Perception, 2002, volume 31, pages 567 ^ 578
DOI:10.1068/p3315
Categorical perception of race
Daniel T Levin, Bonnie L Angelone
Department of Psychology, PO Box 5190, Kent State University, Kent Hall, Kent, OH 44121-0001, USA; e-mail:
[email protected] Received 4 April 2001, in revised form 16 July 2001
Abstract. Traditionally, research demonstrating categorical perception (CP) has assumed that CP occurs only in cases where natural continua are divided categorically by long-term learning or innate perceptual programming. More recent research suggests that this may not be true, and that even novel continua between novel stimuli such as unfamiliar faces can show CP effects as well. Given this, we ask whether CP is dependent solely on the representation of individual stimuli, or whether stimulus categories themselves can also cause CP. Here, we test the hypothesis that continua between individual faces that cross the categorical boundary between races show an enhanced CP effect. We find that continua running from a black face to a white face do, indeed, show stronger CP effects than continua between two black faces or two white faces. This suggests that CP effects are enhanced when continua run between two distinctly represented individual stimuli, and are further enhanced when those individuals are, in turn, members of different stimulus categories.
1 Introduction Categorical perception (CP) refers to the ability to discriminate between-category but not within-category differences along a stimulus continuum. It occurs when the cognitive or perceptual system treats a continuous set of stimuli as belonging to two or more discrete categories. CP is typically demonstrated with stimuli arrayed along a continuum between two endpoints. Using such stimuli, subjects typically complete two tasks that jointly support the conclusion that the continuum is perceived categorically. First, a classification task defines the boundary between classes associated with each endpoint. Once this is known, it is possible to predict that discrimination between pairs of stimuli will be most accurate when the stimuli straddle the empirically defined category boundary, and least accurate for stimulus pairs that fall within one of the categories. CP was first thought to be limited to `hardwired' perceptual and/or overlearned categories. Only recently have researchers shown that CP can occur for other quickly learned continua between a wide variety of individual stimuli. The goal of the current experiment will be to extend this research and demonstrate CP of race. Color, which is based on a continuum of wavelengths, presents a good example of CP. Even when two pairs of stimuli share the same differences in wavelength, it is much easier to discriminate between two colors of different categories (eg green and yellow) than two shades within the same category (eg green and green; Bornstein 1987). Initial research on CP focused on the categorization of naturally occurring continua. For example, Liberman et al (1957) observed CP on sound continua between different phonemes, and argued that the phenomenon reflects perceptual categories deeply embedded in language-specific components of the auditory system. Accordingly, they predicted that CP should not occur on novel continua, and confirmed this by showing that continua between `inverted' phonemes were not perceived categorically. Their own and other findings initially appeared to confirm the hypothesis that CP was indicative of an innate or overlearned tendency to create categories in naturally occurring continua (Liberman et al 1961; Stevens 1981), and even recently researchers have
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argued that CP on continua between facial expressions is a sign that these distinctions are innate (Calder et al 1996; Etcoff and Magee 1992). Although innate perceptual categories may induce CP, a fair number of findings suggest that CP is not limited to these categories. Infrahuman animals show CP effects with phoneme categories similar to those shown by humans (Kuhl 1981; Kuhl and Padden 1982), and CP has also been observed for continua between musical intervals or the first two notes of familiar tunes (Burns and Ward 1978; Smith et al 1994). More recently, CP has been observed on continua between artificial perceptual categories learned in the context of a single session experiment (Goldstone 1994; Livingston et al 1998). In addition, CP has been observed on continua between familiar faces (Beale and Keil 1995), and on continua between unfamiliar faces, even when these are inverted and of a different race from the observer (Levin and Beale 2000). Combined, these findings suggest that CP might be considered an index of the degree to which any pair of representations are learned beyond some criterion, and therefore can reflect either short-term or long-term learning. One issue that becomes apparent, especially in research testing