have argued that Besner et al.'s (1 997) methodology of colounng a single letter is, by itselE

not a plausible way of eliminating the Stroop effect. To make this argument, 1 have pointed to

various replications of their Experiment 2, as well as a number of unique methodological choices

that they made. It remains possible. however, that colouring a single random letter of an


incongment colour word actual does eliminate approximately 20-30 ms of Stroop interference,

with this reduction in Stroop interference offset by a slowing in response time as participants

search for the coloured letter. If this is the case, it would explain why response times in the

control condition of Besner et al. (1997) appeared to rise whereas response times in the

incongruent word condition appeared not to change. The 20-30 ms reduction in Stroop

interference in the incongment condition would be offset by a slowing in response. thus

providing the appearance that these response times were unaffected when a single letter was

coloured.

The same explanation does not hold for control items. however, as no Stroop interference

should be present in this condition and, therefore, response times should only be affected by the

eara time it takes to search for the coloured letter. This two-process account is plausible, but is

also difficult to test. This is because there is no way to demonstrate that colouring a single letter

of a colour word reduces Stroop interference, independently of an overall slowing of response

times. The two processes seem dependent, and therefore, there is no real way to separate thern.

One can investigate, however, whether colouring a single letter (or character) slows responding

relative to colouring al1 letters. This has already been demonstrated in Experiment Za, but there

is a possibility that even these noncolour words evoke Stroop interference (see MacLeod, 199 1,

pp. 172- 174 for a review).

Experiment 3 was designed to fimher test the notion that colounng a single character slows

colour naming relative to colouring dl characters in a stimulus item. To test this prediction

independently from distractors that may elicit Stroop interference, 1 used a variation of the

praaice task that was used in Expenments 1 and 2. The stimuli on al1 trials were rows of

asterisks with al1 or only one random asterisk in colour. It was predicted that participants would


take longer to indicate the display colour when a single asterisk was coloured relative to al1

asterisks being coloured. This slowing in response should be somewhere in the range of 20 to 30

ms. Such a result would support the idea that colouring a single character reduces Stroop

interference relative to colounng al1 characters. but is accompanied by an overall slowing in

response.

Method

Participants. Twenty-four university undergraduates. both male and fernale, participated for

course credit. Each participant had normal or corrected to normal vision as determined by self-

report and none of the participants had participated in any of the other experiments. Testing time

for each participant was approximately 5 minutes. Due to the very brief duration of this

expenment, it was added to the end of another unrelated experimental session taking place in our

laboratory.

Materials. The materials were rows of astensks which were equivalent in length to the

colour and noncolour words used in the first two expenments (i.e., 3.4. 5. or 6 asterisks in

length).

Procedure. To familiarize participants with the button pressing procedure that was to be

used during the experiment, participants initially perfonned 24 practice trials that were identical

to the types of trials contained within the expenment. With the exception of the number of

practice trials. the procedure was identical to the Stroop tasks in Experiments 1 and 2a. with

participants identifjmg the display colour of a fully coloured row of asterisks or of a single

coloured asterisk within the row. rather than words and nonwords. Following the familiarization

task participants completed the actual64 experirnental items.

Resul ts


Mean naming times and standard deviations were calculated for each participant. Colour

naming times for rows of asterisks appeanng fully coloured (677 ms) were 20 ms faster than for

rows of asterisks appearing with a single astensk coloured (697 ms) (See Table 20). A one-way

within participants ANOVA was perfomed with colour type (al1 asterisks or a single coloured

astensk) as the independent variable and colour narning times as the dependent variable. This

analysis indicated that the 20 ms difference in naming times between fùlly coloured rows of

asterisks and rows of asterisks with a single asterisk coloured words was significant. F ( 1. 24) =

7.90, MSe = 597.84, p = . O 1 (see Appendix W 1). There was no effect of the position of the

coloured asterisk (see Table 2 1 for means and standard deviations), F (5, 120) = 1.65, MSe =

7 186.22, p = .15 (see Appendix W2).

Error rate analyses demonstrated no difTerence in error rates as a function of colour type, F ( 1.

24) = 1.23. MSe = 0.0006, g = 0.28, or as a function of coloured asterisk position, F (5, 120) =

1.16, MSe = 0.003, p = 0.33 (see Appendices XI and X2).

Discussion

Due to the nature of the present distractors, it was not necessary to examine error rates as a

function of colounng type or position of the coloured asterisk because asterisks provided little, if

any, distraction to participants. In addition, though there was a marginally significant effect of

the position of the coloured character, there was no intuitive or interesting pattern of colour

naming times as a function of the position of the coloured character. Again, none of the

anticipated patterns of positional results were prevalent, thus lending credence to the notion that

participants either fail to develop a strategy for colour naming or develop a strategy that does not

lend itself to an easily analyzable pattern of results regarding coloured character position.

The results of the present experiment were exactly as expeaed. Participants took longer to

Reduced S troop Interference - 1 O7

Table 20

Ewenment 3 : Mean Reaction Times (in ms). Standard Deviations. and Enor Rates to Name

Colours as a Function of Condition (Al1 Asterisks Coloured vs. Single Asterisk Coloured).

AI1

Single

D iffe rence

Note: RT = response tirnes; SD = standard deviation; O/oE = percentage of errors


Table 18

Emcriment 3: Mean Reaction Times (in ms). Standard Deviations. and Error Rates

to Name Colours as a Function of Coloiired Asterisk Position in the Single Asterisk

Coloured Position.

Coloured Letter Position RT SD %E

1 674 1 14 2.7

2 685 116 6.8

3 71 1 12 1 3 .O

4 702 142 1.9

5 698 128 1.3

6 737 18 1 O

Note: RT = response tirnes; SD = standard deviation: %E = percentage of errors

name the display colour when a single astensk was coloured relative to al1 asterisks being

Reduced Stroop Interfierence - 109

coloured. This slowing in response was right around the 20 ms range that was demonstrated for

control items in the Besner et al. (1997), and MacLeod (1999) studies. as well as the present

Experiment 1. It should be noted that this slowing in response times would have been larger,

except that two participants showed a pattern of results that were in the direction opposite

expectation (these participants were approximately 100 ms faster indicating the display colour

when a single character was coloured relative to al1 characters being coloured). Without these

participants. the difference in response times would actually be closer to 35 ms.

Thus, the evidence does suggest that colouring only a single letter of a coloured item slows

down responding to colour by approximately 20-30 ms. This leads to the illusion that colouring

a single letter of a colour word does not affect response times relative to colouring al1 letters.

For control items, where there is ostensibly no Stroop interference occurring, colouring a single

letter leads to a slowing in a response which causes response times to nse and match incongruent

word response times. Again though, despite a relative ease in demonstrating that colouring a

single letter leads to an increase in response time, it is difficult to show that colouring a single

letter leads to a reduction in interference when it is always accompanied by this increase in what

would appear to be search time. Due to the fact that response time increases have been

demonstrated for al1 irrelevant word types except colour words. it seems reasonable to conclude

that an additional process is leading to a corresponding response time decrease when a single

letter is coloured.

These results could be seen as suggesting that the Stroop effect has been eliminated as Besner

et al. (1997) conclude, but 1 have already suggested why this may not be the case. A strong

argument for my position seems to be a high probability that some Stroop interference is

occumng in Besner et al.3 (1997) control condition (e-g. E T , BLAT), as was suggested by the


pattern of results in Experiment 1, as well as by past findings regarding the first letter of an

incongruent colour word (e.g., Regan, 1978). Consequently, Stroop interference would still be

present in each stimulus type (colour words and colour nonwords), despite an equivalence in

response times across conditions. A more detailed explanation of the possible problems

associated with these control items, as well as other Besner et al. (1997) procedures, will be

reserved for the General Discussion section.

One other possible explanation for the results of the Besner et al. (1997). MacLeod (1999).

Marmurek (1999). and the present studies is that colour words are in some way immune to the

slowing in response that seems to occur when a single letter is coloured relative to colouring al1

leners in other distractor words. There are, however. at least two pieces of evidence that would

suggest that colour words are not immune to this slowing in response time associated with

colounng a single letter. First. there is nothing in the literature to suggest that there is anything

"special" about colour words that should make them immune to the slowing in response

associated with colounng a single letter. Second. in seneral. there was a slight rise in response

times for colour words when a single letter was coloured in the present expenments. and in

Besner et al. ( 1997). MacLeod ( 1999), and Marmurek ( 1999). indicating that colour words were

at least being slightly influenced by the single letter coloured manipulation. Thus. it may be that

when one colours a single letter of a colour word. the average reduction in Stroop interference is

approximately 20 ms whereas the average increase in response times is approximately 30 ms.

