Categorical similarity may affect colour pop-out in infants after all

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British Journal of Developmental Psychology (2002), 20, 185–2032002 The British Psychological Society

Categorical similarity may affect colour pop-outin infants after all

Ian Davies* and Anna FranklinUniversity of Surrey, UK

Gerhardstein, Renner, and Rovee-Collier (1999) reported an investigation of pop-outin infant visual search with targets differing in colour from the distractors. They variedthe perceptual distance between target and distractors and their categoricalrelationship. Targets were either in the same category as distractors or in a differentcategory. They reported that at 3 months, infants showed increased pop-out asperceptual distance increased, but there was no categorical effect. Here, we argue thattheir stimuli were not adequate to address these issues. Use of incandescent lightrather than the normal illuminant C differentially affected the perceptual differencesamong their stimuli, and there may have been an unintended category boundarypresent in an intended within category pair. We argue that these faults in the stimulican account for their pattern of results. They were aware of the possible consequencesof using incandescent light and ran a preliminary study on adults comparing categorymembership and perceived similarity among their stimuli under incandescent light andilluminant C. They report that the illuminant had no effect. We replicated andextended their adult study and found that there were effects of the illuminant as well asevidence consistent with the unintended boundary. Reasons for the discrepant resultsare discussed, and the requirements for a valid investigation outlined.

We perceive colour categorically. The physical continuum of light is perceived asqualitatively different categories denoted in English by terms like red, yellow, green,and blue. Moreover, cross-category discriminations are easier than equivalent within-category discriminations (Bornstein & Korda, 1984; Harnad, 1987). These categoricaleffects vary with linguistic structure, suggesting that categorical perception is learned(Kay & Kempton, 1984). However, there is also evidence that categorical perception iseither innate or acquired before language. In a classic study, Bornstein, Kessen, andWeisskopf (1976) found that 4-month-old infants showed categorical perception forcolour. For instance, after habituating to a green stimulus, they looked more frequently

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* Requests for reprints should be addressed to Ian Davies, Department of Psychology, University of Surrey, Guildford GU27XH, UK (e-mail: [email protected]).

at a novel blue stimulus than at a novel green stimulus, even though the two test stimuliwere the same distance away (in wavelength) from the standard.

Gerhardstein, Renner & Rovee-Collier (1999) exploited Bornstein et al.’s finding toinvestigate the nature of ‘pop-out’ in infants. Pop-out occurs when a target is detectedamong distractors pre-attentively; phenomenally, search is effortless and targets pop-out(Catherwood, Skoien & Holt, 1996; Treisman & Gelade, 1980). Gerhardstein et al.addressed the question of whether pop-out was determined by the perceptual similarityof the target and distractors, or their categorical relationship, or both. They report thatpop-out increased with perceptual distance between target and distractors, but it wasnot affected by their categorical relationship. Here, we argue that there were technicalflaws in their methods that render their case not proven, and we present new data thatsupport our argument.

The logic of the methods usually used to investigate categorical perception requiresthat the inter-stimulus ‘distances’ for between and within category pairs are equal.Gerhardstein et al., like many others (e.g. Bornstein & Korda, 1984), used Munsellstimuli, and the Munsell Metric to equate inter-stimulus distances (see Appendix).Munsell colours are standardized under illuminant C (north light); if they are viewedunder any other illuminant, then the perceptual uniformity of the metric breaks down.For practical reasons, Gerhardstein et al. tested their infants under normal domestic(incandescent) light. Colour constancy may dampen the perceptual effect of theilluminant change, but nevertheless, the inter-stimulus distances could not have beenequal.

Gerhardstein et al. knew that using incandescent lighting weakened the perceptualuniformity of the Munsell metric. Accordingly, they ran a preliminary experiment(Experiment 1) to assess the extent of the problem using the stimuli intended for themain study (Experiment 2) and a follow-up study (Experiment 3). Figure 1 showsGerhardstein et al.’s stimuli and illustrates the use of Munsell distances to balanceperceptual distances. In the main study, there were four pairs of stimuli varying inperceptual distance (‘near’ or ‘far’) and categorical status (within or between category).Far pairs were three Munsell Hue steps (each step is 2.5 units) apart and near pairs twosteps apart.

Figure 1. Munsell codes, categorical status and Munsell distances for Gerhardstein et al.’sExperiment 2 stimuli. The relative perceptual distances (near/far) and categorical relationships (within/between) of the experimental pairs are also shown.