for CP in faces, is that CP appears to reflect representations of individual stimuli or dimensions, as opposed to categories representing groups of stimuli. In most cases, the categories referred to when considering CP are actually variants of individual stimuli. Few studies have tested whether CP is stronger in continua crossing the boundary between categories of stimuli that have within them identifiable individuals. One exception is provided by Newell and Bu«lthoff (2000) who tested for CP on continua between different objects in the same basic-level category, or in different categories. They found CP in both cases and, from their table 1, it appears that for their correlation tests the effect was stronger in the within-category continua than in the between-category continua (t13 3:513, p 0:0038). Experiments demonstrating CP of facial expression might also serve as exceptions to this rule if one assumes that expressions represent categories of faces that have individuals within them, although expression is a transitory feature of faces thereby disallowing one central function of categories which is induction over time (Gutheil and Rosengren 1996). More important, though, is the fact that these experiments do not necessarily demonstrate CP effects that depend on existing representations of expression. If it is possible for CP to occur on continua between novel faces, as demonstrated by Levin and Beale (2000), then the CP effects observed by Etcoff and Magee (1992) and Calder et al (1996) could reflect ad hoc representations of specific individual faces or face configurations learned during the experiment. Given that faces may be processed by a functionally (and perhaps anatomically) dissociable system that is specialized for processing individual identity, it is possible that CP in this domain is limited to continua that differentiate individual faces. This conclusion appears to receive some support from findings suggesting that gender is not perceived categorically (Bu«lthoff and Newell 2000). However, given that this is an inherently negative finding, it is important to ask whether some variation in task or stimuli would allow CP of face categories. For example, although they are perceptible (Wild et al 2000), the cues specifying gender may be subtle enough to be overwhelmed by other information specifying individual identity. This is particularly true for faces with no hair as used in the study by Bu«lthoff and Newell (2000). Brown and Perrett (1993) asked subjects to classify hair-deleted prototype faces and found considerable between-subject variability in classifications, leading them to suggest that ``the perception of gender is subjective rather than absolute'' (page 838). Therefore, other potentially more easily perceived face categories might make better candidates for CP effects. In the present report, we test for CP of race categories. Although there is no evidence directly comparing the perceptibility of race and gender, race is easily and quickly
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perceived (Levin 1996), and the features specifying it are reasonably well understood by subjects (Shepard and Deregowski 1981). We tested for CP of race by comparing CP effects on continua that do and do not cross race boundaries. Given that Levin and Beale (2000) observed CP on continua between individual faces within races, we needed to create conditions where individual stimuli would be more difficult to code, while race categories would remain salient. Therefore, in the present experiment we used the paradigm developed in Levin (2000) to test for CP in mixed blocks of trials that included discrimination pairs from three different continua: one that crossed race-boundaries (with a black face and a white face as endpoints), and two within-category continua (one between two black faces and one between two white faces). It is well known in the memory literature that mixing stimulus categories reduces memory for individual items (see for example, Cofer et al 1966), and we expect that it will help reduce the learning that we presume causes CP on continua between individual faces. In addition, in Levin and Beale (2000), the individual faces that were tested in each block were shown to the subjects who were instructed to carefully examine them and to associate each with the name given in the instruction screen. Levin and Beale (2000) also used a `better likeness' task in which subjects were shown the A and B discrimination pair on each trial and were asked to determine which looked more like one of the named endpoint faces. Both of these features may have also contributed to Levin and Beale's positive finding for withinrace continua, and both were eliminated here via an ABX task in which subjects saw the A and B faces, then were asked which of the two matched a third face (X). We predicted that these differences would reduce CP effects for within-race continua because the presence of six unfamiliar, unnamed, individuals in the same block of trials would preclude subjects from learning them sufficiently to allow CP. If CP of race depends on previously known categories, then it should be stronger than CP on within-race continua in this mixed-block design. Before continuing, it is important to note that this experiment includes two conditions, a short-exposure condition [in which the A and B stimuli were visible for 1000 ms as in Levin and Beale (2000)] and a long-exposure condition (A and B stimuli exposed for 1300 ms). Ultimately, this difference had only a small effect, but because the conditions were run at different times, results are reported for each separately. 