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Laboratory of Experimental PsychologyKULeuven - Campus Kortrijk
General doctoral seminar 16/04/2008
The mechanisms of subliminal stimuli: A meta-analysis and new experimentsEva Van den BusscheBert Reynvoet & Wim Van den Noortgate
2
Overview• PART I: the moderators of masked semantic priming
– Study 1: Masked priming effects in semantic categorization are independent of category size
– Study 2: Masked semantic priming: a picture prime study– Study 3: Mechanisms of masked semantic priming: a meta-
analysis • PART II: conscious versus unconscious processing
– Study 4: Conscious and unconscious proportion effects in masked priming
– Study 5: The interaction between consciousness and attention: an empirical study using the priming paradigm
• General conclusions
3
Part I• PART I: the moderators of masked semantic
priming– Introduction– Study 1: Masked priming effects in semantic
categorization are independent of category size – Study 2: Masked semantic priming: a picture prime
study– Study 3: Mechanisms of masked semantic priming: a
meta-analysis– Conclusions
4
Introduction• Can unconsciously presented
information influence our behaviour?Subliminal advertising:– “Drink Coca-cola, Eat popcorn” James
Vicary (1957)– McDonald’s
5
Introduction• Can unconsciously presented information
influence our behaviour?Masked priming paradigm (Marcel, 1983)
– Masked/subliminal/unconscious semantic priming~> an unidentifiable visually masked stimulus (prime) facilitates the subsequent semantic classification of a visible related stimulus (target)
– Priming effect ~> improved responding when primes are congruent/related to targets, relative to when prime-target pairs are incongruent/unrelated
6
Introduction• Example: is target smaller or larger than 5?
Congruent/related trial
Incongruent/unrelated trial
Target 1 9
Prime 4 4
Both < 5Faster RTs
One <, one > 5Slower RTs
7
Introduction• Can unconsciously presented
information influence our behaviour? – Masked priming paradigm (Marcel, 1983)– Mid-1990s: subliminal perception exists!
YES!• BUT…
8
Introduction• But…• How deep can subliminal stimuli be
processed? – Dehaene et al. (1998): semantically
processed semantic account
9
Introduction• Semantic account, congruent trial:
Smaller than 5!
Response = press left
Congruent faster RT!
Prime4
Target1 Smaller than 5!
Response = press left
10
Introduction• Semantic account, incongruent trial:
Smaller than 5!
Response = press left
Incongruent slower RT!
Prime4
Target9 Larger than 5!
Response = press right
11
Introduction• How deep can subliminal stimuli be
processed? – Dehaene et al. (1998): semantic account– Damian (2001): NOT semantically
processed, but due to S-R mappings S-R account
12
Introduction• S-R account, repeated primes, congruent trial:
Smaller than 5!
Response = press left
Congruent faster RT!
Prime4
Target1
Response = press left
13
Introduction• S-R account, repeated primes, incongruent trial:
Response = press left
Incongruent slower RT!
Prime4
Target9
Response = press RIGHT
14
Introduction• S-R account, novel primes:
Response = press left
Congruent faster RT!
Prime3
Target1
Response = press left
15
Introduction• How deep can subliminal stimuli be
processed? – Dehaene et al. (1998): semantic account– Damian (2001): S-R account– Recent findings: significant priming for
novel primes semantically processed!– Kunde et al. (2003): NOT semantically
processed, but due to action triggers action trigger account
16
Introduction• Action-trigger account, congruent trial:
Congruent faster RT
Response = press left
Target1
Response = press left
Task: categorize numbers
between 1 and 9 as smaller or larger than 5
Action triggers for: 1,2,3,4 = press left6,7,8,9 =press right
Prime3
17
Introduction• Action-trigger account, incongruent trial:
Incongruent slower RT
Response = press left
Target9
Response = press RIGHT
Task: categorize numbers
between 1 and 9 as smaller or larger than 5
Action triggers for: 1,2,3,4 = press left6,7,8,9 =press right
Prime3
18
Introduction• Action-trigger account, large categories:
Congruent faster RT
Response = press left
Targetcat
Response = press left
Task: categorize words
as animals or objects
Action triggers for: Dog, cat,…= press leftBall, pot,…= press right
Primecow
19
Introduction• How deep can subliminal stimuli be
processed? – Dehaene et al. (1998): semantic account– Damian (2001): S-R account– Kunde et al. (2003): action trigger account– Recent findings: significant priming for
large categories and large target sets semantically processed!
