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Study 1- Categorical perception of gender information

CATEGORICAL PERCEPTION OF FACIAL GENDER INFORMATION : BEHAVIOURAL EVIDENCE AND THE FACE-SPACE METAPHOR

Campanella, S., Chrysochoos, A., Bruyer, R.

Cognitive Neuropsychology Unit (NECO), University of Louvain-la-Neuve (UCL), Belgium

Running title : Categorical perception of gender information

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Study 1- Categorical perception of gender information Abstract

Categorisation is a fundamental property of the human brain. We used an imagemorphing procedure to investigate the categorical perception of facial gender information. Three experiments, an identification and two matching tasks, were reported. First, we showed that, even when facial image information changes linearly across unfamiliar male and female faces, gender is perceived categorically. This holds only when faces are presented in an upright orientation. Second, subjects discriminated more easily two unknown morphed faces presenting a gender change as compared to two unknown morphed faces belonging to the same gender, even when the physical distance between the pairs was identical. We discuss the results in terms of how representations of faces are encoded and stored in long-term memory.

Key-words : categorical perception, gender discrimination, face-space metaphor

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Study 1- Categorical perception of gender information 1. Introduction The human brain is not able to process all the stimuli present in the environment. Processing can be simplified, however, by grouping different stimuli, which share common properties, into a single category (Rosch, Mervis, Gray, Johnson and Boyes-Braem, 1976). This categorisation process can operate at several levels of information processing. For example, we can establish a set including the items "wife", "children", "dog" and "money". This set is not defined by the common physical attributes of its members but rather by its abstract meaning such as "what we have to save if

our house is on fire". The abstract meanings of the stimuli are needed to operate this

categorisation while physical attributes are not: this is a form of "semantic categorisation". Categorisation processes are also used at a perceptual level, as is illustrated by our perception of colour. Colours can be defined on a physical continuum by a gradual variation of light wavelenghts. Nevertheless, we perceive chunks of colours in spite of linear changes in the physical signal. Moreover, Bornstein and Korda (1964) showed that if we consider two pairs of equidistant light frequencies, subjects discriminate more easily the pair which is constituted by two stimuli belonging to two different categories (green-yellow) than the pair defined by two stimuli belonging to the same category (two different tones of yellow). Categorisation processes can thus modulate the way we perceive the physical attributes of a stimulus : this is "perceptual categorisation". More precisely, the advantage for inter- over intra-categorical discriminations (the better discrimination between two stimuli belonging to different categories relative to stimuli from the same category) is known as "the categorical perception effect" (Harnard, 1987). This latter type of perceptual categorisation defines the scope of this paper. The categorical perception effect was initially observed on unidimensional stimuli such as speech sounds and colours (Liberman, Harris, Hoffman and Griffith, 1957; Bornstein & Korda, 1964). However, recent developments and applications of computer image-manipulation techniques have made the investigation of multi-dimensional stimuli, such as the human face, possible. This is of interest because even though the ability to recognise specific individuals must be learned and the continua between individual faces are not naturally occuring, there might be general constraints on category formation that also apply to individual face recognition (Harnad, 1987). Thus, during the last few years, several studies have been published showing categorical perception of familiar 3