2 Method 2.1 Subjects A total of seventy-five undergraduate General Psychology students from Kent State University completed this experiment. Of these, thirty-three completed a short-exposure condition, and forty-two completed a long-exposure condition. Two subjects failed to meet the error criterion (significantly above-chance responding in the discrimination task) and were eliminated from the analysis, leaving a total of thirty-two subjects in the short-exposure condition, and forty-one in the long-exposure condition. Of the remaining subjects, four were black, two were Asian, sixty-seven were white, and fiftyone were female. The two conditions were run successively. All subjects completed the experiment in exchange for credit in their General Psychology class, none had participated in related experiments, and none had seen the specific faces used in the experiment. 2.2 Stimuli The stimuli and continua for this experiment were those used by Levin (2000). They include a total of sixteen faces, eight black and eight white. These faces were paired and used as endpoints in twelve continua, four within each of three types of continuum: black ^ black (BB), white ^ white (WW), and black ^ white (BW). To equalize similarity
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among these faces, ten judges rated the overall similarity of all possible pairs of stimuli using a 7-point likert scale (1 very similar, 7 very dissimilar). The eight faces within each race were grouped to make four pairs within the BB and WW continua. Also, four faces from each race were paired to create four additional continua in the BW condition. The set of face pairs was chosen so that rated similarity was equalized for between-race and within-race continua (mean similarity of between-race face pairsö 4.05, mean similarity of within-race face pairsö4.12). Once paired, all faces (including those of different races) were equalized for mean luminance and contrast. See Levin (1996) and Levin (2000) for additional details. The faces in each pair served as endpoints for continua created using the morph program from Gryphon Software (now defunct). This software was used to linearly interpolate the shape and luminance values of the two faces on 10% intervals between the two endpoints. In the end, subjects viewed a total of eleven stimuli from each continuum that represented controlled blends of the two endpoints. For ease of discussion, the continua began with 100% of one face and 0% of the other face and each intermediate face represented a percentage blend of the two faces. 2.3 Apparatus All stimuli in this experiment were presented in 256-level grayscale on 15-inch monitors controlled by MacOS computers. Subjects were run individually or in groups ranging in size from two to six, each on their own computer in the same room. 2.4 Procedure 2.4.1 Discrimination task. Subjects first completed a block of discrimination trials that included mixed trials from three different continua (one BB, one WW, and one BW). Thus, there were four groups of subjects in each condition (ie short exposure/long exposure). Each group saw one of the four continua for each BB, WW, and BW continuum-type. For a given subject, the endpoints of each continuum were different faces which meant that a given face was not an endpoint on two different continua. Recall that in the stimulus set as a whole, the endpoints on a given BW continuum also served as endpoints on other BB or WW continua. However, this `double duty' occurred between subjects. Discrimination accuracy was tested with an ABX task in which subjects first saw two faces from a given continuum (faces A and B, separated by 20% of the total continuum) presented side by side for 1000 ms in the short-exposure condition, or for 1300 ms in the long-exposure condition. Then they saw a third face (face X) for 1000 ms in the short-exposure condition, and for 1300 ms in the long-exposure condition. Once face X had disappeared, subjects decided whether face X matched face A or B. They responded by hitting the `1' key if the third face matched the one on the left and the `2' key if it matched the one on the right. Subjects completed 8 trials corresponding to each of nine AB pairs on each of the three continua for a total of 216 trials. 