20
Introduction• How deep can subliminal stimuli be
processed? – ???– Contradictory research findings– Diverging theories
21
Introduction• Which factors moderate masked priming
effects?- Task- Prime novelty- Category size- Target set size- Nature of primes and targets- Prime duration and SOA- Masking- Prime visibility- Sample size- Population
22
Aims1. Assess whether subliminal stimuli are
semantically processed
2. Assess the influence of potential moderators on subliminal semantic priming effects
23
Study 1
Study 1: Masked priming effects in semantic categorization are independent of category size
Van den Bussche & Reynvoet, 2007
24
Introduction1. Semantic account: priming effects, no matter
what2. S-R account: priming effects only for repeated
primes and small target sets3. Action trigger account: priming effects only for
small categories and small target setsGoal = Clarify the role of category size and target set size on subliminal priming effects using novel primes
25
Design
Larger than dog
Fig. 2
Incongruent trial
LION
######
mouse
######
17ms
33ms
480ms
time
Smaller than dog
Fig. 1
Congruent trial
LION
######
horse
######
17ms
33ms
480ms
time
Larger than dogSmaller than dog
26
Design• In all 3 experiments, targets never
appeared as primes (novel primes)• Prime visibility was always objectively
measured “apply same task to prime”d’ measures never differed from 0!
27
Design• Experiment 1(N = 16): 3 separate parts:
– 1a: categorize numbers as smaller or larger than 5 (small category, small target set)
– 1b: categorize body parts as below or above the pelvis (small category, small target set)
– 1c: categorize animals as smaller or larger than a dog (large category, small target set)
• Experiment 2 (N = 14): categorize words as animals or objects (large category, large target set)
• Experiment 3 (N = 16): categorize words as animals or not (large category, large target set)
28
Results
440
460
480
500
520
540
560
580
Experiment 1a Experiment 1b Experiment 1c
RT
(ms) Congruent
Incongruent
Fig. 3
20 ms, p < .001
25 ms, p < .001
8 ms, p < .05
Experiment 1
29
Results
520
530
540
550
560
570
580
590
600
Experiment 2: animal targets Experiment 2: object targets
RT
(ms) Congruent
Incongruent
520
530
540
550
560
570
580
590
600
610
Experiment 3: animal targets Experiment 3: non-animal targets
RT
(ms) Congruent
Incongruent
Fig. 5
Experiment 3
11 ms,p < .01
Fig. 4
Experiment 2
15 ms,p < .001
30
Conclusions• Significant priming effects for novel
primes, across different tasks, irrespective of category size and target set size!
• Evidence in favor of the semantic account ~> subliminal primes are semantically processed!
31
Study 2
Study 2: Masked semantic priming: a picture prime study
Van den Bussche, Notebaert & Reynvoet, in preparation
32
Introduction• Abrams (in press) priming effects of
Van den Bussche & Reynvoet (2007) did not stem from semantic processing of the primes, but from subword processing, since substantial orthographic overlap between primes and targets was present (e.g. been – teen, kat – rat)
33
Design• N = 21• Task: categorize words as animals or not
(large category, large target set)• To eliminate overlap: use picture primes
and word targets!
34
Design
Fig. 6
Congruent trial
time
AnimalNo animal
400ms +######
horse
######
LION
4x13ms
13ms
26ms
200ms
4x13ms
35
Design
Fig. 7
Incongruent trial
time
AnimalNo animal
400ms +######
horse4x13ms
13ms
26ms
200ms
4x13ms ######
LION
36
Results and conclusions• Significant priming effect of 9 ms (F(1,
20) = 15.65, p = .001)Even when overlap is eliminated, significant priming was still observed for a large category and large target setEvidence in favor of the semantic account ~> subliminal primes are semantically processed!