Regardless, it does not seem likely that colour words are immune to slowing when a single ietter

is coloured.

Reduced Stroop Interference -1 1 1

General Discussion

The present series of experiments was conducted to further examine the possibility that the

Stroop effect can be eliminated, a claim put forth in articles by Besner et al. (1997) and Besner

and Stolz (1 999a,b). These researchers claimed that the Stroop effect could be eliminated by

colouring only a single letter and not al1 of the letters of an incongment colour word. I have

pointed out a number of possible problems with the Besner et al. (1997) procedure (e-g., control

stimuli, response mode), and have reviewed a number of replications and partial replications of

the Besner et al. (1997) work that have not completely elirninated the Stroop effect. 1 have

argued that to elirninate the Stroop effect via preventing word reading. one would have to show

that distractor words are not being read (and thus. not providing interference).

Summary of the Findings

Although Besner et a1.k (1997) results demonstrate an equality in colour naming response

times between fully coloured words and words with a single letter coloured, a simple reliance on

naming time differences is not definitive in demonstrating whether distractor words are beinç

read. Thus, the present Experiment 2a was designed to provide a more explicit test as to whether

distractors were being read on Stroop trials. A two-process account was introduced to account

for the results of Besner et al. (1997) and two subsequent expenments were conducted to test this

view (Experiments 2a and 3).

The purpose of Experiment 1 was to replicate and extend Experiment 2 from Besner et al.

(1997). with the additional goal of examining the effect of the position of the single coloured

letter. Although the key aspect of the basic pattern of Besner et al.'s (1997) results replicated. -

interference decreased for single-letter-coloured words relative to fully coloured words - there

was no evidence to suggest that the Stroop effect had been eliminated. Colounng only a single


letter appeared to decrease, but not to elirninate, Stroop interference. Thus, the present

Experirnent 1 adds to a growing literature of replications and partial replications of Besner et al.

(1997) and Besner and Stolz (1999a/b), that have not demonstrated an elhination of the Stroop

effect (e.g., MacLeod, 1999; Marmurek, 1999).

Error rate analyses from Experiment 1 also discounted the Besner et al. (1997) claim that

colounng a single letter leads to a disruption in semantics. Overall, participants made more

errors when incongruent colour words appeared with a single letter coloured relative to fully

coloured. Were Besner et al. (1997) correct in claiming that colounng a single letter disrupts

semantic processing of distractors, then fewer errors would be expected when a single letter is

coloured, assurning that errors are the result of an incorrect response to the distractor words.

The purpose of Experiment 2 was to provide a more explicit test of how distractor words are

processed under these conditions. Experiment 2a examined recognition rates for distractor

words that were presented dunng an initial Stroop colour naming study phase. Rather than using

colour words and colour nonwords, each Stroop trial consisted of a different English language

word. Participants were then subjected to a surprise recognition test to examine tneir memory

for the to-be-ignored distractor items. Collectively. the recognition accuracy and response time

data indicated that colouring a single letter does not disrupt semantic processing of distractor

words and it is therefore more likely that colounng a single letter reduces Stroop interference

while simultaneously increasing response time. Consequently, the two-process account was

given further scrutiny in Expenments 2b and 3 .

Enperiment 2b further examined the marner in which words are processed when they appear

with a single letter coloured relative to having al1 lerters coloured. Using an intentional word

reading paradigm, there was a mere 7 ms benefit for fully coloured words over single Ietter


coloured words in terms of reading time. The most plausible explanation for this is the

possibility that different processes occur when individuals read words intentionally rather than

incidentally. This is of little consequence to the present iine of research, however, given that

Stroop research deals exclusively with colour naming of only incidentally read words.

Expenment 3 was an examination of the two-process account that has been posited to account

for Besner et ale's (1 997) results. In an attempt to tease apart these two processes, a Stroop task

was used in which rows of astensks appeared as distractors rather than words. The results of the

expenment indicated that it does take participants approximately 20-30 ms longer to name a

display colour when only a single character is coloured relative to al1 characters being coloured.

This result supports the notion that colounng a single letter may lead to a reduction in Stroop

interference. which is in tum offset by a slowing in response.

The present experiments were also designed to test the influence of the position of a coloured

Ietter on response. Previous research (e-g. Regan. 1978) had demonstrated that much of Stroop

interference may be attributable to the first letter or letters of an incongruent colour word (e-g..

the letter 'Y in blue) and, therefore. 1 sought to determine the influence of letter position on the

present response time and error data. Although positional analyses were useful in further

demonstrating that the Besner et al. (1997) control stimuli may be inappropnate (a point which

will be expanded upon shortly), there were no other sijgificant effects of letter position. Nor

were any interesting or intuitive patterns of results observed for letter position across the set of

experirnents. This is only a minor disappointment to the present line of research and does not

detract frorn the other important findings that have already been reviewed. In the upcorning

section, 1 will hrther outline the issues that have been raised to this point, as well as considering

the implications and interpretations of the present results. Possible methodological problerns


will be considered, and ideas for future research in this area will be suggested.

Where Does the Present Study Fit?

The present Expenment 1 is now the third known attempted replication of Besner et al.

(1 997). Experiment 2, in addition to the further studies by Besner and Stolz ( 1999% b).

Previously, both Marmurek (1 999) and MacLeod ( 1999) have performed replications of the

original Expenment 2. as well as partial replications in which different control stimuli and

response modes were used. In al1 of the straight replications of Besner et al. ( 1997). Besner et

al.3 ( 1997) basic pattern of reduced interference was replicated. though Marmurek ( 1999) failed

to demonstrate an elimination of Stroop interference. as was also the case in the present

Experiment 1. MacLeod (1999) and Besner and Stob (1 999) both observed an "elimination" of

Stroop interference though this elimination was wiped out by MacLeod (1999) when he switched

to different controls and vocal response.

Marmurek (1 999) and the present Experiment 1 both observed the difference between control

and incongrnent trials to be closer to 20 ms than O ms in the sinlge letter coloured condition. In

addition, when Marmurek ( 1999) and MacLeod ( 1999) performed partial replications of Besner

et al. (1 999) using different control items (animal names or rows of X's) and different response

modes (vocal rather than button pressing). substantial Stroop interference was always observed.

even when only a single letter was coloured. Thus. the present Experiment 1 adds to the growing

literature that demonstrates that colouring a single letter of a distractor word does not. by itselt

eliminate Stroop interference. although interference is consistently reduced by this manipulation.

The present Expenment 2a, however. is the first attempt to apply an explicit memory test to

the Besner et al. (1997) paradigm in an attempt to determine the manner in which distractor

words are processed when a single letter is coloured relative to the entire word. 1 have argued

Reduced Stroop Interference - 1 1 5

that Besner et d.'s (1997) reliance on equivalence in response times is insufficient grounds to

conclude that distractors are not read when a single letter is coloured. Indeed. the present results

demonstrate that distractor words are being read, regardless of the manner in which they are

coloured. Recognition rates for words appearing with a single letter coloured during an initial

Stroop task were equal to those for words appearing fully coloured during the same task. In

addition, each of these word types was recognized at a rate well above baseline (as detennined

by false dams to words not presented during the Stroop task).

Error rate analyses funher supplemented the present claim that colouring a single letter does

not lead to a dismption of semantics for distractor words. In the present Experirnent 1, the error

rate for incongment colour words increased when they appeared with a single Ietter coloured

relative to fully coloured. Had colouring a single letter disrupted semantics, then it would be

expected that error rates for incongment colour words would decrease when a single letter is

coloured. assuming that errors are the result of an incorrect response to irrelevant words.

Marmurek (1999) observed a sirnilar (if not greater) increase in error rates for incongruent colour

words when a single letter was coloured relative to al1 letters, and Besner et al. (1997) obtained

no change across these two conditions. Clearly, these results suggest that distractor words are

being read when a single letter is coloured and. therefore. this method of colouring does not lead

to a dismption of semantics nor to an elimination of Stroop interference, contrary to Besner et al.