186 Ian Davies and Anna Franklin

Adults rated the similarity of each pair, and judged whether or not they were from thesame category. Judgments were made once under ‘natural daylight’, and once underincandescent light. They found that, for both illuminants, category judgments accordwith the experimental manipulation (their data are reproduced in Table 3).Furthermore, similarity judgments were correlated across the two illuminants(r = 0.82), and there were no significant differences between judgments under thetwo illuminants. There were, however, appropriate highly significant effects ofperceptual distance, and of category membership. Far pairs were judged as less similarthan near pairs, and cross-category pairs as less similar than within category pairs. Theyconclude that ‘neither the category membership nor the perceived similarity of thesame Munsell stimuli was affected by the artificial light’ (pp. 407–408).

In Experiment 2 (the main study), infants viewed a mobile of seven coloured disks.The display was linked to the infant’s ankle, and they were able to control the positionof the display by kicking. Infants were trained to this contingency, and then habituatedto seven discs of the baseline colour. They were then tested for pop-out with six discsin the habituated colour and one novel colour. Infants showed pop-out if theyresponded less to the test array than at baseline. Note that not responding left thedisplay unchanged, thus allowing the infant to look longer at the novel stimulus.

Infants showed no pop-out for the near perceptual distance condition, but they didfor the far condition—a perceptual distance effect. However, there was no categoricaleffect. Infants kicked (or did not) just as much when the test colour was in a differentcategory as when it was from the same category. In a follow-up experiment(Experiment 3) using only within-category stimuli, infants again showed a perceptualdistance effect for the pairs 10RP–5R (near, red) and 10R–7.5R (far, orange). Pop-outwas found for the far pair but not the near pair.

Problems with the studyThere are two key problems with the stimuli, which we describe next. In addition,infant colour vision may be sufficiently different to adult colour vision to invalidateother aspects of the methods, and these are discussed in the next section.

The first key problem is due to the effect of incandescent light, already mentioned.We measured the CIE co-ordinates of each stimulus under incandescent light andilluminant C and calculated the perceptual distance in each pair in CIE L* u* v* space(see Appendix). This colour space is approximately perceptually uniform such thatequal separations in different regions of the space correspond to equal perceptualdistances. Figure 2 shows how the loci of the stimuli shift under incandescent lightrelative to illuminant C. (All stimuli have increased lightness, not shown here, but theincreases are more or less uniform.) In general, the stimuli become redder (increasedu*) and less yellow (decreased v*). And crucially, the perceptual distances among theexperimental pairs change differentially between the two illuminants. Table 1summarizes these effects. Under illuminant C, the CIE measures are more or less inaccord with the Munsell metric, as expected. The near distances are about the same aseach other, as are the far distances. The far distances are also greater than the neardistances. Note, however, what happens under incandescent light. First, all distancesare smaller than their equivalent under C. Second, cross-category distances are nowgenerally larger than their equivalent within-category distances. That is, there is now abuilt in bias favouring cross-category discriminations. In addition, the nature of thedifferences between stimuli changes differentially. Most importantly, the within–far pair

187Categorical similarity

is no longer balanced in Chroma: 7.5RP has a lower Chroma than 5R (see Table A1 inthe Appendix). There was a similar reduction of perceptual distances in Experiment 3:from 38.2 to 30.0 for the far pair and 27.8 to 15.2 for the near pair.

CIE distance should be strongly associated with similarity judgments. Thus, it issurprising that Gerhardstein et al. found no significant effects of lighting. The reasonmay be found in the second flaw in the design. Stimuli may not have had the categoricalstatus intended. The stimuli were intended to be either red or orange. However, one of

Table 1. CIE distances (L*u*v*) for both perceptual distances (near/far) and categorical relationships(within/between) for both illuminants

Illuminant C Incandescent

Category Near Far Near Far

Within 27.51 35.65 15.99 19.63Between 25.71 34.26 25.01 28.43

Figure 2. Loci of the stimuli in CIE (u*v*) colour space for both illuminants. Lines link the twostimuli in each pair: within/near (w/n); within/far (w/f); between/near (b/n); and between/far (b/f).Equivalent stimuli under the two illuminants are labelled with the same codes: O1–O3; R1–R2 (see alsoFig. 1).


the two putative red stimuli (7.5 RP) seems to be pink. Thus, the comparison of thewithin–far and between–far pairs may have been invalid. Both pairs seem to be cross-category: red–pink and red–orange, respectively. Gerhardstein et al. found that both farpairs popped out, and the magnitude of the effect was about the same (Fig. 2: 412 in theoriginal). In view of the previous argument, this finding may conceal a categoricalperception effect. Pop-out may be due to perceptual distance, or categoricalperception, or both.

Recall, however, that Gerhardstein et al. asked their adult participants to judge thecategorical status of each pair. They found that these judgements were, by and large, inaccord with the intended categorical status. We assume that if colours are labelled withdifferent ‘basic’ terms (Berlin & Kay, 1969), this indicates that they are, in some sense,in different categories. However, in another sense, being labelled differently may not besufficient for categorical difference.