37
Study 3
Study 3: Mechanisms of masked semantic priming: a meta-analysis
Van den Bussche, Van den Noortgate & Reynvoet, submitted
38
AimsStatistically combine published and unpublished data using meta-analytic techniques to:
1. Assess whether the literature to date provides clear evidence in favor of semantic processing of subliminal information.
2. Assess the influence of potential moderatorson subliminal semantic priming effects
39
Moderators• Which factors moderate masked priming
effects?- Task- Prime novelty- Category size- Target set size- Nature of primes and targets- Prime duration and SOA- Masking- Prime visibility- Sample size- Population
40
Method• Search criteria:
– Published between 1998 – 2006– Language: English, Dutch, French or German– Unconscious semantic priming
• Search string:– (SEMANTIC OR ASSOCIATIVE) AND
(PRIMING OR PRIME) AND (MASKED OR SUBLIMINAL OR UNCONSCIOUS OR AUTOMATIC)
41
Method• Four databases were searched
– Web of Science– ScienceDirect– PubMed– PsycInfo
• Cross-references, reviews, experts• 477 published articles were selected• Two independent reviewers coded these
articles
42
Method• Inclusion criteria:
1. Prime-target relation of a semantic nature in the visual domain (e.g. cat - dog)
2. Primes had to be presented subliminally3. Semantic categorization, lexical decision or naming
task4. Standard priming procedure 5. Centrally presented single word or symbol primes 6. Healthy sample 7. Sufficient statistical information to compute an effect
size
43
Method• 37 published and 8 unpublished studies
containing 135 separate experiments / conditions
• Effect sizes were computed:
RT (unrelated trials) – RT (related trials)ES =
SD– ES = 0: no effect– ES > 0: positive ES (~ priming effect)
44
Method
• 3 standard tasks• 2 separate meta-analyses for:
– Conditions using a semantic categorization task
– Conditions using a lexical decision or naming task
45
Introduction• Semantic categorization
congruent trial: incongruent trial:
Animal!
Response = press left
Primecat
Targetcow Animal!
Response = press left
Animal!
Response = press left
Primecat
Targettable Object!
Response = press RIGHT
Congr faster RT
Incongrslower RT
Congr faster RT
Incongr slower RT
More relatedfaster RT
Less relatedslower RT
46
Introduction• Lexical decision
related trial: unrelated trial:
Word!
Response = press left
Primecat
Targetcow Word!
Response = press left
Word!
Response = press left
Primecat
Targettable Word!
Response = press left
= =
= =
More relatedfaster RT
Less relatedslower RT
47
MethodImportant discrepancy in processing of congruent/incongruent trials! Furthermore, different set of moderators2 separate meta-analyses for:
– Conditions using a semantic categorization task
– Conditions using a lexical decision or naming task
48
Meta-analysis 1
Meta-analysis 1:Semantic categorization conditions
49
MA1: Descriptives22 studies containing 87 conditions (k = 87)
PopulationStudents: k = 62, Adults: k = 25
Prime duration M = 42, SD = 70.7, range = 17-72
TargetsSymbols: k = 34, Words: k = 37, Both: k = 16
Obj/SubjObj: k = 56, Subj: k = 10, Both: k = 7 (k = 73)
SOA M = 96, SD = 35.0, range = 41-273
MaskingBM and FM: k = 70, BM: k = 1, FM: k = 16
No: k = 14, Yes: k = 73
M = 0.19, SD = 0.20, range = -0.06-0.66 (k = 58)
Visibility measured
d’
N M = 20, SD = 12.1, range = 6-80
PrimesSymbols: k = 32, Words: k = 37, Both: k = 18
NoveltyRepeated: k = 40, Novel: k= 43, Both: k = 4
Category size Small: k = 44, Large: k = 43
Target set size M = 19, SD = 22.8, range = 4-90
50
MA1: Effect sizes
Fig. 8
Observed effect sizes
-1,5
-0,5
0,5
1,5
2,5
3,5
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88
Experiment
ES
51
MA1: Empty model• Overall mean ES = 0.79 (SE = 0.10)• Significant differences between studies• Significant differences between
conditions, within studiesLook for moderators!