(1 997).

It does seem entirely reasonable. however, to suggest that colouring a single letter of a

distractor word leads to a rediictioii in Stroop interference, likely in the 20-30 ms range. This

reduction in Stroop interference is then offset by an increase in overall colour narning response

time in the single letter coloured condition which, from the present expenments, not

Reduced Stroop Interference -1 16

Experiment 3, we can determine to be somewhere in the range of 20-40 ms. This explanation

represents the two-process account for the results of Besner et al. (1997), an account which has

been argued for extensively throughout this thesis.

There are a number of aspects of the present data that provide suppon for the two-process

view. First, there is a consistent increase in response time when a distractor appears with a

single letter coloured relative to appearing with al1 letters coloured. In Experiment 1. there was a

colour naming increase of 13 ms for incongruent colour words and of 36 ms for colour nonwords

when a single letter was coloured relative to al1 letters being coloured. Note that this increase is

much srnaller for incongruent colour words, the condition most likely to be offset by a reduction

in Stroop interference. In Experiment 2% there was a naming increase of 40 ms when a single

letter was coloured relative to al1 Ietters being coloured. Finally. in Experiment 3, the result is

especially clear. There was a narning increase of 20-30 ms when a single asterisk was coloured

relative to al1 asterisks being coloured. Thus, an increase in the colour naming response time has

now been documented for four separate types of lexical and nonlexical stimuli when a single

character is coloured relative to al1 characters: Colour words, colour nonwords, assoned 3-6

letter English words, and rows of asterisks. These naming time data provide unequivocal support

to the argument that colounng a single letter increases naming response time relative to

colouring al1 letters of the irrelevant word.

As has already been pointed out, it is difficult to determine whether colouring a single letter

reduces Stroop interference in the absence of an increase in response time. There are. however,

key aspects of the present data that suggest that this may be the case. Consider again the just-

reviewed differences in colour naming time when a single letter is coloured relative to al1 letters.

Of the four registered increases in naming response, the srnallest increase occurs when a single


letter of an incongruent colour word is coloured. The incongruent colour word condition is the

one most associated with Stroop interference and. therefore, the one most likely to evidence a

decrease in interfierence when a single letter is coloured. This appears to be the case in the

present Experiment 1. Were Stroop interference not being reduced, there would be no reason to

think that incongruent colour words should be any less affected by colouring a single Ietter than

are colour nonwords or other English language words. The only other possible explanation for

the relatively small increase in naming response when a single letter is coloured is that colour

words are in some way immune to these effects relative to colour nonwords and other English

words. It has already been pointed out, however, that there is no reason, either empirically or

theoretically, to believe this to be the case.

The second aspect of the present data that supports the possibility that colouring a single letter

reduces Stroop interference is the pattem of response time resuits in the present Experiment 1. as

well as in Besner et al. (1997) Experiment 2, and in MacLeod (1999). In Besner et al. ( 1 997)

and al1 replications and extensions. response times in the control condition rise to match (or

nearly match) response times in the traditional incongnient condition, whereas incongruent times

appear to remain unchanged. To argue that the Stroop effect can be eliminated. one would

expect that the opposite pattem of results would be obtained: Response times in the incongnient

condition should have decreased to match response times in the control condition. This unusual

pattern of results is readily explained by the two-process account. In the control condition, there

is ostensibly no Stroop interference to be elirninated and, therefore, response tirnes are only

afi?ected by the increase associated with switching From a fully coloured word to a single letter

coloured word. In the incongruent colour condition, however, there is an abundance of Stroop

interference that cm be reduced. Colouring a singie letter leads to the expected increase in


colour naming but is offset by an approximately equal decrease in S troop interference, presenting

the illusion that naming times have remained unchanged.

Final Words on Besner et al. (1997)

The present results provide support for a two-process account. but also demonstrate that it is

inappropriate to think of the Besner et al. (1997) and Besner and Stolz (1999ah) results as

representing an eiirnination of the Stroop effect. Colouring a single letter in fact does not appear

to dismpt the semantic processing of irrelevant words, contrary to the claim of Besner et al.

(1 997). These words are clearly being read when a single letter is coloured, despite explicit

instructions to the contrary, as evidenced by recognition rates for these words that are far above

baseline. In addition. recognition rates for words appeanng with a single letter coloured were

equal to rates for words appeanng fully coloured. opening up the possibility that colouring a

single letter does not even lead to differential processinç of distractors relative to colouring al1

letters.

If colouring a single letter leads to a reduction in Stroop interference, then why is it that

Besner et al. (1997) obtain what appears io be a complete elimination of Stroop interference

when their response time data are considered. A number of aspects of the Besner et al. ( 1997)

and Besner and Stolz (1999ah) studies have already been questioned in the present research and

will be reiterated here as the response time and error rate data in Experiment 1 seem to confinn

these concerns.

The first concem with the Besner et al. (1997) and Besner and Stolz (1999ah) rnethodologies

was their use of button pressing as a response mode. Though the present study also used button

press as a response mode (for consistency). I have reviewed two partial replications of Besner et

al. (1997) where the researchers used vocal response. In both Marmurek (1999) and MacLeod


(1999). the Besner et al. (1997) stimuli and methodology were used with vocal response; in both

of these studies, substantial Stroop interference was recorded in the single-letter-coloured

condition. Thus, it seems reasonable to conclude that the button press response was a key part of

the reason that Besner et al. (1997) observed a mere 30 ms of initial difference between

incongment colour words and control items - Stroop interference - when words appeared fully

coloured. Of course, smaller amounts of initial interference are also easier to elirninate.

The second main concern with the Besner et al. (1997) methodology revolves around the

appropriateness of their control stimuli. Although there is no universally agreed upon control

item for Stroop research - indeed. there could not be because controls always hinge on the

question being asked - I have suggested that the Besner et al. (1997) colour nonwords rnay be

inappropriate as they could readily elicit Stroop interference themselves. If the first letter (or

letters) of an incongruent colour word can elicit Stroop interference, then it would be expected

that the response time difference between incongment colour words and Besner-type colour

nonwords would be minimal because colour nonwords contain the first two letters of incongruent

colour words. This is indeed the case in the Besner et al. (1997) data, as there is only 30 ms in

response difference between incongment colour words and their controls when the words appear

fully coloured. Had different controls been used (rows of X's or other English words), then a

larger difference in response time would be expected between incongruent colour words and

controls appearing fully coloured.

There are a number of aspects of the present response time and error rate data that seem to

corroborate the daim that the first lener (or letters) of an incongnient colour word elicit Stroop

interference. First. in the present Experiment 1, there was no difference between incongnient

colour words and colour nonwords when the first or second letter appeared in colour. When the


third or fourth letters were coloured, however, significantly longer response times were observed

for incongment colour words relative to colour nonwords. No difference was recorded when the

fifth or sixth lener was coloured, but there may have been too few triais containing these letter

position to rnake useful conclusions. Nonetheless, the response time data for the first four letter

positions are consistent with what would be expected if the first letters of incongruent colour

words elicit Stroop interference. The first two letters of colour nonwords are identical to the first

two letters of incongment colour words and, therefore, this response time equivalence is to be

expected. The third letter of Besner-type colour nonwords, however. is the point at which these

words are confirmed not to be colour words and this is also the point where response times and.

therefore. Stroop interference. drop relative to incongruent colour words. Were colour nonwords

appropriate controls (Le.. they did not elicit Stroop interference themselves), response times

should be greater for incongruent colour words in every letter condition.

The second piece of evidence for colour nonwords eliciting Stroop interference is denved

€rom the pattern of error rate data as a function of the position of the coloured letter. The error

rate data are actually very similar to the just reviewed response time data. There was no

difference in error rates between incongrnent colour words and colour nonwords when either the

first or second letter was coloured. When the third letter appeared coloured. however.

substantially more errors were observed for incongruent colour words than for colour nonwords.

Again. this is the point at which colour nonwords are confinned not to be colour words and.

therefore. the point at which error rates should be expected to drop.