Gerhardstein et al. ascribe special status to four ‘primary’ chromatic categories: red,yellow, green, and blue. This is based in part on Hering’s (1878/1964) four unique hues,and partly on Boynton and Gordon’s (1965) naming experiment. Phenomenally, aunique hue includes no experience of any other hue, whereas other colours seem to beperceptual blends of two unique hues. For instance, prototypical orange seems tocontain both red and yellow. Boynton and Gordon found that if responses are restrictedto the four primary terms (red, green, yellow, and blue), most colours can still benamed consistently. Secondary colours, such as orange, required the use ofconjunctions of the primary terms, e.g. orange = red–yellow. Gerhardstein et al. areless clear about the status of secondary terms or categories such as orange. Theysometimes treat their category boundary as red–yellow (as Bornstein et al., did), andsometimes as red–orange. In contrast, Kay and McDaniel (1978), in their influentialtheory of the origin of basic colour categories, give a prominent role to secondarycategories (derived categories in their terms). They ascribe privileged status to theprimary categories, (including white and black,). However, they also regard orange,pink, purple, brown, and grey as basic universal categories.

Whether the short-wavelength category is yellow or orange, there is nevertheless acategory boundary falling at 7.5R (Fig. 1). Our concern here is whether there is also anunintended boundary (red–pink) in the long-wavelength group. There is a prima faciecase for a nominal boundary, but none based on unique hues. Perhaps pink producesonly the red experience, and in that sense, it falls in the red category. Gerhardstein etal.’s participants certainly judged the putative red pairs to be in the same category.However, as they did not name the stimuli, and no elaboration of the meaning of sameor different category was given, we do not know whether there was any dislocationbetween linguistic categories and perceptual categories, or categories in some moregeneral sense.

Infant colour visionInfants are said to show true colour vision when they can discriminate between twostimuli of different wavelength, but equal luminance. Infants as young as 2 months candetect chromatic differences at adult isoluminance (e.g. Knoblauch, Vital-Durand, &Barbur, 2001; Teller, 1998). However, colour vision does not reach its adult form untilthe teenage years at the earliest. The most obvious developmental change is inchromatic sensitivity. There is a 30-fold reduction in thresholds from infancy toadolescence (Teller, 1998). Relative sensitivity to different wavelengths probably


changes little across that time span, although the yellow–blue mechanism may developmore slowly than the red–green mechanism (see Teller, 1998). The infant is alsoconsiderably less sensitive to spatial and temporal chromatic variations than the adult.

The spectral sensitivity function is at the core of the CIE system. Although, as wenoted above, the shape of the infant spectral sensitivity function is similar to the adults’,there may be small, but possibly significant, differences. If this is so, the perceptualdistances we have used will be invalid for infants to the extent of the differencebetween the two functions. Moreover, the uniformity of the Munsell metric will bedistorted to the same extent.

As pointed out earlier, true colour vision can detect wavelength differences atisoluminance. Thus, usually, when testing colour vision, stimuli are equated forluminance1. However, according to Werner and Wooten (1979), there were residualbrightness differences in Bornstein et al.’s. (1976) study (see also Bornstein, 1981;Werner & Wooten, 1985), and all Gerhardstein et al.’s differed in lightness (Value).Rather than equating their stimuli for brightness, Gerhardstein et al. used stimuli thatdiffered by one Munsell Value unit. They assumed that as this was so for all pairs,lightness differences could not account for their results2. Nevertheless, they took theprecaution of checking whether lightness differences might have affected their results.They tested whether infants could detect a difference between achromatic stimulidiffering by one Value unit (Experiment 4) and found that infants did not responddifferentially to them. They concluded that therefore lightness differences could nothave been responsible for the effects in the main experiments. However, perceptualdistance combines lightness and hue differences. A below-threshold Hue differencecould combine with a below threshold Value difference to produce a suprathresholddifference. Provided that the Munsell metric holds for infants, the lightness differencesprobably do not matter as they are the same size for all pairs. If, however, infant relativesensitivity to wavelength differs from adults, the equating of lightness differences couldbe violated, and this could have affected the results.

Even if the Munsell metric is perceptually uniform for infants, the perceptualdistances among stimuli are considerably less than for adults, because infant chromaticthresholds are markedly lower than adults, as mentioned above. For adults, a perceptualdifference of more than about 20 CIE units (in L*u*v*) is required for pop-out to occur(Carter, 1989). It seems probable that much larger CIE distances would be needed toproduce pop-out in infants. Gerhardstein et al.’s perceptual distances ranged fromabout 16 to 30 (Table 1). Thus, some of the perceptual distance effect may be owing todistances below the pop-out threshold.