52
MA1: Main effects (1)• Prime novelty (F(2,65.8) = 14.21, p < .0001)
Fig. 9
Observed effect sizes for novel and repeated primes
-1,5
-0,5
0,5
1,5
2,5
3,5
Repeated primes Novel primes Both
Effe
ct S
izes
ES = 1.07, p < .0001
ES = 0.70, p = .006
ES = 0.54, p < .0001
53
MA1: Main effects (2)• Category size (F(1,14.8) = 47.63, p < .0001)
Fig. 10
Observed effect sizes for small and large categories
-1,5
-0,5
0,5
1,5
2,5
3,5
Small category Large category
Effe
ct S
izes
ES = 0.35, p = .0003
ES = 1.08, p < .0001
54
MA1: Main effects (3)
• d’ (β = 1.04, F(1,54.1) = 8.09, p = .006)• Positive slope higher visibility ~ higher
ES• BUT: at intercept (where d’ = 0), still
significant ES (β = 0.44, t(14.3) = 3.73, p= .002)
55
MA1: Interaction effects and multiple models
• 2-way interaction effects: none were significant (p-values ranging from .20 to .93)
• Add several main effects in one model • Using several strategies, the same optimal
model always emerged: a model containing prime novelty and category size (adding d’slightly improved the model)
56
MA1: Publication bias• Meta-analysis: published AND unpublished data!
Fig. 11
0
20
40
60
80
-1 -0,5 0 0,5 1 1,5 2 2,5 3
Observed effect sizes
Sam
ple
size
57
MA1: Conclusions
Less priming for novel primes, large categories and completely invisible primesStill, significant priming was always observed, even under circumstances where S-R accounts are less likely (for novel primes and large categories)Subliminal primes can be processed semantically
58
Meta-analysis 2
Meta-analysis 2:Lexical decision and naming
conditions
59
MA2: Descriptives24 studies containing 48 conditions (k = 48)
PopulationStudents: k = 38, Adults: k = 10
Task Lexical decision: k = 39, Naming: k = 9
TargetsSymbols: k = 9, Words: k = 39
Visibility measured No: k = 21, Yes: k = 27
d’ M = 0.05, SD = 0.04, range = -0.01-0.08 (k = 4)
Prime duration M = 51, SD = 17.5, range = 13-84
SOA M = 91, SD = 67.0, range = 33-340
BM and FM: k = 26, BM: k = 5, FM: k = 13, none: k = 4
Obj: k = 19, Subj: k = 6, Both: k = 2 (k = 27)
Masking
Obj/Subj
N M = 37, SD = 29.1, range = 11-132
PrimesSymbols: k = 2, Words: k = 46
NoveltyRepeated: k = 2, Novel: k= 46
Category sizeSmall: k = 2, Large: k = 38 (k = 40)
Target set size M = 117, SD = 109.1, range = 6-320 (k = 40)
60
MA2: Effect sizes
Fig. 12
Observed effect sizes
-1
0
1
2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
Experiment
ES
61
MA2: Empty model• Overall mean ES = 0.46 (SE = 0.06)• Significant differences between studies• NO significant differences between
conditions, within studiesLook for moderators!4 variables could not be included (nature of primes, prime novelty, category size and d’)
62
MA2: Main effects
• Target set size (β = 0.002, F(1,24.3) = 25.24, p < .0001)
• Positive slope more targets ~ higher ES
• BUT: at intercept, still significant ES (β = 0.26, t(15.3) = 4.27, p = .0006)
63
MA2: Interaction effects and multiple models
• 2-way interaction effects: none were significant (p-values ranging from .60 to .74)
• Add several main effects in one model• Using several strategies, the same
optimal model always emerged: a model containing only target set size
64
MA2: Publication bias• Meta-analysis: published AND unpublished data!