Taken together, the error rate and response tirne data as a fùnction of letter position seem to

corroborate the claim that Stroop interference is present in Besner et al.'s (1997) control

condition. It c m therefore be concluded that the equivaience in response time data in Besner et


al. (1997) Experiment 2 is not solely attributable to the colouring of a single letter. Rather, it

appears as though the initially srnall difference between fûlly coloured incongment colour words

and their controls is a function of bunon responding and the presence of Stroop interference in

the somewhat incongment control condition. It is reasonable to believe that colounng a single

letter reduced approximately 30 rns of Stroop interference From incongruent colour words, but it

surely cannot be claimed that simply colouring a single letter of a distractor eliminated Stroop

interference.

Thus, 1 am confident in concluding that whereas colounng a single letter of an irrelevant word

reduces Stroop interference, it is not the case that this colouring methodology elirninates Stroop

interference. Clearly, distractors are being read in the present study when they appear with a

single letter coloured. In fact, there does not seern to be any diflerence in the manner in which

single letter coloured words are processed relative to hl ly coloured words because Experiment 2

shows that recognition rates for these two word types are equal. The present error rate and

response time data are also indicative of problems in the basic Besner et al. (1997) methodology.

Clearly there is reason to believe that the response modality and control items used by Besner et

al. (1997) may be inappropriate, and shouid be replaced if one wishes to hnher examine the

effects of colouring a single letter of distractors. To this extent, the studies of both Marmurek

( 1999) and MacLeod ( 1999) are usehl in presenting a more complete story of how distractors

are processed when a single letter is coloured.

As has already been outlined, there is a findamental problem in interpreting a change in

amount of interference (a difference score) when there is also a change in the control condition

(against which the difference score is computed). Did interference decrease in the incongment

condition or did it increase in the control condition, or was there just some overall slowing in the

Reduced Stroop Inteference - 122

single-letter coloured condition, perhaps due to the necessity to search for the colour

information? This complicates interpretation yet also forms the basis for the two-process

explanation set out here. The obtained pattem of results in the present experiments (as well as in

the other previously reviewed replications of Besner et al.) are what would be expected under the

two-process account that has been outlined. In particular, the finding that the colouring of a

single letter leads to a response time increase in al1 conditions (colour nonwords, noncolour

English words, animal words, rows of X's) other than colour words seems to be strong evidence

that a second process is contnbuting to the colour word condition. This process likely

counteracts the increase in response time that would be expected due to extra search tirne. 1 am

therefore confident in concluding that this is the most appropriate explanation for the present

results, as well as the results of Besner et al. ( 1 997) and their successors.

Why Does Letter Position Not Matter?

Due to the work of Regan (1978) and Logan and Zbrodoff (1998). 1 had expected to find an

interestindintuitive pattem of response time and error rate results as a fùnction of the position of

the coloured letter. Specifically, 1 was expecting to observe substantial Stroop interference when

the first letter of an irrelevant colour word appeared coloured and then a systematic decrease

when the second through sixth letter positions were coloured. Though positional analyses were

worthwhile in demonstrating that the Besner et al. (1997) control items appear to elicit Stroop

interference, there were no other significant positional effects observed.

I outlined a view that is still highiy efficient, but does not lend itself to a predictable pattern of

results, seeing as this pattern would be a function of the random order in which each triai is

presented. Under this new view. participants do not retum their attention to the left side of the

screen when each new stimulus trial is encountered. rather their attention remains fixated on the


general area where the previous response colour was encountered. For example, imagine that on

the first trial the word GREEN appears with the fourth letter coloured. Initially, participants

attentionally scan across to the first letter so that a response may be made. Once the naming

response is made, however, participants remain attentionally fixated on the area where the fourth

letter of GREEN appeared. If the word that follows is either hlly coloured (and at least four

letters long), or also appears with the fourth Ietter coloured, then no additional search is needed

before a response can be made. Because half of the trials contain fuily coloured words.

participants will not to have to engage in any search at least 50 % of the time (and probably more

than this seeing as the same letter position might occasionally appear in colour on two

consecutive trials). Search will only be engaged when a word appears with a single letter

coloured that is not the fourth letter. Response time should then be a function of how far away

from the last coloured position the current coloured letter is.

Originaily, the goal of exarnininç Ietter position data was to examine the influence of the

position of the coloured letter solely due to the possibility that this was an integrai aspect of

response. Though the current pattern of results does not support this notion, it has not interfered

with formulating a reasonable explanation for the current results, as well as the results of Besner

et al. (1997). This explanation is independent of the position of the coloured letter and therefore,

it is reasonable to suggest that coloured letter position is not necessarily integral in colour

namng.

Where then does this leave the works of Regan (1978) and Logan and Zbrodoff (1998)?

These researchers have argued for the importance of the position of the coloured letter in Stroop

interference. In addition, Logan and Zbrodoff have argued that the first letter of an incongruent

colour word may be solely attributable for Stroop interference. Clearly the present research


demonstrates that Stroop interference can occur at other letter positions and, therefore, their

claim is too strong. There are, however, a number of aspects of the present data that suggest that

the first lener of incongruent colour words may play an important a role Stroop interference.

First, among al1 letten, the first letter is the position at which errors occur most fiequently.

Assurning that errors are the result of irnproper response to the distractor worddetter, the error

rate data seem to corroborate the claims of Logan and Zbrodoff A sirnilar pattern, though one

Iesser in magnitude, is observed when the second letter is coloured relative to al1 later colour

positions.

It is also the case that substantial Stroop interference is observed with regard to response

times when the first two letters of incongruent colour words are examined. Recall that error rates

and response times for words with the first or second letter coloured did not differ when 1

compared incongruent colour words to Besner et al.3 (1997) colour-like nonwords. When

comparing these two item types with the third letter coloured, however. there was a response

time and error rate advantage for the colour-like nonwords (meaning that response times are

faster and error rates lower for colour-like nonwords relative to incongment colour words). The

third letter is also the point at which colour-like nonwords are confirmed not to be incongruent

colour words and therefore it is intuitive that Stroop interference should be reduced here. Earlier

1 used this as support for the daim that the Besner et al. (1997) nonwords were an inappropriate

baseline as they too elicit Stroop interference. In the fiame of the current argument, however,

the present results can be taken as evidence that although the first letter of incongment colour

words does elicit Stroop interference, interference is not solely attributable to the first letter.

Clearly Stroop interference is observed at other letter positions, despite the fact that more of it is

observed for the first letter.


Recently in Our lab, MacLeod (2000) perforrned an expenment where Stroop interference was

cornpared for colour words, rows of X's, and rows of repeated first letters frorn words (e.g.

BBBB, GGGGG, YYYYYY). The prediction was that rows of repeated first letters might

actually elicit more S troop interference than incongruent colour words, if in fact Stroop

interference is due solely to the first letter. This, however, was not the case. Although more

Stroop interference was observed for rows of repeated letters relative to rows of X's. the most

Stroop interference was stiil observed for colour words. Incongrnent first letters did elicit a level

of Stroop interference but are clearly not solely responsible for Stroop interference given that

longer response times were recorded for colour words.

It may be that there is an individual letter component to Stroop interference but there is also

an effect of the entire word/semantics above this effect. Again, these findings do not add or

detract from the present results. Although the present studies have not found any interesting

pattern of results as a fùnction of the position of the coloured letter. it is also the case that they

have done nothing to detract from the work or theory of Regan (1978) or Logan and Zbrodoff

(1998). The full effect of the position of the coloured letter within an incongruent colour words

still remains an issue, despite being able to form an explanation for the current results that

operates independent of letter position.

Suggestions for Future Research

To test the possibility that the Besner et al. (1997) controls elicit Stroop interference, it would

also be interesting to perforrn a traditional Stroop task in which the Besner et al. (1997) colour

nonwords are used in place of incongruent colour words, with either the MacLeod (1 999) rows

of X's or the Mamurek (1999) animal words as controls. Due to the fact that the Besner et al.

(1997) controls are nonwords, they should not elicit any Stroop interference if they ostensibly


have no meaning attached to them. It has been shown, however, that even the first letter of an

incongruent colour word can produce Stroop interference (e.g., Regan, 1978; see MacLeod,

199 1, pp. 174- 175 for a review ). Because the colour-like nonwords contain the first two letters

of colour words, 1 have argued that these words rnay be eliciting Stroop interference.

Thus, it would be worthwhile to design a study where colour nonwords, animal names, and

rows of X's are a11 used as distractors. The pattern of results of such a study would demonstrate

whether Stroop interference was present for colour nonwords relative to these other control

types. Other English language words have been shown to elicit some Stroop interference (e-g..