We are not arguing that differences between infant and adult colour vision aresufficient to account for Gerhardstein et al.’s data. We are arguing that because ofuncertainty about the precise form of infant colour vision, that there is correspondinguncertainty about whether luminance differences were equal across all pairs, andwhether perceptual differences were large enough to afford pop-out in all cases.Accordingly, over and above the two main technical problems that we focus on, theresults need treating with caution.

1Alternatively, because of uncertainty about the position of the infant isoluminant point, discrimination can be tested across arange of luminance differences including the likely isoluminant point. If discrimination is shown across the whole range, thisindicates chromatic discrimination.2 Incandescent light also changes the luminance of the stimuli relative to illuminant C, but fortunately, although all Valuesincrease, the difference between pairs is preserved at about one. See Table A1 in the Appendix.


The current studyIn order to try to resolve our concerns, we replicated and extended Gerhardstein et al.’sfirst experiment. Participants named each stimulus and judged whether they were inthe same category or not. For short hand, we will call these nominal and perceptualcategorization, respectively. They also judged the perceptual similarity/dissimilarity ofeach pair. These three measures are unlikely to be independent of each other, but weassume that the perceptual loading of the tasks increases from naming to perceptualcategorization, to similarity judgments, and that the linguistic load decreases. Keyquestions addressed were as follows. First, what name is given to each stimulus; inparticular, is the within–far stimulus (7.5RP) named pink? Second, do the namingjudgments correspond to the category judgments? Third, to what extent do CIEdistances predict similarity judgments? Fourth, does the within–between categoryfactor further predict similarity judgments? Fifth, if the within–far pair is treated asbetween-category, does this add to the predictive power? Finally, is there any residualeffect of lighting or intended perceptual distance (near–far)?

Method

ParticipantsThere were two groups of 10 participants with normal colour vision, as indicated by theCity University test (Fletcher, 1981). The gender balance was the same in both groups(five men and five women), and the mean ages of the two groups were approximatelythe same (23 and 25 years). All participants were first-language English speakers fromthe University of Surrey. One group made judgments under incandescent lighting, andthe other under illuminant C.

StimuliThe colours used were the same as those used by Gerhardstein et al. They consisted of2-cm squares of Munsell paper mounted on 7.6-cm squares of grey card. The Munsellcodes were as follows: 7.5RP 6/12, 5R 5/12, 10R 6/12, 2.5YR6/12, 5YR 7/12, 7.5YR 7/12 (note that 10RP 6/12, 7.5YR were used by Gerhardstein et al. in follow-ups to themain experiment, and they are not shown in Fig. 1).

ProcedureParticipants did the same similarity and category judgment tasks as in Gerhardsteinet al.; in addition, they named each stimulus. For similarity and category judgments, apair of stimuli was presented, and they rated their similarity/dissimilarity on a linearanalogue scale. The far left of the line indicated a high similarity, and the right of theline high dissimilarity. They then judged whether the pair was in the same or differentcategory. Each possible pair of stimuli was presented in a different random order foreach person. In the naming task, stimuli were presented singly, and participants wereasked to name the colour. For both samples, half did the naming task first, and half did itlast.


Results

Naming and category membershipTable 2 summarizes the naming data and shows the intended category (red or orange)for each stimulus. It can be seen that although the majority name for most stimuliaccords with the intended categories, the majority name for 7.5RP is pink. Thus, thewithin–far pair (7.5 RP–5R) is nominally pink–red.

Table 3 shows the percentage of participants that judged Gerhardstein et al.’sstimulus pairs to be in different categories. We also include Gerhardstein et al.’s data forcomparison (Table 1 in the original). If participants’ judgments were in accord with theexperimental design, then the within scores should be small (ideally 0), and thebetween scores should be large (ideally 100). By and large, the scores tend to be in theright direction, but they are less polarized than the original. In particular, the within–farscore (7.5RP–5R) under incandescent light is 50%rather than the ideal score of 0%.

Perceptual similarity/dissimilarityTable 4 shows the mean dissimilarity scores for the four experimental pairs under eachilluminant (i.e. scores are ‘distances’). Athree-way mixed design ANOVAon the factorsof perceptual distance (near, far), category membership (between, within) and lightingcondition (illuminant C, incandescent) revealed that all main effects and two-wayinteractions were significant. Far pairs were judged more dissimilar than near pairs(F(1,18) = 218.9, p < 0.001); between pairs were judged more dissimilar than withinpairs (F(1,18) = 626.1, p < 0.001); and judgments under incandescent light were moresimilar than under illuminant C (F(1,18) = 81.7, p < 0.001).