2.4.2 Classification task. After completing the discrimination task, subjects completed three blocks of classification trials. Each block included faces from one of the three types of continua used in the discrimination trials. For half of the subjects, the BW block came first, and for the other half one of the two within-race continua was first. Within these two groups the order of the within-race blocks was counterbalanced. To begin each block, the subjects were presented with two endpoint faces associated with arbitrarily assigned names. They were told that they would see a series of individual faces and would have to decide which of the two endpoint faces from the initial instruction screen the face looked more like. On each trial, subjects responded by hitting the `1' or `2' keys to indicate which of the two endpoints each face was
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most similar to. Each of the eleven faces was presented three times for a total of 33 randomly ordered trials within each of three blocks.(1) Other than instructions specific to responding in the task, subjects were not told the hypotheses under test until after they had completed the experiment. 3 Results 3.1 Contrast tests CP effects were tested in two different ways. First, the difference between the `crossboundary' discrimination pair and the other within-category pairs was tested. From the classification task, the 60%=40% pair crossed the boundary for each continuum type. The particular criterion used to select this pair was that one member of the pair had to be classified with the continuum endpoint on fewer than 33% of trials, while the other member of the pair had to be classified with the endpoint on more than 66% of trials. To test for CP, mean performance on within-category pairs was subtracted from performance on the cross-boundary pairs. CP effects were considered to have occurred if this measure was significantly greater than zero. This procedure follows that used by Etcoff and Magee (1992), Beale and Keil (1995), and Levin and Beale (2000). Relative success on the cross-boundary pairs for each continuum type was also entered into an ANOVA with continuum type (BB, BW, WW) as the single within-subjects factor. Analysis of the short-exposure condition shows that the CP occurred only on continua that crossed race boundaries (see figure 1). The advantage for cross-boundary pairs was significantly greater than zero for the BW continua (12.16%, t31 3:625, p 0:001) while it was nonsignificant for the BB and WW continua (BB: 2.44%, t31 0:87, ns; WW: 0.29%, t31 0:105, ns). The continuum-type effect was significant (F2, 62 4:014, MSE 0:032, p 0:0229). Pairwise comparisons show that the advantage for cross-boundary pairs was larger for BW continua when compared with BB and WW continua ( p 5 0:05, Duncan test). Similarly, results from the long-exposure condition show that CP effects were stronger for the BW continua. The advantage for cross-boundary pairs was significantly greater than zero for the BW continua (11.32%, t40 5:072, p 0:0001) while it was nonsignificant for the BB and WW pairs (BB: 3.73%, t40 1:825, p 0:0754; WW: ÿ3:96%, t40 ÿ1:49, p 0:144). The continuum-type effect was significant in the continuum-type ANOVA (F2, 80 11:335, MSE 0:021, p 5 0:0001). Pairwise comparisons show that the advantage for cross-boundary pairs was larger for BW continua when compared with BB ( p 5 0:05, Duncan test) and WW continua ( p 5 0:01, Duncan test). To compare the short-exposure and long-exposure conditions directly, the relative advantage for cross-boundary pairs was entered into a mixed-factors condition (long versus short exposure)6continuum type (BB, BW, WW) ANOVA. There was no main effect of condition, nor an interaction between condition and continuum type (Fs 4 1). (1) This classification task has the disadvantage of being focused on individual identity for all continua, including the between-race continuum. It is possible that had subjects been classifying the BW continua on the basis of race, the results may have turned out differently. Therefore an additional group of eight subjects classified all of the faces by race. In this task, subjects viewed a set of individual faces including all stimuli from all continua (including the BB, WW, and BW continua) and were told simply to classify them by race. This experiment produced a classification function that was similar to that observed in the main task. The 40%=60% pair again straddled the between-category boundary. In addition, global correlation tests were again computed collapsing across all BW continua, and were similar to those reported above. The predicted ^ actual discrimination correlation for BW continua was 0.87 in the short-exposure condition (as compared with 0.89 in the main analysis), and 0.86 in the long-exposure condition (as compared with 0.92 in the main analysis).