Fig. 13
0
20
40
60
80
100
-1 -0,5 0 0,5 1 1,5 2
Observed effect sizes
Sam
ple
size
65
MA2: Conclusions
4 variables (nature of primes, prime novelty, category size and d’) could not be included in the meta-analysis: caution warranted! Significant priming was observed, even under circumstances where S-R accounts are less likely (lexical decision and naming) Subliminal primes can be processed semantically
66
MA1+MA2: Conclusions• Can unconscious stimuli be processed in depth?
– YES! Priming effects are found in conditions where S-R accounts are unlikely (naming/lexical decision tasks, semantic categorization with large categories and/or novel primes)
– BUT, S-R accounts can also play a prominent role (significantly smaller ES for naming/lexical decision tasks, novel primes, primes from large categories)
These findings reconcile the two reigning theories regarding subliminal semantic priming: when given a chance, automatic S-R effects will clearly boost the priming effects. However, when S-R influences are minimized or avoided, subliminal information can be genuinely semantically processed!
67
Part I: Conclusions• Can subliminal stimuli influence our behavior?
YES!• Can subliminal stimuli be processed semantically?
YES! Significant priming effects are found in conditions where S-R accounts are unlikely:– Naming and lexical decision tasks– semantic categorization tasks with large categories,
large target sets and novel primes• Priming effects are moderated by several factors for
semantic categorization (prime novelty, category size) and lexical decision/naming (target set size)
68
Part II• PART II: conscious versus unconscious
processing– Introduction– Study 4: Conscious and unconscious
proportion effects in masked priming – Study 5: The interaction between
consciousness and attention: an empirical study using the priming paradigm
– Conclusions
69
Introduction• Understand unconscious processing by
contrasting it to conscious processing• Look at the effect of a manipulation at
the conscious vs. the unconscious level on priming effects– Study 4: proportion of notations– Study 5: attention
70
Study 4
Study 4: Conscious and unconscious proportion effects in masked
priming
Van den Bussche, Segers & Reynvoet, in press
71
Introduction• Global Neuronal Workspace theory (Dehaene
& Naccache, 2001) 2 assumptions:1. A fully consciously perceived stimulus can be
used strategically to improve task performance- Top-down manipulations can have an effect on
subliminal processing (e.g. spatial and temporal attention, the target set and target notation)
- E.g. Kunde et al. (2003)Hypothesis: A manipulation on target level will influence observed priming effects!
72
Introduction2. An unconscious stimulus can NOT be
used strategically to improve task performance- Dehaene et al. (2006): unconscious
primes have insufficient strength to result in a bottom-up effect
- Never explicitly investigated!Hypothesis: A manipulation on prime level will NOT influence observed priming effects!
73
Aims• Investigate the 2 assumptions of the
GNW by manipulating the proportion of Arabic numbers (e.g. 1) versus number words (e.g. “one”): – Consciously (target level): experiment 1– Unconsciously (prime level): experiment 2
• All other task conditions were controlled
74Fig. 14
Congruent trial
nine
#$#$#$
7
$#$#$#
33ms
33ms
300ms
time
Larger than 5Smaller than 5
Design
Larger than 5
Fig. 15
Incongruent trial
nine
#$#$#$
two
$#$#$#
33ms
33ms
300ms
time
Smaller than 5
75
Design• Exp1: manipulation of the proportion Arabic
targets vs. number word targets• 3 conditions (N = 46):
– 1a: 50% Arabic / 50% number word targets– 1b: 25% Arabic / 75% number word targets– 1c: 75% Arabic / 25% number word targets
• Proportion Arabic primes vs. number word primes was always identical (50% / 50%)
76
Design• Exp2: manipulation of the proportion Arabic
primes vs. number word primes• 3 conditions (N = 47):
– 2a: 50% Arabic / 50% number word primes– 2b: 25% Arabic / 75% number word primes– 2c: 75% Arabic / 25% number word primes
• Proportion Arabic targets vs. number word targets was always identical (50% / 50%)
77
Predictions• Exp1: priming effect for a certain prime