The three two-way interactions were explored using protected t-tests. All differencesreported here were significant at at least the 1%level. First, from Fig. 3a, it can be seen

Table 2. Most frequent names and the corresponding frequencies (%) for both illuminants

Intended category Stimulus Illuminant C Incandescent

Red 7.5RP Pink 90 Pink 1005R Red 80 Red 100

Orange 10R Orange 100 Orange 1002.5YR Orange 100 Orange 1005YR Orange 70 Orange 100

Table 3. Percentage of ‘different-category’ responses for both illuminants for both perceptualdistances (near/far) and within and between categories (Gerhardstein et al.’s equivalent data are inbrackets)



Within 30 (0) 40 (11) 10 (0) 50 (11)Between 60 (79) 80 (100) 70 (84) 70 (95)


that the main reason for the significant category by lighting inte raction(F(1,18) = 247.2, p < 0.001) is that the effect of lighting is much larger for withinthan between. Second, Fig. 3b shows that the main reason for the lighting by distanceinteraction (F(1,18) = 72.5, p < 0.001) is that the effect of lighting is greater for nearpairs than for far pairs. Finally, Fig. 3c shows that the main reason for the category bydistance interaction (F(1,18) = 77.9, p < 0.001) is that there is no categorical effect forfar pairs (within not different to between), whereas there is for near pairs (within lessthan between).

Similarity judgments, CIE distances, and categorical statusFigure 4 shows the relationships among the similarity judgments and colorimetricdistances for the four experimental pairs (scores are dissimilarities). Note that the twosets of four mean scores for the similarity judgments are the same as in Table 4, but theyare now presented in relation to colorimetric distance (Table 1). The categorical statusrepresented is the intended categorical relationship; the within–far pair may actually bebetween category. It can be seen that under illuminant C, there appears to be acategorical effect for near pairs (within–near less than between near (t(9) = 8.82,p < .01) but not for far pairs (t(9) = 1.52, p > .05). However, under incandescent light,there are categorical effects for both near pairs (t(9) = 49.17, p < .001) and for far pairs(t(9) = 22.64, p < .001). The size of the categorical effect is greater for near pairs thanfar pairs.

Predictors of similarity/dissimilarityWe investigated whether colorimetric distance and categorical status were sufficient toaccount for the pattern of similarity judgments using multiple regression. Colorimetricdistance on its own produced an R2 of 0.46; adding intended categorical statusincreased the R2 to 0.68; treating the within–far pair as actually between–far increasedR2 further to 0.71. Adding lighting and distance increased R2 to 0.82.

DiscussionOur main concerns have been the possible effects of the use of incandescent lightrather than illuminant C, and whether there is an unintended category boundary (red–pink) in the putatively red category. At the end of the introduction, we set out a numberof tests as a way of addressing these issues; we will deal with these in turn.

Table 4. Means (SD) of dissimilarity judgments (mm) for the four experimental pairs under the twoilluminants for both perceptual distances (near/far) and within and between categories



Within 35.00 (1.85) 46.16 (3.52) 17.30 (2.70) 33.59 (2.11)Between 48.70 (4.28) 42.70 (4.23) 46.46 (2.09) 49.27 (1.16)


Figure 3. Two-way interactions for dissimilarity ratings. (a) Mean dissimilarity ratings for category-by-lighting interaction. (b) Mean dissimilarity ratings for the lighting-by-distance interaction. (c) Meandissimilarity ratings for the category-by-distance interaction.


Naming and categorical statusUnder incandescent light, the pattern of naming was unanimous (Table 2). Naming wasconsistent with the intended categories with the exception of 7.5RP, which was namedpink. Thus, the data support our suspicion that the within–far pair is nominally cross-category (red–pink). However, judgments of the same–different category are not fullyconsistent with naming (Table 3). Although, for most pairs, the majority judgment isconsistent with the experimental assignment, on average, there is about 30%disagreement. In every case, the level of agreement is lower than that found byGerhardstein et al. Disagreement is highest for the within–far pair (7.5RP–10R) underincandescent light. About half of our participants judged this to be a between-categorypair (i.e. consistent with naming), rather than the intended within-category pair.

It is clear that nominal categories and perceptual categories are not completelyequivalent. However, equivalence is greater for the red–orange boundary than for thered–pink boundary. This may be because the red–orange difference is largely a huedifference, whereas red–pink differences can be due to lightness or saturationdifferences. Differences in hue seem to be more intrinsic to our concept of colour thanlightness or saturation. In fact, it is common in categorical perception studies to varyhue only (e.g. Bornstein & Korda, 1984). In Gerhardstein et al.’s study, the main

Figure 4. Dissimilarity judgements as a function of categorical status and CIE distances. (a) IlluminantC. (b) Incandescent light.


variable was Munsell Hue, but Value (lightness) also differed in each pair. In the within–far pair 7.5RP had Value 6, and 5R had Value 5, where higher scores indicate greaterlightness. This lightness difference is partly responsible for 7.5RP appearing pink, as isthe Chroma difference under incandescent light.