Study 1- Categorical perception of gender information identities (Beale & Keil, 1995; Stevenage, 1998). Beale and Keil (1995) used stimuli morphed between two familiar identities (i.e. Presidents Kennedy and Clinton). Subjects were confronted with both identification and matching tasks. The identification task allows the identification of the categorical boundary of a continuum while the matching task shows a better discrimination of faces straddling this categorical boundary as compared to faces stemming from the same category. The results strongly suggested that there was categorical perception of familiar facial identities. In other words, subjects discriminated more easily the identities of two faces belonging to two different people than the identities of two faces belonging to the same person, even when the physical distance between the stimuli within each pairs was kept constant. Moreover, Stevenage (1998) performed an experiment where she used photographs of twin faces (Rosie and Elizabeth). Subjects had to rate the similarity of pairs of photographs (same : Rosie-Rosie; Elizabeth-Elizabeth; different : Rosie-Elizabeth) before and after a category learning session of the two distinct faces. She found evidence for (1) a "compression effect", i.e. subjects judged the same-twin pairs as more similar after than before the learning session and (2) an "expansion or separation effect", i.e. subjects judged the different-twin pairs as more different after than before the category learning phase. These data provide some evidence about the mechanisms responsible for categorical perception effects. Indeed, a categorical perception effect could arise from acquired equivalence or distinctiveness (Goldstone, 1994a; Livingston, Andrews & Harnad, 1998). According to acquired equivalence, there is an increase of perceptual sensitivity to similarities (among instances of a same category) that are relevant for a categorisation (Nosofsky, 1986). This mechanism would be responsible for the within-categorical compression effect. According to acquired distinctiveness, there is an increase in perceptual sensitivity to differences that are relevant for a categorization (Gibson, 1969; Nosofsky, 1986). This mechanism would be responsible for the between-categorical separation effect. Therefore, the categorical perception effect observed for familiar identities is due to the way faces are stored in memory, such that differences and similarities can be extracted through comparisons between stored exemplars, or relative to a facial prototype (Valentine, 1991). Accordingly, categorical perception is correlated with face familiarity (Beale & Keil, 1995) and should not be observed with unfamiliar faces (Goldstone, 1998).

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Study 1- Categorical perception of gender information Whilst identity is an important dimension of facial information, it is not the only dimension used in face recognition (Bruce & Young, 1986). Over the last few years, most of the published studies on categorical perception of faces have concentrated on facial expressions (Etcoff & Magee, 1992; Calder, Young, Perrett, Etcoff and Rowland, 1996; Granato, Bruyer and Révillion, 1996; de Gelder, Teunisse and Benson, 1997; Young, Rowland, Calder, Etcoff, Seth, and Perrett, 1997; Bruyer & Granato, 1999). These data are particularly important because they allow us to define the perceptual basis of how emotions are recognised. Strong evidence has been reported showing that facial expressions are perceived as belonging to qualitatively discrete categories and not as varying continuously along certain underlying dimensions (Woodworth & Schlosberg, 1954 ; Ekman, 1982). A further dimension of faces is gender. Contrary to the case with facial expressions (which can be accounted for by a number of 6 emotional categories, see Ekman, 1994) or facial identities (which are potentially defined by an infinite number), the categories implied in the gender discrimination process are limited to two. Moreover, we can imagine that,

from a sexual

reproduction viewpoint, the categories "attractive/unattractive" might be better psychological constructs than "male/female". With this in mind, it could be that the male/female distinction could as easily be continuous rather than categorical. Accordingly, we can wonder whether subjects confronted with facial gender information that varies linearly will perceive it categorically or not. This was investigated here. By means of a morphing procedure, we generated continua of facial stimuli in which gender information was varied linearly. The aim of this paper was to evaluate whether a categorical perception effect can be evidenced for facial gender information. Categorical perception needs two stages to be assessed : (1) an identification task, which has to show non-linear responses to linearly manipulated stimuli and to define boundaries within each continuum, and (2) a delayed matching task, which has to evidence an enhanced discriminability for between- as compared to within-category pairs of morphed faces. Both pieces of evidence were collected in the present study in three separate experiments.

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Study 1- Categorical perception of gender information EXPERIMENT 1 The goal of Experiment 1 was to show, by means of an identification task, that even if subjects are confronted with upright morphed faces in which gender information varies linearly, they will perceive this gender information in discrete categories. However, it is possible that this categorical effect was (1) an artifact due to experimentator instructions, which forced subjects to classify morphed faces into two pre-defined categories(male/female), or (2) a "technical" artifact linked to the use of a morphing procedure. To overcome these objections, the same experiment was run with a new set of subjects but with faces presented upside-down. Inversion has frequently been used in research on face perception as a control for the role of nonface-specific properties of the material due to the fact that inversion is supposed to alter the perception of the configurational information conveyed by faces (Valentine, 1988; de Gelder et al., 1997). Indeed, it is nowadays well-accepted that humans are experts in the recognition of faces (Carey, 1992). We are also experts in the gender recognition of male and female faces. Experts differ from novices in their enhanced sensitivity to the configural properties of a stimulus (Tanaka & Gauthier, 1997). Then, if inversion "disturbs" the categorical perception effect evidenced with upright faces, the information relevant to induce this effect on upright faces should be carried by configural information and it does not represent the result of the experimentator's instructions or the morphing technique. However, if the same categorical results are obtained in the UPRIGHT and INVERTED conditions, it would mean that the categorical perception effect is unrelated to configural cues and may rather be due to author's instructions or to technical artefacts due to the use of the morphing procedure.

