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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D T Levin, B L Angelone
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 ˆ 3:513,
p ˆ 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
Categorical perception
569
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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D T Levin, B L Angelone
among these faces, ten judges rated the overall similarity of all possible pairs of stimuli
using a 7-point likert scale (1 ˆ very similar, 7 ˆ 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
Categorical perception
571
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 ˆ 3:625,
p ˆ 0:001) while it was nonsignificant for the BB and WW continua (BB: 2.44%,
t31 ˆ 0:87, ns; WW: 0.29%, t31 ˆ 0:105, ns). The continuum-type effect was significant
(F2, 62 ˆ 4:014, MSE ˆ 0:032, p ˆ 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 ˆ 5:072, p ˆ 0:0001) while it
was nonsignificant for the BB and WW pairs (BB: 3.73%, t40 ˆ 1:825, p ˆ 0:0754;
WW: ÿ3:96%, t40 ˆ ÿ1:49, p ˆ 0:144). The continuum-type effect was significant in
the continuum-type ANOVA (F2, 80 ˆ 11:335, MSE ˆ 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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D T Levin, B L Angelone
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Figure 1. Classification and discrimination data from short-exposure and long-exposure conditions
averaged over all stimulus sets.
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 ˆ 10:694, MSE ˆ 0:03, p ˆ 0:0042 and F2, 9 ˆ 2:884, MSE ˆ 0:004, p ˆ 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 ˆ 13:709, MSE ˆ 0:003, p ˆ 0:0049 and F1, 9 ˆ 5:720, MSE ˆ 0:004,
p ˆ 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
Categorical perception
573
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 ˆ 0:3101, p ˆ 0:417; WW: r ˆ 0:5636, p ˆ 0:114), while
the correlation was significant for the BW continua (r ˆ 0:8938, p ˆ 0:001). In the
long-exposure condition, the correlation for the WW continua was nonsignificant
(r ˆ 0:0837, ns), while the correlations for the BB and BW continua were significant
(BB: r ˆ 0:8205, p ˆ 0:007; BW: r ˆ 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 ˆ 11:09, MSE ˆ 0:029, p ˆ 0:0038,
and F2, 9 ˆ 1:606, MSE ˆ 0:185, p ˆ 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 ˆ 9:680, MSE ˆ 0:029, p ˆ 0:0125 and F1, 9 ˆ 3:075, MSE ˆ 0:185,
p ˆ 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 ˆ 2:956,
MSE ˆ 0:003, p ˆ 0:0650), and a planned contrast comparing the between-race advant-
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Difference in accuracy=%
20
16
Figure 2. Discrimination advantage for between-race
continua by AB pair. In this case, cross-race and withinrace variants are distortions of the same faces. The figure
shows only half of a continuum, constituting variants
of a single face. Therefore, the 0%=20% and 10%=30%
pairs constituted the within-category pairs which were
contrasted with the 30%=50% and 40%=60% pairs, which
constituted the between-category pairs.
12
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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 ˆ 13:434, MSE ˆ 0:003, p ˆ 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 ˆ 2:10,
p ˆ 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 ˆ 2:36, p ˆ 0:021). For WW continua, the advantage was 5.1% in block 1 (t72 ˆ ÿ1:92, p ˆ 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 ˆ 3:65, p 5 0:001; block 2: 14.6%, t72 ˆ 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 ˆ 7:282, MSE ˆ 0:043, p ˆ 0:0087), and that there
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Figure 3. Discrimination performance in the first (a) and second (b) block of discrimination trials.
Categorical perception
575
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 ˆ 0:11) for
within-race continua for block 2. The correlation for BB continua was 0.57 for block 1
( p ˆ 0:11), and 0.80 for block 2 ( p ˆ 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 ˆ 6:133, MSE ˆ 0:010, p ˆ 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.
576
D T Levin, B L Angelone
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.
Categorical perception
577
Acknowledgements. Portions of this work were presented at the November 1998 Psychonomics
Conference in Dallas, Texas. Thanks to D J Angelone for reading and commenting upon drafts
of this report. Preparation of this report was supported by National Institute of Mental Health
grant #1-R03-MH60137-01 to DTL.
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