Dissimilarity judgments CIE distances and categorical statusAs expected, CIE distances are associated with dissimilarity judgments. Recall that, onaverage, incandescent light reduces the intra-pair distances relative to illuminant C. Thisreduction in CIE distance is not uniform. Rather, the difference between within pairsand between pair distances is increased, whereas the distance between near pairs andfar pairs is reduced (Table 1). These changes are reflected partially by associatedchanges in dissimilarity judgments. First, mean dissimilarity judgments are significantlylower under incandescent light (Table 4). Second, the category effect is greatest underincandescent light (Fig. 3a). However, the near–far effect is greater under incandescentlight than under illuminant C(Fig. 3b). Further support for the influence of CIEdistancecomes from the regression analysis, where it was the strongest predictor ofdissimilarity.

Intended categorical status is also associated with dissimilarity. Although thecategorical effect is greatest under incandescent light (as above), overall, between-category dissimilarities are greater than within-category dissimilarities, as borne out byANOVA and regression.

There is also evidence that the within–far pair behaves like a between-category pair.Under illuminant C, where the within–between comparison is equal in CIE distance(Fig. 4a), there is no difference between within–far and between–far pairs, whereasthere is between within–near and between–near pairs. In other words, there may be acategorical effect, but both far pairs show it. Under incandescent light, there areapparent categorical effects for both near and far comparisons, but the size of the effectis smaller for the far pair. The comparisons in this case are not fair colorimetrically, andthe colorimetric imbalance between the within and between far pairs may be sufficientto account for the greater dissimilarity of the between pair.

However, there are also inconsistencies in the data. Whereas dissimilarity scores arerelated generally to perceptual distance and categorical status, the score for between–near under illuminant Cis inconsistently high (Table 4). We have no simple explanationfor this, and it may just be an error of measurement owing in part to the relationshipsamong the three dependent variables. Nominally, at least 80%of participants judgethem to be in different categories, whereas only 60%judge them to be in differentperceptual categories. It is unclear exactly what people are doing in the perceptualcategorization task. Some may just rely on naming, whereas others may rely more onperceptual qualities. Similarly, the precise nature of the dissimilarity judgments is notclear. They could be influenced directly by naming, perhaps amplifying judgmentswhen the pair differ nominally. We followed Gerhardstein et al. in using the perceptualcategorization task, and some clarification of what participants think they are doing isneeded, particularly given the discrepancy between the results (see next section).However, this same pair seems anomalous in the infant study (see section Implicationsfor the infant studies).


Discrepancies with the original studyAs we have noted several times, our results are inconsistent with Gerhardstein et al.’s tovarying degrees. Our judgments of same or different categories are generally less inaccord with the experimental assignments than theirs. This is particularly so for theintended within–far pair. It is not clear why the results should differ. There was oneprocedural difference between the two studies. We used an independent groupsdesign, whereas the original was repeated measures. It is possible that this differenceinfluenced category judgments. Perhaps, if you see the full set of stimuli, as in theoriginal, red–pink are judged as more categorically similar, than if you only see thestimuli under one illuminant. In terms of the CIE colour space for the two illuminants(Fig. 2), the original participants would have been exposed to a larger region than ourparticipants. The distance in the within–far pair (and other pairs) relative to themaximum distance is smaller for their participants than for ours. We tested this possibleexplanation using 20 participants who named the stimuli and judged the categoricalrelationship of each pair under both illuminants. The results were essentially the sameas in the independent groups design.

It is also difficult to understand why the studies differ in their comparison ofdissimilarity judgments under the two illuminants. Again, it is possible that theprocedural difference contributes to the discrepancy. By essentially the same argumentas above, intra-pair judgments might be made relative to the maximum distance in theset. This would compress Gerhardstein et al.’s participants’ judgments relative to ours.This could have produced some kind of floor effect. That is, the effect of the differentialCIE distances under the two illuminants might be masked by the compression.However, in the original study, lighting conditions were blocked. Thus, if there was arange effect, it could only have affected judgments in the second block.

Implications for the infant studiesThe imbalanced CIE distances are likely to have affected the results. Underincandescent light, the near–far differences are reduced to about three CIE unitsrelative to about nine units under illuminant C. In contrast, the within–betweendifferences are about nine units, relative to effectively zero under Illuminant C (Table1). This makes it surprising that there was a near–far effect, but no effect of within–between. This might be, of course, because of the unintended category boundary. If weassume that the categorical effect is in essence a perceptual distance effect (‘warping’of psychological colour space; Harnad, 1987) and adds to ‘raw’ perceptual distance,most of the near–far effect should be due to the within-category condition. In this case,there is a CIE difference of about three units plus whatever the unintended categoryeffect may have produced. For the between comparison, the difference is just three CIEunits. By the same argument, the unintended boundary may have partially masked anycategorical effect by reducing the difference between within–far and between–far.However, the latter argument is relatively weak, as the difference in raw distances forthese pairs is about nine CIE units.