2. Method 2.1. Subjects Thirty-two volunteers (16 females) of the Department of Psychology (Louvain-la-Neuve) took part in this experiment. Sixteen (8 females) were given the identification task with upright faces, the other half received inverted faces (180°) to identify. All were aged between 18 and 25 years, reported no neurological disease and had normal or corrected-to-normal vision.

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Study 1- Categorical perception of gender information 2.2. Stimuli Three unfamiliar male faces (M1, M2 and M3) and three unfamiliar female faces (F1, F2 and F3) were photographed (Figure1). Nine continua of male/female pairs were possible (M1/F1, M1/F2, M1/F3, M2/F1, M2/F2, M2/F3, M3/F1, M3/F2 and M3/F3). By this way, each male face was paired to each female face, and vice-versa.

Figure 1-

F1

F2

F3

M1

M2

M3

Photographed source faces of three unfamiliar females (F1, F2 and F3) and three

unfamiliar males (M1, M2 and M3).

Five morphed images were created for each continuum. These were prepared by blending two faces in proportion 90:10 (i.e., 90% M and 10% F), 70:30, 50:50, 30:70 and 10:90. We will refer to these as 90%, 70%, 50%, 30% and 10% morphs along the appropriate continuum (see Figure 2 for illustration).

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Study 1- Categorical perception of gender information

90%

70%

30% Figure 2-

50%

10%

Illustration of the morphed faces for continuum M2-F3 with, from upper left, M2

90%/F3 10%, M2 70%/F3 30%, M2 50%/F3 50%, M2 30%/F3 70% and M2 10%/F3 90%.

The preparation of each continuum involved five stages. First, photographic quality images (digital camera) of faces were chosen as source-images. Models were selected as being devoid of beard, moustache and glasses. All faces were taken from a frontal view and with a neutral facial expression. Second, these photographs were downloaded onto a Macintosh computer where they were edited by Adobe Photoshop 4.0.1. to remove backgrounds, everything below the chin, hair and ears. Coloured-scale images were created, matched for skin tone and colouration and scaled to 150 X 191 pixels. Third, morphed stimuli were generated using the Morph 2.5. program. One hundred and fifty points were located manually onto the sources. The locations of these points were specified in terms of facial features such as corners of the mouth, tip and rest of the nose, outlines of the eyes and various anatomical landmarks. The same method was applied to each other source so that there was a correspondance of the 150 points for all sources. Fourth, a vector equation for each of the 150 points was computed on the sources to determine which position a point on, say, M1's 8

Study 1- Categorical perception of gender information face, will have on the morphed image after moving to 10, 30, 50, 70 or 90% of the distance to the position of the corresponding point on, say, F1's face. Fifth, the Morph program used a warping procedure to move from one source to the other by allowing the shift of the 150 control points from their initial position (in one source) to their final position (in the other) along linear changes. For example, in the 90% M2 /10% F3 morphed face, the pixel intensities have deformed the M2's face 10% toward the F3's face and the F3's face 90% toward the M2's face. In total, 45 images were drawn (5 from each of the 9 continua) and were prepared in upright and upside-down (180°) orientations.