notation (Arabic numbers or number words) will be larger if the proportion of targets in that notation is large as opposed to when the proportion of targets in that notation is small
• Exp2: manipulation of the proportion of Arabic/number word primes will have no effect and thus no differences in are expected between the priming effects observed for the 3 conditions of experiment 2
78
Results
-5
0
5
10
15
20
1a: baseline 1b: 75% numberw ord targets
1c: 75% Arabictargets
2a: baseline 2b: 75% numberw ord primes
2c: 75% Arabicprimes
Condition
prim
ing
effe
ct (m
s)
Arabic primes Number word primes
F(2,43) = 3.70, p = .03 F(2,44) = 0.22, p = .81
Fig. 16
79
Conclusions• The results are in line with the 2
assumptions of the GNW theory:– The weak activation evoked by an
unconscious stimulus is insufficient to be used strategically to enhance task performance
– A fully consciously perceived stimulus can be used strategically to enhance task performance
80
Study 5
Study 5: The interaction between consciousness and attention: an empirical study using the priming
paradigm
Van den Bussche, Hughes & Reynvoet, in preparation
81
Introduction• Dehaene et al. (2006): Conscious,
preconscious and subliminal processing: a testable taxonomy
• Both bottom-up stimulus strength (conscious vs. unconscious) and top down attention (attended or not) will define these different brain states
82
Introduction
Fig. 17
83
Aims• “A testable taxonomy” test it!• Can we distinguish these different brain
states? Do we observe differential priming pattern for these different states?
• Behavioral study using the masked priming paradigm
84
Design• N = 52• Categorize numbers as smaller or larger than
5 • Stimuli could appear on 2 locations• A cue always indicated the target location• The prime could appear on the same place as
the target (i.e. attended) or on the other place (i.e. not attended)
• The prime could either be presented subliminally or clearly visible
85
Design
Fig. 18
Congruent trial, subliminal condition
time
Larger than 5Smaller than 5
480ms ####
##++
####
8§
####
##7
120ms
26ms
26ms
80ms
86
Design
Fig. 19
Congruent trial, conscious condition
time
Larger than 5Smaller than 5
480ms ####
##++
####
§8
##7
120ms
26ms
106ms
87
Design• This design created 4 conditions:
Unattended Attended
Subliminal Subliminal, unattended
Subliminal, attended
Conscious Conscious, unattended
= “preconscious”
Conscious, attended
88
Predictions
Unattended Attended
Subliminal Subliminal, unattended
Subliminal, attended
Conscious Conscious, unattended
Conscious, attended
<
<< <
89
Results
Fig. 20
subliminal
conscious
0
5
10
15
20
25
30
35
40
45
50
55
60
not attended attended
Prim
ing
effe
ct (m
s)
2 ms
10 ms*11 ms**
50 ms***
90
Results1. No priming for subliminal, unattended
primes2. Priming for all other states3. Attention modulates the observed
priming effects 4. Consciousness modulates the observed
priming effects
91
Conclusions• Completely in line with the taxonomy of
Dehaene and colleagues• Significant priming can be obtained for
attended, subliminal primes• Significant priming can be obtained for
unattended, conscious primes Receiving either attention or being consciously perceived is sufficient for a prime to elicit significant priming
92
Part II: Conclusions• Top-down manipulations (manipulation of
proportion target notations, attention) influence subliminal priming effects
• Subliminal primes have insufficient bottom-up strength to influence subliminal priming effects
• For subliminal primes, attention is a prerequisite in order to elicit significant priming
93
General Conclusions• Subliminal primes can be semantically
processed• Several variables moderate the emergence of
subliminal priming effects• Top-down influences have an impact on
subliminal priming, but subliminal primes themselves have insufficient strength to influence priming effects
• Attention is a prerequisite for subliminal priming
94
Contact Info• Eva Van den Bussche
KULeuven - Campus KortrijkE. Sabbelaan 53 8500 Kortrijk BelgiumTel.: +32 56 246074Fax: +32 56 246052 [email protected] info: http://www.kuleuven-kortrijk.be/~u0050367/