If we accept that CIEdistance including possible stretching due to categorical effectsis a valid indicator of infant perceptual distances, the lack of pop-out for the between–near condition for infants is anomalous. From Table 1, it can be seen that the CIEdistance for between–near is greater than for within–far. Both include an adult categorydifference, and adult dissimilarity judgments are consistent with perceptual distance.Yet, within–far pops out, whereas between–near does not. We have no convincing


explanation for this. It may be experimental noise; it may be that for infant colourvision, the relative perceptual distances are inconsistent with CIE distances; perhaps,because of the latter, the between–near perceptual distance is too small to producepop-out.

As well as the magnitude of inter-stimulus differences, the quality of stimulusdifferences was also changed by using incandescent light, particularly for the within–farpair. Hue differences are reduced, and Chroma is no longer completely balanced. Moststudies of categorical perception for colour hold the Munsell Value and Chromaconstant, and vary only hue (e.g. Bornstein & Korda, 1984; Roberson & Davidoff, 2000).Thus, little if anything is known about whether adults, and certainly not infants, showcategorical effects across a saturation or lightness-based category boundary. Ourconcept of colour is closely tied to hue, but even so, colour categories are defined by acombination of all three variables. For instance, yellow is characterized by highlightness as well as hue; pink differs from red by being lighter and less saturated. In fact,blue and green are the only categories that exist at all levels of lightness (Boynton &Olson, 1987).

The foregoing, of course, undermines our argument. This rests in part on infantsshowing a categorical effect across the red–pink category boundary by 4 months of age.We know of no direct test of this in either infants or adults. However, we are alsounaware of any direct test of categorical perception across primary category boundariesin infants based on a perceptually uniform metric. Bornstein et al.’s classic study usedspectral lights varying in hue with constant luminance, and their distance measure waswavelength. Wavelength discrimination varies across the spectrum with minima atabout 490 and 600 nm (e.g. Pokorny & Smith, 1986). The transformation from awavelength representation of colour to a perceptually equal space such as Munsellflattens the function. Nevertheless, adults show categorical perception effects withMunsell stimuli (Bornstein & Korda, 1984; see also Roberson & Davidoff, 2000). It isimportant to establish how early these categorical effects occur and across whatcategory boundaries.

General implicationsCategorical perception occurs in many stimulus domains, such as speech perception(Pisoni & Tash, 1974) and perception of emotional expression (Roberson & Davidoff,2000). The question of whether the effect is due to nature or nurture applies to alldomains, not just to colour. The issue is related to the more general issue of perceptualplasticity, or the extent to which perception is a function of personal history. It alsoprovides a way of testing the linguistic relativity hypothesis: colour cognition isinfluenced by the category structure of language (e.g. Roberson, Davies, & Davidoff,2000). However, there is some uncertainty over whether categorical perception isgenuinely perceptual, rather than a phenomenon of memory or labelling (Roberson &Davidoff, 2000). One reason why investigating category effects in pop-out is importantis that it is probably based on low-level preattentive processes that are cognitivelyimpenetrable. Thus, if there were categorical effects in pop-out, this would suggest thatthe phenomenon is, at least in part, perceptual. Moreover, if they are not present in 3-month-old infants, this implies that they may be learned, unless they arise from latermaturation.

It is partly because Gerhardstein et al.’s results have such wide-ranging implications


for what are essentially questions about human nature that it is important that themethods can provide unequivocal results. One of the flaws in the method has alsoprovided misleading results in the study of categorical perception in speech perception.Evidence seemed to support reduced within category discrimination — the perceptualmagnet effect. However, the results were undermined because, as here, the categoricalstatus of some of the stimuli was ambiguous (Lively & Pisoni, 1997).

ConclusionThe foregoing arguments are sufficient for us to give pause before acceptingGerhardstein et al.’s conclusions. Ironically, perhaps, the lack of a categorical effect,despite the built in advantage for cross-category pairs, may seem to strengthen theirconclusions. However, the reduction in perceptual distances produced by usingincandescent light combined with residual uncertainty about the relative sensitivity ofinfant colour vision means that that conclusion is unsafe. Their data cannot testifyunequivocally to either the presence or absence of a categorical effect in pop-out invisual search by infants. In order to resolve this issue, it is first necessary to establishthat there are categorical perception effects in young infants using reflective stimuli,such as Munsell. If there are such effects, their possible influence on pop-out could beexplored using appropriate stimuli and illuminants.

AcknowledgementsThe work was supported partly by ESRC grant number R000236750 to the first author. Theauthors are grateful for the support. They also thank Sam Boyles and Paul Sowden for constructivecomments on drafts of the paper.