2.3. Design and procedure The 45 morphed faces were displayed for gender identification. Upright faces were used in the upright condition (UPRIGHT) while upside down faces were used in the reversed condition (INVERTED). The images appeared one at a time, in random order, on a 256 coloured scale 15" Macintosh monitor. The viewing distance was 1 meter and every stimulus had a size of 60 X 78 mm. The task involved a two forced-choice decision : the subjects had to make a gender decision for each displayed image. Responses were made by pressing with the right index finger one of two buttons which were labelled accordingly. The labelling of these two buttons was randomized across subjects. Each trial involved : the display of a fixation cross (300ms), then a blank (300ms) and finally the morphed image which remained in view until a button was pressed or a delay of 3400ms. No feedback was given about the subjects's responses. Choice of gender and latency were recorded. Each subject was shown randomly 10 blocks of 27 images. These 270 trials were formed with the 45 morphed faces repeated 6 times each. An additional pre-experiment block of 15 random stimuli was discounted as practice. Note that the source-faces (Figure 1) were never shown in the experiment. To ensure that the source-faces were correctly referred to their gender, 10 additional subjects (5 females) had to take a gender decision for each of the 6 source-faces. All faces were correctly referred to their gender category with a mean latency of 427 ms for F1, F2 and F3 and 426ms for M1, M2 and M3.

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Study 1- Categorical perception of gender information 3. Results Figure 3 illustrates the mean frequencies with which subjects identified each morphed image as male or female in each condition (UPRIGHT vs INVERTED).

Mean frequency of responses 100 90 80 70

UPRIGHT-M INVERTED-M UPRIGHT-F

60 50 40

INVERTED-F

30 20 10 0 10

30

50

70

90

morphs

Figure 3-

Mean frequency of responses (%) in the identification task for subjects of the

UPRIGHT condition and subjects of the INVERTED condition.

These percentages were computed from 4320 data points (16 subjects X 6 occurrences of each 5 morphs from the 9 continua) recorded from a two-way choice. An ANOVA was computed for percentages of "female" responses with morphs (90%, 70%, 50%, 30% and 10%) as a withinfactor and orientation (UPRIGHT vs INVERTED) as between-factor. One subject in the INVERTED condition was rejected as an outlier (70% of the responses were out of time). The analyses showed a significant main effect of morphs (F 4,116 = 929,6; p < .0001) and a significant interaction morphs X orientation (F 4,116 = 30,3; p < .0001), while orientation was not significant (F 1,29 = 0,679; NS). The main effects indicate that the percentages of male and female responses were modulated by the intensity of the morphing defining the images, whilst orientation had no effect on how subjects identified the gender on these morphed faces. More interestingly, the interaction morphs X orientation indicates that the morphing effect changed for upright and inverted faces (Table 1). Post-hoc tests were computed using one-way ANOVAs with a within-subjects 10

Study 1- Categorical perception of gender information factor of morph (with 5 levels). There was a clear effect of the morphs for both the UPRIGHT ( F 4,60 =982,4; p < .0001) and the INVERTED (F 4,56 = 237,7; p < .0001) conditions. However, polynomial tests showed that linear, quadratic and cubic contrasts were significant in the UPRIGHT condition while quadratic and cubic contrasts were not significant in the INVERTED condition (UPRIGHT : linear : F 1,15 = 3680,7; p < .0001; quadratic : F 1,15 = 4,976; p = .041; cubic : F 1,15 = 197,9; p < .0001; INVERTED : linear : F 1,14 = 530,5; p < .0001; quadratic : F 1,14 = 1,748; NS; cubic : F 1,14 = 4,1; NS).

UPRIGHT

INVERTED

10%

30%

50%

70%

90%

4

11

53

85

93

(1,56)

(1,98)

(2,89)

(2,46)

(1,58)

12

31

53

70

86

(1,61)

(2,04)

(2,98)

(2,54)

(1,64)

Table 1- Mean percentages of female responses (SD) to morphed faces composed of 10%, 30%, 50%, 70% and 90% of female information along the nine continua "male-female" (Exp.1). In the UPRIGHT condition, the 10%, 30%, 70% and 90% morphed faces were referred to their correct gender on at least 85% of the occasions. Only the 50% morphed images gave rise to ambiguous responses. Indeed, the significant cubic function suggests the existence of two clear regions, one where the end-points of the continuum (10%, 30%, 70% and 90%) were clearly referred to their sources, and another around the mid-point (50%) of the continnum, which led to an abrupt shift from one gender decision to the other. This is consistent with a categorical perception effect. However, the frequencies of the responses in the INVERTED condition did not delineate clearly similar regions. Now the 10% and 90% morphed faces were correctly referred to their

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Study 1- Categorical perception of gender information gender but gender decisions were difficult to 30% and 70% morphed faces. Here performance seems to vary continuously, as a function of the linear shifts in facial images. The presence of categorical perception can also be assessed statistically by considering response latencies (Figure 4).