Klein, 1964). though not as much as incongnient colour words. Rows of X's. it could be argued.

do not elicit any Stroop interference. though it has been argued (including by Besner et al..

1997). that they are inappropnate controls on the basis that they are unwordlike. Thus. one

might expect that other English language words should elicit more Stroop interference than the

colour nonwords, if the latter are indeed (rue controls with no meaning attached to thern. If these

colour nonwords do elicit Stroop interference. however. one might expect colour naming times

that are equal to or greater than those for other English language words.

The only probiem with this study is that actual colour words would no longer be present.

This is crucial as it may be the presence of colour words in the Besner et al. (1997) study that

would cause participants to view the colour nonwords in a meaningfil manner. On their own.

these nonwords may have little meaning, but when CO-occumng up with actual colour words. it

is easy to see how a comection may be made between these two types of stimuli. For example.

aside fiom the word RED. the nonword RET is very sirnilar to the words RAT. ROT. and PET.

arnong others. Thus, a second experiment (or a second block of trials) could be nin in which

rows of X's were replaced with colour words to investigate whether there is any difference in


colour narning times when colour words serve distractors, relative to colour-like nonwords or

animal words.

One additional way to investigate the manner in which distractor words are processed when a

single letter is coloured would be through the use of tMRI technology. Recently, some

researchers have begun to use technology in their Stroop studies to compare performance

on colour naming in the classic incongruent condition relative to a number of controi conditions

(see MacLeod & MacDonald, 2000, for a review). Such an expenment would be large in

magnitude as it would require a number of different comparison groups, or to have participants

perform a considerable number of tasks. The purpose of the experiment would be to examine

whether there is a difference in the processes that are evident when a single letter of a distractor

word is coloured relative to al1 letters being coloured. The design of the experiment would have

participants perfonn 4-8 tasks with the conditions as follows:

1) colour naming with a full row of asterisks coloured 2) colour naming with a single random asterisk coloured in a row of astensks 3 ) normal word reading (white pnnt on a black background) 4) word reading with the entire word coloured 5) word reading with a single letter of a word coloured (other letters appearing in white) 6) Colour narning with an entire distractor (X's, nonwords or animal words) coloured 7) Colour narning with a single letter of a distractor (X's, nonwords, or animal words)

coloured S) Same as 6 and 7 with colour words used as distractors

Essentially, these conditions comprise the experimental and practice stimuli used in the present

study. though an attempt has been made to present the tasks independently so that the processes

that drive them can be disassociated. This methodology would allow for an examination of the

different regions of the brain that are activated and, therefore, the different processes that would

be invoked dunng each task. Though the present results do not seem to indicate that differential

processing occurs when a single letter is coloured relative to al1 letters, fMRl data would provide


a more explicit examination of this. Do areas associated with reading show the same activation

in the single letter as in the fùlly coloured condition?

Based on the present analyses, a key point of interest would be the processes that are invoked

when participants read words intentionally rather than incidentally. Though £MRI would provide

funher evidence that distractors are being read automatically when a single letter is coloured. the

present data raise the issue of whether differential processes are used when a word is read

intentionally vs. incidentally (i.e.. automatically). tMRl would be a very usehl tool for testing

this possibility and may lead to a new line of research and. consequently, a new way of thinking

about how we process words that we encounter.

The Big Picture

The present thesis has provided sufficient evidence to dispute the claim that colounng a single

letter of an irrelevant word eliminates Stroop inteference. as put fonh by Besner et al. (1997).

Their claim that colouhg a single letter biocks the automatic reading (and therefore. the

semantic processing) of irrelevant words is placed in serious doubt by the results of the present

research. A number of aspects of the present results. as well as the results of partial replications

of Besner et al. (e.g., Marmurek, 1999; MacLeod, 1999) suggest that irrelevant words are. in

fact. read when they appear with a single letter coloured and that it was other methodological

aspects of Besner et al. (1 997) that led to their apparent elirnination of the Stroop effect. The

most convincing argument against this claim is the relatively strong recognition rates for words

appearing during a Stroop task. Although Besner et al.'s (1997) single letter coloured

manipulation definitely seems to reduce Stroop interference, it is clear that a different

explanation (other than that of elimination) is required.

To account for these results. a two-process explanation was introduced, which posits that the


colouring of a single letter forces participants to search for the desired response (display colour),

thus increasing search time. In the case of colour words, however, this search rnay also lead

participants to process irrelevant words in a less wordlike manner, thus leading to a decrease in

interference roughly equal to the increase in search tirne. Although the present experiments

provide considerable evidence that colouring a single letter of an irrelevant word leads to an

increase in response time relative to colouring al1 letters, it is admittedly difficult to show that

colouring a single letter leads to a reduction in interference when it is always accornpanied by

this increase in search time. Experiment 3 does, however, help to reinforce this claim. For the

present, it does seem reasonable to conclude that the single letter coloured manipulation does

lead to a decrease in Stroop interference given that response times rise for al1 distractor types

(cotour nonwords, animal words, noncolour words, rows of X's) other than colour words when a

single letter is coloured in the Stroop task.

Clearly. however, fbrther research is necessary to corroborate or disprove the two-process

account. Nonetheless. the present research restores the notion that certain automatic processes

(in this case, word reading) cannot be prevented from occumng in rnost circumstances.

Specifically, colouring a single letter does not prevent the individual from reading irrelevant

words. despite the fact that it may lead to these words being processed in a dif3erent manner.

This is not to say, however, that there are no situations under which automatic processes will be

prevented from occumng. As has been pointed out. automaticity is clearly contextual in nature.

The present research indicates that simply colouring a single letter of an irrelevant word.

however, is not one of the situations in which automatic processes are prevented. It is, therefore.

viable to conclude that Besner et al. (1997) and Besner and Stolz's (1999ah) claim that the

Stroop efYect can be eliminated and that existing accounts of automaticity are too strong is, in


itselc too strong.



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Appendix A: Word Lists For Experiments 2a and 2b

List One

act cut dog ear fu n hen ice kid lesi lot

mud net oil pad Pea Sun ta P tie van

List Two

cap day dot fan fog hat hog jar lap lie man mob oak owl Pet pie Pot sin top web

cake chip deck edge feet frog hand hill joke la ke mail nest pa* PumP seat shop star tank vine wave

aunt card dart dish farne food hair heat home king loc k mask mile moon nose play sait sin k time wine

chain class dance d raft fault flarne house kn ife laugh nerve month night patch phone queen shape smile thumb train watch

check chant ditch drain earth feast fruit horse judge light lunch mouse match paint peach plane score toast voie wtieel

cannon debate dinner hammer infant jacket kitten ladder lesson middle parcel pe b ble sister sorrow summer tem per tissue tunnel vesse1 walnut

attack candle dollar horror jungle kettle leader manner market matter needle office paddle pillar saddle spider supper tennis travel valley

Reduced S troop Interference - 1 3 8

Appendix B 1 : Participant Consent Form

1 have volunteered to take part in a study conducted by Mike Dodd in the Psychology Department, under the supervision of Dr. Colin MacLeod. 1 understand that my participation will involve attending on session of 30 minutes or less, the time to be arranged by mutual consent. 1 have been informed that for my participation in this experiment, I will receive '/r credit.

I understand that, in this experiment. 1 will be presented with a number of words and will be asked to either identiS, their colour of ink, or read them aloud (instructions will differ for different sections of the experiment). 1 have received instructions and have had the opportunity to ask questions and have had them answered before beginning. 1 understand. too, that I will be given some practice or farniliarization with the task before the actual expenment begins.

1 understand that the results of this study are not expected to be of direct benefit to me. It is hoped, however, that the information obtained will lead to a better understanding of colour naming. It has been made clear to me that, for the purposes of the experiment and in any subsequent reports, I will be identified by arbitrary subject number only.

I understand that following my participation I will be given a more detailed account as to the purpose of this study. I consent to participate in this project with the understanding that I may withdraw fieely and without consequence at any time.

Name Date Signature

Reduced Stroop Interference -1 39

Appendix 82: Participant Feedback Sheet

First and foremost. thank you very much for your participation in the present expenment. It has been very helpful to the research process.