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Received 14 March 2001; revised version received 21 August 2001

Appendix

Notes on colorimetry and colour order systems (see Hunt, 1987)

The Munsell systemMunsell stimuli are standardized colours produced with a high reliability. Munsellcolour space is three-dimensional: Hue, Value (lightness) and Chroma (colourfulness,rather like saturation). In Munsell notation, Hue is specified by abbreviations of fivemain Hues: R (red), Y (yellow), G (green), B (blue) and P (purple). Combinations of themain hues such as YR designate intermediate hues. A number, as in 7.5RP, 10RP, 2.5R,5R, precedes the Hue abbreviation indicating the degree of the Hue. Value ranges from1 (darkest) to 10. Chroma ranges from 0 (achromatic: white, black, or grey) upwards,


with increasing numbers indicating increased colourfulness. The maximum Chromarealizable varies with Hue and Value; but 16 is about the maximum available. Note,however, that the Munsell space is theoretically continuous, so higher Chromas couldexist; indeed, one of the effects of using incandescent light is, in effect to increase theChroma of red and orange stimuli.

The system was standardized so that each dimension was intended to beperceptually uniform. Thus, equal differences in Value anywhere in the space forconstant Hue and Chroma appear the same. The situation with Hue is morecomplicated. For constant Chroma, equal Hue differences appear the same; however,the perceptual distance between Hue steps increases with Chroma. Thus, it isparticularly important to use constant Chroma if the Munsell metric is used to giveequal perceptual distances.

Light sources and colour temperatureMunsell standardization was done under a standard light source, illuminant C. Otherlight sources have different spectra, and the composition of light reflected by thestimuli will vary accordingly with the illuminant. Illuminant C has approximately equalamounts of visible wavelengths and appears more or less white. Incandescent light hasrelatively more long wavelengths (yellow to red) and fewer short wavelengths (blue),and thus appears yellowish. Gerhardstein et al. used ‘natural light at midday’ inExperiment 1. Provided this was from the north on a clear day, this would haveapproximated illuminant C. The spectral composition of lights can be designated by‘colour temperature’. The colour temperatures of the illuminants considered here areabout 2500 K and 7000 K for incandescent light and illuminant C, respectively. Theshort-wavelength component increases, and the long-wavelength component decreasesas the temperature increases from 2500 to 7000 K (red hot to white hot).

CIE (L*u*v*)The CIE (Committee International D’Eclairage) has several systems for describingcolour. The system that we use here is recommended for describing differences incolour appearance. L*u*v*are the axes of the colour space, and equal distances in thespace are intended to correspond with equal perceptual distances. In other words, it isperceptually uniform. L* is lightness; u* is a red–green axis; and v* is a blue–green axis.Figure A1 shows good examples of English chromatic categories as landmarks andillustrates the effect of changing the illuminant from daylight to incandescent. The maineffect is compression along the blue–yellow axis and, to a lesser extent, expansionalong the red–green axis

Representing illuminant changes in Munsell spaceJust as the effect of illuminant can be illustrated by transitions in CIE colour space, itcan also be done in Munsell space. For each Munsell colour viewed under incandescentlight, there is an equivalent Munsell colour viewed under illuminant C, at leasttheoretically. Table 1 shows what happens to Gerhardstein et al.’s stimuli under thistransformation. It can be seen that both Value and Chroma increase for all stimuli, andthere are also shifts in Hue. In most cases, despite these changes, most pairs are stillapproximately equal in Chroma, and the differences in Value is preserved at about one.


Figure A1. Loci of good examples of English basic colour terms in CIE (u*v*) colour space for bothilluminants. Arrows show the changes produced by the shift from illuminant C to incandescent light.

Table A1. Munsell stimuli under illuminant C that would produce the same reflected light as theexperimental stimuli under incandescent light


Hue Value Chroma Hue Value Chroma

Within/near 10R 6 12 9.57R 7.51 21.375YR 7 12 3.92YR 8.58 21.65

Within/far 7.5RP 6 12 6.28R 7.40 19.805R 5 12 7.92R 6.50 21.83

Between/near 5R 5 12 7.92R 6.50 21.8510R 6 12 9.57R 7.51 21.37

Between/far 5R 5 12 7.92R 6.50 21.832.5YR 6 12 2.5YR 7.44 21.45


This is least so for the within–far pair, where there is now a Chroma difference of abouttwo. Hue differences are considerably reduced. Note, however, that the reduction inperceptual distance due to Hue differences is not as large as it first appears. This isbecause the perceptual difference due to a given Hue difference increases withChroma. Multiplying the Hue difference by about 5/3 (actual Chroma divided byintended Chroma) gives a better estimate of the perceptual difference relative to thoseunder illuminant C.


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