Mean responses latency 1200 1100 1000 Ms

UPRIGHT

900

INVERTED

800 700 600 10

30

50

70

90

Morphs

Figure 4-

Mean responses latency (ms) in the identification task for subjects of the UPRIGHT

condition and subjects of the INVERTED condition.

Latencies for each condition and for each continuum were collapsed into those for morphs closest to the sources (10% and 90% = distance 1), morphs further from the sources (30% and 70% = distance 2) and morphs furthest from the sources (50% = distance 3). A 3 X 2 ANOVA with distance (1, 2, 3) as a within-subject factor and orientation (UPRIGHT, INVERTED) as a betweensubject factor showed significant main effects of orientation (F 1,29 = 4,351; p = .046) and distance (F 2,58 = 24,7; p < .0001) along with a significant interaction (F 2,58 = 5,482; p = .007). The main effect of orientation showed that subjects identified more easily the gender of upright morphed faces than the gender of reversed faces. Moreover, the main effect of distance indicated that the distance from the sources had a definite cost on the ability to categorise the morphed images. Subjects categorised more easily (1) the 10%, 30%, 70% and 90% morphed faces as male or female relative to the 50% morphed images (UPRIGHT : t 15 = 3,985; p = .001; INVERTED : t 14 = 12

Study 1- Categorical perception of gender information 3,319; p = .005) and (2) the 10% and 90% morphed faces relative to the 30% and 70% morphed faces (UPRIGHT : t 15 = 4,765; p < .0001; INVERTED : t 14 = 3,916; p = .002). The distance X orientation interaction was assessed by analyzing the data separately for UPRIGHT and INVERTED faces. In the UPRIGHT condition, there was a significant main effect of distance ( F 2,30 = 16,456; p < .0001) with the linear polynomial contrast and the quadratic polynomial contrasts also being significant (respectively, F 1,15 = 17,617; p = .001 and F 1,15 = 5,066; p = .04). The same analysis for the INVERTED condition showed a significant main effect of distance and a significant linear polynomial contrast (respectively, F 2,28 = 12,532; p < .0001 and F 1,14 = 14,61; p = .002), but the quadratic polynomial contrast was not significant (F 1,14 = 2,551; NS). This suggests that the difference between morphs of distance 3 and 2 was more marked on RTs than the difference between morphs of distance 2 and 1, but only in the UPRIGHT condition (Table 2) .

UPRIGHT

INVERTED

Distance 1

Distance 2

Distance 3

747

851

1043

(60)

(72)

(93)

1034

1109

1150

(62)

(74)

(96)

Table 2- Mean latencies (SD) to morphed faces of distance 1 (10% and 90%), distance 2 (30% and 70%) and distance 3 (50%) (Exp.1)

4. Discussion The results of Experiment 1 show the following. First, both response frequencies and latencies in the UPRIGHT condition suggest that each continuum between male and female faces can be defined by two clear regions : one (near the end-points) where morphed faces are clearly

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Study 1- Categorical perception of gender information referred to their gender, and another (around the mid-point) where morphed faces trigger ambiguous responses. Nevertheless, in the UPRIGHT condition, 30% and 70% morphed faces were assigned 87% of the time to the gender which predominantly constituted the images, such as 10% and 90% morphed faces were referred to their sources. However, this tendency to associate 30% and 70% morphed faces to their sources was less clear in the INVERTED condition, decreasing towards 70% for inverted morphed faces. Second, analyses of response latencies showed that, in the UPRIGHT condition, subjects identified more easily the gender of morphs of distance 1 than morphs of distance 2, and morphs of distance 2 more easily than morphs of distance 3, but the difference between morphs of distances 3 and 2 was more marked than the difference between morphs of distances 2 and 1. This was not the case for the INVERTED condition : indeed, for inverted faces, the difference between morphs of distances 1 and 2 and morphs of distances 2 and 3 was not significant. Taken together, these data indicate that, for the UPRIGHT condition, despite the fact that a linear continuum of gender information has been created, there was some tendency towards categorical perception. This tendency was reduced for inverted faces. In the INVERTED condition, the difficulty of the gender decision increased linearly as a function of the linear manipulations affecting the gender information in the morphed faces. The data in the INVERTED condition also show that the information relevant for the categorical perception effect is carried by facial configuration and not the result of a methodological (instructions, morph procedure) artifact (see also de Gelder et al., 1997). It is interesting to note that these data link to the notions of how categorisation develops with expertise. Humans are experts in the recognition of faces (Carey, 1992) and face recognition is disproportionnaly impaired by inversion (Yin, 1969) as compared to other objects. Accordingly, using a manipulation of expertise level with face stimuli, Rhodes, Tan, Brake and Taylor (1989) showed that the effect of inversion was larger for faces of the subjects' own race than for different race faces. These results can be accounted for by the fact that (1) inversion is supposed to alter the configurational information present in faces (Valentine, 1988), and (2) novices use a feature-based strategy and experts use a holistic (configurational) strategy. Accordingly, for normal faces, subjects recognised parts better in the whole face than in isolation while this is not the case when faces are inverted (Tanaka & Farah, 1993). Applied to the current 14