The experiment that you just took part in has to do with a phenomenon known as the Stroop effect (which you have either heard of or will leam as pan of your Introductory Psychology class). The Stroop effect was discovered in 1935 and has received a great deal of attention fiom psychological researchers over the last half century. The basic idea behind the Stroop effect is that if 1 were to present you with a list of colour words and asked you to read them, you could do this quite easily. If. however. I gave you a list of colour words which 1 coloured in incongruent ink colours (e.g. the word RED in the colour blue) and asked you to identify these ink colours, you would have a difficuit time doing this. Even though you are supposed to name the ink colour and ignore the colour word. this is very difficult to do. In essence. the colour word is read "automatically" and it interferes with the colour naming response.

The Stroop effect has long been thought of as key evidence that when we encounter words. we automatically read them. Recently, however, a group of researchers have suggested that the Stroop effect can be eliminated by colounng a single letter of a colour word. rather than fûlly colounng the word. These researchers have then suggested that we do not necessarily read words automatically. and that theones of automaticity are too strong. We believe, however. that these researchers have not actually eliminated the Stroop effect so much as they have created the illusion of eliminating this effect through the use of questionable research rnatenals. These researchers have not demonstrated that colounng a single letter prevents people From automatically reading a word that is presented to them. Thus. the purpose of my research has been to more fully examine how people process words when they appear with only a single letter coloured. You have just participated in the 2nd and 3d portions of my research. The first ponion was a replication of a previous study and was completed last tem. Again. thank you very much for al1 your help.


Appendix C: Written On-Screen Instructions For Subjects In Experiment I

Thanks for helping us by taking part in this experiment. Please read the following instructions carefully so you know what you are going to do. Your task throughout this experiment is to identify the colours in which you see words and nonwords pnnted as fast as you can, avoiding rnistakes. To identify each item's colour, you should just press the button for that colour. Remember to go as quickly as you can without making mistakes. You will be identifjmg the colours of about 200 items; don't worry, it actually won't take very long. There are only four colours: red, blue, green, and yellow. Sometimes the whole item will be printed in colour. and sometimes only one letter will be printed in colour (in that case, the remaining letters will be pnnted in white). Here are two examples:

green yellow

You would respond by pressing the 'red' button to each of these."

The words will be the 4 colour words: red, blue, green, and yellow. There will also be four nonwords: ret, blat, grend, and yenile. Your job is to ignore the written item, whether it's a word or a nonword, and simply identify the colour in which it is printed as fast as you can. Please don't try to read the item; that will just slow you down. We want to see how good you are at ignoring the printed items. Before we begin the 200 items, there will be a short set of 12 practice items so you can get the 'feel' of the task. There will also be a short rest afler 100 items. As well. aeer you identify the colour of each item, you will see a row of stars (******) on the screen, to let you know that the next item will appear in about half a second. Dunng the experiment, please be sure to press only one button for each item. If you have any questions. please ask them now or during practice; we cannot intempt the actual experimental items.

Press the space bar to begin practice.

A i the corrcltrsioti of lhe practice session, the foilowirig t r x ~ apprmd:

Okay, that's how the experiment will work. The only difference now will be that there will be 200 items instead of 13. If you have any funher questions, please ask them now because 1 cannot answer them once the experiment has begun.

Press the space bar to begin this part of the experiment.


Appendix D: Wntten On-Screen Instructions For Subjects In Experiment 2a

Thanks for helping us by taking part in this experiment. Please read the following instructions carefblly so you know what you are going to do. Your task throughout this experiment is to identify the colours in which you see words printed as fast as you can, avoiding mistakes. To identify each item's colour, you should just press the button for that colour. Remember to go as quickly as you can without making mistakes. You will be identiwng the colours of about 30 items. There are only four colours: red, blue, yellow, and green. Sometimes the whole item will be printed in colour, and sometimes only one letter will be printed in colour (in that case, the remaining letters will be printed in white). Here are two examples:

i!iinh copper

You would respond by pressing the 'red' button to each of these.

Each trial will consist of a different English word. Your job is to ignore the written item. and sirnply name the colour in which it is printed as fast as you can. Please don't try to read the item: that will just slow you down. We want to see how good you are at ignoring the printed items. Before we begin the 30 items, there will be a short set of 12 practice items so you can get the 'feel' of the task. M e r you name the colour of each item. you will see a row of stars (* * * ** * ) on the screen to let you know that the next item will appear in about half a second. During the expenment, please be sure to press only one button for each item. If you have any questions, please ask them now or during practice; we cannot intempt the actual experimental items.

Press the space bar to begin practice.

At the cortclz~sion of the practice session, the foliowbg tex! appeared:

Okay, that's how the experiment will work. The only difference now is that there will be 30 items instead of 12. If you have any funher questions, please ask them now because 1 cannot answer them once the experiment has begun.

Press the space bar to begin this part of the experiment.

AI the conclzision of the Stroop rnsk, the followir~g text npprared:

You will now be presented with 64 words. some of which appeared dunng the colour narning task and others of which did not appear. Your job will be to identie the words that you DID see in colour (either al1 in colour by pressing the YES button. For words that you did NOT see in colour (new words), you should press the NO button. Even if you are not positive, but you think you saw a word in colour before, you should press the YES button. Remember to try to be as accurate as you can. If you have any further questions, please ask thern now because I cannot answer them once the experiment has begun. 1 will now show you which buttons to use for YES and NO.


Appendix E: Wntten On-Screen Instructions For Experhent 2b

Thanks for helping us by taking part in this experiment. Please read the following instructions carefully so you know what you are going to do. Your task throughout this experiment is simply to read the words which are presented to you as fast as you can. avoiding mistakes. To identiQ each item you should just read that word aloud into the microphone. Please read loud enough so that the microphone picks up your response. Remember to go as quickly as you can without making mistakes. You will be reading about 60 words. These words will appear in one of four colours: red, blue, yellow, or green. Sometimes the whole word will be printed in colour, and sometimes only one letter will be pnnted in colour (in that case, the remaining letters will be printed in white). Here are two examples:"

t hi tiL cop per

Your task is to read the word out loud as quickly as you can. Please ignore the colour and simply read the word aloud.

Before we begin the 60 items, there will be a shon set of 12 practice items so you can get the 'feel' of the task. After you read each item, you will see a row of stars (******) on the screen to let you know that the next item will appear in about half a second. If you have any questions. please ask them now or during practice; we cannot intempt the actual experimental items.

At the concli<siotr of the practice session, the foi~owitzg trxt appemed:

Okay, that's how the experiment will work. The only difference now is that there will be 60 items instead of 12. If you have any further questions, please ask them now because 1 cannot answer them once the experirnent has begun.


Appendix F: Wntten On-Screen Instmctions For Expenment 3

Thanks for helping us by taking part in this experiment. Please read the following instructions carefully so you know what you are going to do. Your task throughout this experiment is to identifL the colours in which you see rows of asterisks printed as fast as you can, avoiding rnistakes. To identify each item's colour, you should just press the bunon for that colour. Remember to go as quickly as you can without making rnistakes. You will be identi*ng the colours of about 60 items. There are only four colours: red, blue, yellow. and green. Sometimes the whole item will be printed in colour, and sometimes only one astensk will be pnnted in colour (in that case, the remaining asterisks will be printed in white). Here are two examp les:

You would respond by pressing the 'red' button to each of these.

Here are the buttons that you will be using to respond to the colour of the pnnted items:

If you see the colour red. please press z on the keyboard If you see the colour blue. please press x on the keyboard If you see the colour yellow, please press . on the keyboard If you see the colour green, please press I on the keyboard

Please place one hand on each side of the keyboard so that you may use your lefi hand (normally your rniddle and index fingers) to press the "z" and "x" keys, and your right hand (again. your index and middle fingers) to press the "." and "/" keys. While volunteers usually use their index and rniddle fingers, feel fiee to use your middle and ring fingers if you are more comfonable doing so. Before we begin the 60 items, there will be a short set of 24 practice items so you can get the 'feel' of the task. M e r you name the colour of each item, you will see a row of dashes (-- ----) on the screen to let you know that the next item will appear in about half a second.

Dunng the experiment, please be sure to press only one button for each item. If you have any questions, please ask them now or during practice; we cannot intempt the actual experiment al items.