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In addition, mean discrimination accuracy was not different between conditions (67.1% correct in the short-exposure condition and 68.8% correct in the long-exposure condition; Fs 5 1). Finally, contrast tests were recomputed with continuum as the unit of analysis. For both the short and long-exposure conditions, the CP effects for the four betweenrace continua were significantly greater than the effects for the eight within-race continua. For each of the twelve continua in the experiment, the difference in mean accuracy for the cross-boundary pair and the rest of the pairs was computed, then entered into two three-level one-factor (continuum type: BB, WW, BW) ANOVAs, one for each condition. The continuum-type effect was significant in the longexposure condition, and was a nonsignificant trend in the short-exposure condition (F2, 9 10:694, MSE 0:03, p 0:0042 and F2, 9 2:884, MSE 0:004, p 0:1077, respectively). Planned contrasts comparing the within-race continua with the betweenrace continua were significant for both the long-exposure condition and the short-exposure condition (F1, 9 13:709, MSE 0:003, p 0:0049 and F1, 9 5:720, MSE 0:004, p 0:0405). 3.2 Correlation tests In addition to testing for CP using the contrast between cross-boundary pairs and within-category pairs, CP was assessed by testing the correlation between actual discrimination performance on each pair and performance as predicted by the classification task. This method avoids the pitfalls inherent to selecting a single cross-boundary pair
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and using it as the basis for comparison with all other pairs. For example, had the classification data been slightly different a CP effect would have been observed on WW continua if the 50%=70% pair had been selected as the single cross-boundary pair. Therefore CP effects were also tested with a method that does not involve this kind of contrast. Following Calder et al (1996), and Levin and Beale (2000), we generated performance predictions for each discrimination pair by multiplying the difference in percentage classification in the continuum endpoint category by 0.25, then adding the product to the mean performance for the two pairs at the continuum ends. The test for CP was the correlation between predictions and actual performance for the nine discrimination pairs on each continua. In the short-exposure condition, the correlations for the BB and WW continua were nonsignificant (BB: r 0:3101, p 0:417; WW: r 0:5636, p 0:114), while the correlation was significant for the BW continua (r 0:8938, p 0:001). In the long-exposure condition, the correlation for the WW continua was nonsignificant (r 0:0837, ns), while the correlations for the BB and BW continua were significant (BB: r 0:8205, p 0:007; BW: r 0:9230, p 5 0:001). Correlation tests with continuum as the unit of analysis show that correlation coefficients were significantly greater for between-race continua than within-race continua for the short-exposure condition, while this effect was a nonsignificant trend for the long-exposure condition. Correlation coefficients were entered into two threelevel one-factor (continuum type: BB, WW, BW) ANOVAs, one for each condition. The continuum-type factor was significant for the long-exposure condition, and nonsignificant for the short-exposure condition (F2, 9 11:09, MSE 0:029, p 0:0038, and F2, 9 1:606, MSE 0:185, p 0:2532, respectively). Planned contrasts comparing the within-race continua with the between-race continua were significant for the long-exposure condition, and revealed a nonsignificant trend for the short-exposure condition (F1, 9 9:680, MSE 0:029, p 0:0125 and F1, 9 3:075, MSE 0:185, p 0:1134). 3.3 Constant-face tests One feature of the stimulus set used in this experiment is that each endpoint face on the four between-race continua also served as an endpoint on a within-race continuum in a different group of subjects (eg if subject A saw the faces in set A, the black face endpoint on the BW continuum for this subject would also serve as an endpoint on a within-race BB continuum in stimulus set B seen by subject B). Therefore, it was possible to test whether variants of a given face showed stronger CP effects when they lay upon a between-race continuum as compared with when they fell on a within-race continuum. To compute this test, data were collapsed across the two conditions, and four sets of difference scores were generated. Each set was based on one of the four between-race BW continua. Discrimination accuracies for each of the five AB pairs closest to each face on each between-race continuum were compared with accuracies for analogous pairs for same faces when they fell upon within-race continua. These difference scores represent the discrimination advantage for variants of a given face when it fell on a between-race continuum. Difference scores for the black and white faces on the between-race continua were averaged to avoid unit-of-analysis problems (eg we do not have eight independent observations corresponding to the eight endpoints on the four BW continua because each endpoint on a given continuum affects variants of the other endpoint), producing a total of four sets of difference scores, one associated with each between-race continuum. These four sets of five scores were entered into a one-factor ANOVA with pair (0%=20%, 10%=30%, 20%=40%, 30%=50%, 40%=60%) as the single factor. The pair effect was nearly significant (F4, 12 2:956, MSE 0:003, p 0:0650), and a planned contrast comparing the between-race advant-