A t the cor~clicsio~~ of the practice sesszor~, the foilowb~g text apprared:

Okay, that's how the experiment will work. The only difference now is that there will be 60 items instead of 24. If you have any further questions, please ask them now because we cannot answer them once the expenment has begun.


Appendix G: Summary Table for a 2 x 2 Within Subjects ANOVA With Word Type (W) and Colouring Type (C) as the Independent Variables and Colour Naming Times as the Dependent Variable

Source df MS F

W 1

Error 34

C 1

Error 34

w x c 1

Error 34

Note: * denotes p < .10; ** denotes p 5 .OS; ***denotes p 5 .O 1


Appendix H: Surnmary Table for a 2 x 2 Within Subjects ANOVA With Word Type (W) and Colouring Type (C) as the Independent Variables and Error Rates as the Dependent Variable

Source d f MS

W 1 0.000 1 O. 135

Error 34 0.0009

C 1 0.00 18 3.406"

Error 34 0.0005

W x C 1 0.0008 1 .O63

Error 34 0.0007

Note: * denotes p < .IO; ** denotes p 5 .05; ***denotes p < .O 1


Appendix 1 I : Summary Table for a One Way Within Subjects ANOVA With Letter Position as the Independent Variable and Colour Naming Times as the Dependent Variable (For Control Words Only).

Source SS d f MS F

Letter 547 14.6 1 5 10952.92

Error 13 70665 170 8062.73

Total 1425379.6 1 175

Note: * denotes p ( .10; ** denotes p 5 .05; ***denotes p 5 . O 1


Appendix 12: Summary Table for a One Way Within Subjects ANOVA With Letter Position as the Independent Variable and Colour Naming Times as the Dependent Variable (For Incongment Colour Words Only).

Source SS d f MS F

Total 1917673.59 175

Note: * denotes p < .IO; ** denotes p 5 .OS; ***denotes p 5 -01


Appendix J I : Surnmary Table for a One Way Within Subjects ANOVA With Letter Position as the Independent Variable and Error Rates as the Dependent Variable (For Incongruent Colour Words Only).

Source SS d f MS

Letter 0.0557 5 0.01 1 1

Error 1.2000 170 0.0070

Total 1.2557 175

Note: * denotes p 5 .IO; ** denotes p < .05; ***denotes p 5 .O 1


Appendix J2: Sumrnary Table for a One Way Within Subjects ANOVA With Letter Position as the lndependent Variable and Error Rates as the Dependent Variable (For Control Words Only).

Source SS d f MS F

Letter

Error

Total

Note: * denotes p 5 -10; ** denotes p 5 .05; ***denotes p 5 .O 1


Appendix 53: Summary Table for a One Way Within Subjects ANOVA With Word Type as the Independent Variable and Error Rates as the Dependent Variable (For Distractors With the First Letter Coloured Only).

Source SS df MS

Word 0.000 1 1 0.000 1

Error O. 1770 34 0.0052

Total 0.1771 35

Note: * denotes p 5 .IO; ** denotes p < .Os; ***denotes p 5 .O 1


Appendix J4: Summary Table for a One Way Within Subjects ANOVA With Word Type as the Independent Variable and Error Rates as the Dependent Variable (For Distractors With the Second Letter Coloured Only).

Source SS d f MS

Word 0.001 1 1 0.001 1

Error O. 1530 34 0.0045

Tot al O. 153 1 35

Note: * denotes p 5.10; ** denotes p < .05; ***denotes p < .O1


Appendix 55: Summary Table for a One Way Within Subjects ANOVA With Word Type as the Independent Variable and Error Rates as the Dependent Variable (For Distractors With the Third Letter Coloured Only).

Source SS d f MS F

Word O. 0266 1 0.0266 8.226***

Error O, 1 IO0 34 0.0324

Total O. 1365 35

Note: * denoies p 5 .IO; ** denotes p 5 .OS; ***denotes p < -01


Appendix 16: Surnrnary Table for a One Way Within Subjects ANOVA With Word Type as the Independent Variable and Error Rates as the Dependent Variable (For Words With the Fourth Letter Coloured Only).

Source SS d f MS F

Word 0.00 17 1 0.00 17 0.380

Error O. 1420 34 O. 0042

Total O. 1437 35

Note: * denotes p 5 .IO: ** denotes p 5 -05; ***denotes p 5 .O1


-4ppendix 57: Surnrnary Table for a One Way Within Subjects ANOVA With Word Type as the Independent Variable and Error Rates as the Dependent Variable (For Distractors With the Fifth Letter Coloured Only).

Source SS d f MS F

Word 0.0022 1 0.0022 O. 195

Error 0.3980 34 0.01 17

Total 0.4002 35

Note: * denotes p 5 .10; ** denotes p < .05; ***denotes p 5 .O 1


Appendix 58: Sumrnary Table for a One Way Within Subjects ANOVA With Word Type as the Independent Variable and Error Rates as the Dependent Variable (For Distractors With the Sixth Letter Coloured Only).

Source SS d f MS F

Word 0.0036 1 0.0036 1 .O00

Error O. 12 10 34 0.0036

Totai O. 1246 35

Note: * denotes p 5.10; ** denotes p 5 .05; ***denotes p 5 -01


Appendix K1: Summary Table for a One Way Within Subjects ANOVA With Word Type as the Independent Variable and Colour Narning Times as the Dependent Variable Fo r Words With the First Letter Coloured Ody).

Source SS d f MS F

Word 21.73 I 21.73 O. 004

Error 206670.8 34 6078.55

Total 206692.53 35

Note: * denotes p 5 .IO; ** denotes p < .05; ***denotes p < .O 1


Appendix K2: Sumrnary Table for a One Way Within Subjects ANOVA With Word Type as the Independent Variable and Colour Narning Times as the Dependent Variable (For Words With the Second Letter Coloured Only).

Source SS d f MS F

Word 2366.4 1 1 2366.4 1 0.35

E rro r 332942.1 34 685 1.24

Tot al 235308.94 35

Note: * denores p 5 .IO; ** denotes p < .05; ***denotes p < .O I


Appendix K3 : Summary Table for a One Way Within Subjects ANOVA With Word Type as the Independent Variable and Colour Naming Times as the Dependent Variable (For Words With the Third Letter Coloured Only).

Source SS d f MS

Word 46853. 16 1 46853.16

Error 149653 . 3 34 440 1.57

Total 196506.46 35

Note: * denotes p 5 -10; ** denotes p < .05; ***denotes p < .O 1


Appendix K4: Summary Table for a One Way Within Subjects ANOVA With Word Type as the Independent Variable and Colour Naming Times as the Dependent Variable (For Words With the Fourth Letter Coloured Only).

Source SS d f MS F

Word 17569.73 1 17569.73 3.46*

Total 190237.53 35

Note: * denotes p 5 .10; ** denotes p 5 .05; ***denotes p 5 .O1


Appendix K5: Summary Table for a One Way Within Subjects ANOVA With Word Type as the Independent Variable and Colour Naming Times as the Dependent Variable (For Words With the FiRh Letter Coloured Only).

Source SS d f MS F

Word 1536.9 1 1 1536.9 1 . 1 3

Error 404283.1 34 1 1890.68

Total 405820.0 1 35

- -

Note: * denotes p Ç .IO; ** denotes p < -05; ***denotes p 5 -01


Appendix K6: Summary Table for a One Way Within Subjects ANOVA With Word Type as the Independent Variable and Colour Narning Times as the Dependent Variable (For Words With the Sixth Letter Coloured Only).

Source SS d f MS

Word 10.41 1 10.41

Enor 846905.1 34 24908.97

Total 846915.5 1 35

Note: * denotes p 5.10; *+ denotes p 5 -05; ***denotes p < .O1


Appendix L: Summary Table for a One Way Within Subjects ANOVA With Colouring Type (Al1 Letters or One Letter) as the Independent Variable and Colour Naming Times as the Dependent Variable .

Source SS d f MS F

Colour 28087.12 1 28087.12 9.494***

Enor 97623.88 33 2958.30

Total 12571 1 34

Note: * denotes p 5 -10: ** denotes p 5 .05: ** *denotes p 5 .O 1


Appendix M: Summary Table for a One Way Within Subjects N O V A With Letter Position as the Independent Variable and Colour Naming Times as the Dependent Variable.