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age for the two pairs in the middle of the continua with the two pairs at the end was significant (F1, 3 13:434, MSE 0:003, p 0:0351), suggesting that variants of a given face that lay upon between-race continua showed a stronger CP effect than analogous variants that lay upon within-race continua (see figure 2). 3.4 CP effects in the first and second half of the discrimination task Given that some small CP effects may have occurred on within-race continua, we tested whether learning during the experimental trials could have accounted for these effects. Discrimination trials were run in two blocks of 108 trials, so we compared CP effects in the first and second half of the experiment under the assumption that learninginduced CP would be weaker in the first half of trials. Data were collapsed across the short-exposure and long-exposure conditions. The analysis revealed no CP for within-race continua in the first block of trials (figure 3). In the first block of trials, the cross-boundary advantage for all withinrace continua was 1.8% (t 5 1), and in the second block it was 3.5% (t72 2:10, p 0:039). The cross-boundary advantage for BB continua was 1.6% in block 1 (t72 5 1), and 5.6% in block 2 (t72 2:36, p 0:021). For WW continua, the advantage was 5.1% in block 1 (t72 ÿ1:92, p 0:058), and 1.5% in block 2 (t72 5 1). In contrast, between-race continua showed strong CP effects in both blocks (block 1: 9.3%, t72 3:65, p 5 0:001; block 2: 14.6%, t72 6:57, p 5 0:001). A two-factor ANOVA with block (1, 2) and continuum type (BB, BW, WW) as within-subjects factors confirmed that CP increased across blocks (from an average of 1.9% in block 1 to an average of 7.2% in block 2; F1, 72 7:282, MSE 0:043, p 0:0087), and that there 100
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was no interaction between block and continuum type (F 5 1) suggesting that CP increased equally across blocks for all continuum types. Correlation tests produced a similar pattern although, in this case, the CP effect for BW continua was strong and equivalent across blocks. The discrimination ^ classification correlation was 0.14 (ns) for within-race continua in block 1, and 0.57 ( p 0:11) for within-race continua for block 2. The correlation for BB continua was 0.57 for block 1 ( p 0:11), and 0.80 for block 2 ( p 0:01). The correlation for WW continua was ÿ0:27 (ns) for block 1, and 0.37 (ns) for block 2. The correlation for BW continua was 0.93 ( p 5 0:001) for block 1, and 0.95 for block 2 ( p 5 0:001). 3.5 Within-category discrimination on BW continua In the interest of completeness, and to allow comparison with the very similar paradigm used by Levin (2000), we analyzed within-category discrimination on BW continua. Discrimination was more accurate for AB pairs falling within the black end of the BW continuum than the white end of the continuum for the long-exposure condition (71.0% versus 65.5%; F1, 40 6:133, MSE 0:010, p 0:0176), but not the short-exposure condition (64.7% versus 65.4%, F 5 1). Although it is not possible to be certain why there was no black-end discrimination advantage in the short-exposure condition [which matched the timing parameters used in Levin (2000)], we would like to note that the subject populations in the previous experiment and this one are quite different, and that replications of the black-end discrimination advantage [and of the relationship between the discrimination advantage and poor recognition of black faces; see Levin (2000)] in the new population have been repeatedly successful with slower presentations and discrimination tasks with fewer trials (Levin and Lacruz 2000). 4 Discussion When stimuli from different continua are mixed in a single block of trials, CP on between-race continua is stronger than CP on within-race continua. This is true both in tests using subjects and in those using stimuli as the unit of analysis. Accordingly, CP is facilitated both when individual faces are adequately represented (Levin and Beale 2000), and also when they represent different face categories. This result replicates across the two separately run conditions, especially for the contrast tests. The correlation tests are similar with the exception that the BB continua also show a small CP effect for the long-exposure condition (which was nearly significant). This latter result was, however, not significant in the first block of trials. The small CP effect observed for some within-race continua, if it proves consistent, suggests that subjects may be able to represent more than one face in a mixed block of trials, and that this representation might be sufficient to drive a small CP effect. Thus, it appears that CP effects may vary continuously with the burdens placed on the subjects' ability to represent the endpoints. A number of factors seem to affect the short-term representability of face endpoints. Most clearly, if the faces are already familiar to some degree, CP effects are larger (Beale and Keil 1995), and the present report suggests that if they represent different categories the effect is also facilitated. Other factors may include the similarity of the faces, and the degree to which they can be coded effectively. This would converge with Levin and Beale (2000), who found that CP effects were reduced considerably in magnitude when half continua were tested. These continua were subsets of their original continua between unfamiliar faces which began near the original midpoints, and continued to the original endpoints. Thus the new endpoints were more similar to each other given that one was distorted to look more like the other. It is interesting to note that reduced CP on continua with similar endpoints contrasts with at least one finding that CP is increased by increased shape similarity.
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Newell and Bu«lthoff (2000) tested for CP in continua between computer-generated images of everyday objects. They found that CP effects decreased in low-similarity object pairs. Logically, at least two differences between this finding and the findings in faces could account for the discrepancy. First, it is possible that CP only occurs within a limited range of raw perceptual similarity, and that increases or decreases in similarity beyond this range eliminate the effect. This makes sense if CP represents a differentiation process which is not necessary if representations are sufficiently distinct, and is impossible if the objects are too similar. The other possibility is that qualitative differences between faces and other objects are responsible. Clearly, one hypothesis is that different perceptual subsystems are responsible for processing faces and other objects (for review see Farah 1995). Broadly, these findings expand the scope of CP. The most basic way to conceptualize CP is that it reflects a psychological remapping of physical variation into psychological categories. Thus, some physical variation is deemphasized and some is enhanced. Traditionally, this has meant that within-category variation is deemphasized because it represents perceptually irrelevant noise. In addition, this variation usually reflects variation around a single exemplar or higher-level perceptual/conceptual token. Thus, the categories on the continuum between /b/ and /p/ constitute the set of possible variants in the instantiation of single phonemes. The same is true of CP on continua between individual faces. Again, the category is the set of variants on a single exemplar (eg a single face). Variation within the category is assumed to consist of small, psychologically unimportant variations in view, lighting, and even adiposity. CP on continua between facial expressions (Calder et al 1996; Etcoff and Magee 1992) might be considered a similar case in that within-category variation putatively reflects relatively unimportant variation in a given attribute or feature of some object. CP based on race categories extends the CP phenomenon beyond variance-reduction to include categorical effects that derive from categories more like the generic knowledge categories discussed by Medin and Barsalou (1987). In particular, Medin and Barsalou point out that the typical sensory ^ perceptual categories underlying CP are made up of exemplars that have no individual identity, and are transient (for example, two different instantiations of a particular phoneme, or some set of exemplars instantiating a given face). In contrast, races are made up of individuals who are permanent, and have individual identities. Accordingly, faces represent a case where CP can be driven by a nested set of categories with varying levels of generality. CP occurs on continua between individual faces, and it occurs on continua between sets of those faces. A given face can therefore serve as an endpoint on different categorically perceived continua, each representing a different level of classification. In a sense, this is similar to CP of facial expressionöone could argue that happy faces and sad faces constitute categories that include a set of individual faces. However, in the case of race categories, both the category and the identity of the faces are permanent features, thus allowing characteristic functions of generic knowledge categories such as induction over time (Gutheil and Rosengren 1996). These findings suggest the need for a comprehensive understanding of the factors that influence the short-term learnability of new object representations. Similarity, categorical differences, and perhaps the cognitive context where the new stimuli are recognized all probably contribute to CP. On this view, the notion that CP represents a perceptual effect limited to some stage of advanced learning is untenable. Rather, CP seems more like an index of the degree to which object representations are differentiable. In addition, however, CP may be affected by more stimulus-bound constraints in that difficulty of creating (or even conceptualizing) a continuum between endpoints with different part configurations may preclude an adequately controlled test of the degree to which category membership affects discriminability.
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