Source SS d f MS F

Letter 282423.2 5 56484.63 2.173*

Total 457058 1.3 170

Note: * denotes p 5.10; * * denotes p 5 .05; **+denotes p 5 -01


Appendix N: Summary Table for a One Way Within Subjects ANOVA With Colounng Type as the Independent Variable and Error Rates as the Dependent Variable .

Source SS df MS F

Colour 0.000 1 1 0.000 1 0.033

Error 0.060 1 33 0.00 18

Total 0.0602 34

Note: * denotes p 5 .10; ** denotes p 5 .OS; ***denotes p < .O1


Appendix O: Summary Table for a One Way Within Subjects ANOVA With Letter Position as the Independent Variable and Error Rates as the Dependent Variable

Source SS d f MS F

Letter

Error

Total

Note: * denotes p 5-10; ** denotes p 5 .05; ***denotes p < .O1


Appendix RI: Summary Table for a One Way Within Subjects ANOVA With Word Type (Fully Coloured vs. Single-Letter-Coloured vs. Not Presented) as the Independent Variable and Recognition Rates as the Dependent

Variable.

Source SS d f MS F

Word 1 S860 2 0.7930

Error 0.9750 66 0.0 148

Total 2.5610 68

&te: * denotes p < .lO; ** denotes p 5 -05: ***denotes p 5 .O1


Appendix R2: Summary Table for a One Way Within Subjects ANOVA With Letter Position as the Independent Variable and Recognition Rates as the Dependent Variable.

Source SS d f MS F

Letter 0.6 180 5 O. 1240 I .O36

Note: * denotes p 5 .IO; ** denotes p 5 -05; ***denotes p 5 .O1


Appendix S 1: Summary Table for a One Way Within Subjects ANOVA With Response Type (Fully Coloured Hits vs. Single-Letter-Coloured Hits vs. Unstudied False Aiarms) as the Independent Variable and Recognition Response Times as the Dependent Variable.

Source SS d f MS F

Word 407 196 2 203597.8 1.45

Error 92672 12 66 1404 12.3

Total 9674408 68

Note: * denotes p 5 -10; ** denotes p < .05; ***denotes p 5 .O1


Appendix S2: Summary Table for a One Way Within Subjects ANOVA With Letter Position as the Independent Variable and Recognition Response Times as the Dependent Variable.

Source SS d f MS

Letter 1661074 5 3322 14.8

Error 4530202 1 165 274557.7

Total 46963095 170

Note: * denotes p < - 1 O: * * denotes p 5 .05; ** *denotes p < .O 1


Appendix T: Summary Table for a One Way Within Subjects ANOVA With Letter Position as the Independent Variable and Recognition Response Times as the Dependent Variable.

Source SS d f MS F

Letter 1662943 5 332588.7 1.21

Error 45780190.5 165 274557.7

Total 47443 133.5 170

Note: * denotes p 5-10; ** denotes p < .OS; ***denotes p 5 .O1


Appendix U: Summary Table for a One Way Within Subjects ANOVA With Colouring Type as the Independent Variable and Word Reading Times as the Dependent Variable.

Source SS d f MS F - -

Colour 1 169.47 1 1 169.47 2.767 (!)

Error 13945.53 24 422.59

'Total 151 15 25

Note: * denotes p 5 -10; ** denotes p < .05; ***denotes p < .O 1; ! actual p = . IO6

Reduced S troop Interference - 1 72

Appendix V: Summary Table for a One Way Within Subjects ANOVA With Letter Position as the Independent Variable and Word Reading Times as the Dependent Variable.

Source SS d f MS F

Letter 20693.80 5 4138.76 3 . IO***

Error 220663.9 165 1337.36

Total 241357.7 170

Note: * denotes p 5.10; ** denotes p < .05; ***denotes p 5 .O1


Appendix W 1 : Summary Table for a One Way Within Subjects ANOVA With Colounng Type as the Independent Variable and Colour Naming Times as the Dependent Variable (For Rows of Astensks).

Source SS d f MS F

Colour 4723,92 1 4723.92 7.9 1 ***

Enor 14348.48 24 597.84

Total 19072.4 25

Note: * denotes p 5 -10; ** denotes p 5 .05; ***denotes p < .O1


Appendix W2: Surnmary Table for a One Way Within Subjects ANOVA With Letter Position as the Independent Variable and Colour Naming Times as the Dependent Variable (For Rows of Astensks).

Source SS d f MS F

Letter 59406.3 8 5 11881.28 1.65

Total 921752.88 125

Note: * denotes p < -10: ** denotes p 5 -05; ***denotes p 5 .O1


Appendix XI : Summary Table for a One Way Within Subjects ANOVA With Colouring Type as the Independent Variable and Error Rates as the Dependent Variable (For Rows of Asterisks).

Source SS df MS F

Colour O. O007 1 0.0007

Error 0.0 138 24 0.0006

Total 0.0 145 25

Note: * denotes p 5 -10; ** denotes p 5 .OS: ***denotes p < 0 1

Reduced S troop Interference - 1 76

Appendix X2: Summary Table for a One Way Within Subjects ANOVA With Letter Position as the Independent Variable and Error Rates as the Dependent VanabIe (For Rows of Asterisks).

Source SS d f MS

Letter 0.0 167 5 0.0033

Error 0.3440 120 0.0029

Total 0.3607 125

Note: * denotes p < .IO; ** denotes p 5 .OS; ***denotes p < -01

Applicants for admission for Sept. 2001 (as of January 30.2001)

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Anderson, Rheanne Arsalidou, Marie Bayrami, Lisa Bebiroglu. Neda Bender, Danielle Bharadia, Vinay Birze. Arija Borch, Shannon Burney. Atiya Campos. Jennifer Cantanese, Dana Catanzaro. Anna Maria Chan, Kimberley Chard, Diana Chukwuemeka-aju ba, Gregory Coleman, Julie Cornier, Natalie Craig, Kristin D'Angelo. Francesca Duval. Cara Dyke, Danielle Eliav, Jasmine Fahy, Louise Favor, Stacey Fung, Francis Glenn. Julie Grant, Sabrina Harrison, Jererny Hemog, Ludmila Hitzig, Sandra Hopyan. Talar Huizinga. Carolyn lshmael. Tracy Jeansson. Vivian Kalpakis, Elaine-Marie Keay, Pamela Kemmotsu. Nobuko Kendall, Julia Knight. Melanie Krete, Oana Launeanu, Mihaela Leal, Elizabeth Loumbeva. Nadejda Luo, Lin MacDonald, Heather Mahler. Stephen Martin, Nicholas McCou brey. Gail McGillivray, Shannon McHugh, Tara Mclsaac. Caroline Mohebat. Emis Moradi. Farshad Niiya, Yu O'Connor. Charlene Orprecio. Jazmine Saelhof, Michael Sarkar, Christopher Semenya. Antoinette Sethian, Taline Shen. Ching-Ting Shen Sheppard. Michael Smith, Kathleen Soule. Michelle Spence, Men's Steinberg, Daniel Ta-Min, Rachelle TaIrni, Deborah Taylor. Chnstopher Telner, Jason Tirtabudi, Prisilia Tosco. Anna Maria Trottier. Kathryn

Devel PICICN Devel Devel SPA B&B SPA SPA Devel 688 SPA Devel 888 SPA B&B SPA SPA SPA PICICN SPA Devel Devel Devel SPA Devel Devel SPA SPA SPA Devel Ba8 SPA SPA PICICN Devel SPA SPA SPA SPA SPA Devel SPA SPA PICICN B&B SPA SPA PICICN SPA 888 SPA B&B PlCICN SPA B&B PICICN SPA SPA SPA SPA SPA SPA Devel Devel SPA SPA PICICN PICICN PICICN PICICN PICICN SPA SPA

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PIClCN B&B Devel SPA SPA

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888 SPA

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Applicants for admission for Sept. 2001 (as of January 30.2001) dlarm lsuhuh aîc&Q

Vasilescu, Maria-Cristina Velasquez, Ana Vishram, Julie Wager. Brandon Walsh. Andrew Wareham, Stacey Weir, Kelly A. Wells, Elizabeth Np, Wan Hung Yuval. Linda

B&B Devel 888 6&B P/C/CN SPA SPA SPA PICICN SPA

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