Study 1- Categorical perception of gender information context, we may presume that people are experts in recognising (upright) male and female faces. The present data suggest that (1) when processed configurally (in the UPRIGHT condition), face gender is perceived categorically, and (2) when processed featurally (in the INVERTED condition), face gender is perceived linearly. This fits with there being a principal role for configural information in perceptual expertise and suggests that configurational cues are important for gender category formation. In sum, Experiment 1 achieved a triple goal : (1) the first stage, to assess categorical perception of gender information (i.e. subjects giving non linear responses to linearly manipulated stimuli) was fulfilled; (2) the UPRIGHT condition allows us to define on the basis of the identification task the categorical boundary of each continuum individually (see later), and (3) the INVERTED condition assessed the validity of these results. In Experiment 2, we assessed categorical perception of gender information using a matching task. The hallmark of categorical perception is better discrimination across category boundaries than within categories (Harnard, 1987; Young et al., 1997). We tested whether there was an enhanced discriminability for stimuli crossing the categorical gender boundary in a delayed "samedifferent" matching task.

EXPERIMENT 2

Experiment 2 used a delayed "same-different" matching task on two morphed images (A, B) successively shown to subjects who had to decide whether B was physically identical or not to A. This task shares the same goal as the usual ABX discrimination task used in the categorical perception literature, with the advantage that the memory load component is reduced. Categorical perception is defined as an enhanced discriminability of between-category relative to withincategory stimuli (Young et al., 1997). We asked whether the discrimination of morphs crossing the subjective categorical boundary (as defined by Experiment 1) is facilitated relative to the discrimination of morphs that remain within the same category, even if the physical differences between each pair is kept constant. If there is categorical perception, then responses for "different pairs" will be better for "between-category" (2 stimuli crossing the boundary) than for "within15

Study 1- Categorical perception of gender information category" pairs (2 stimuli being closer to the source), the difference between A and B being held constant (20%) in the two conditions.

5. Method 5.1. Subjects Fourteen new volunteers (7 females) of the Department of Psychology took part in the experiment. All were aged between 18 and 25 years, reported no neurological disease and had normal or corrected-to-normal vision.

5.2. Stimuli Data of Experiment 1 were used to define boundaries between categories. For example, Figure 5 shows the percentages of "male" and "female" responses for each stimuli (10%, 30%, 50%, 70% and 90%) of one continuum (M1-F1). The intersection of the two curves (UPRIGHT condition) indicates the point where half of the subjects would respond "male" and the other half "female" (48% in this example). This point was taken as the subjective categorical boundary of the continuum M1-F1.

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Study 1- Categorical perception of gender information Categorical boundary 100 90 80 70 60

Male

50

female

40 30 20 10 0 10

30

50

70

90

Morphs

48% Figure 5-

Frequency of responses (%) in the identification task for subjects of the UPRIGHT

condition for the continuum M1-F1. The categorical boundary was situated at 48%.

The same procedure was applied to each of the 9 continua in order to obtain their own categorical boundary. For the nine continua, categorical boundaries were always situated between 44% and 56%. Subsequently, between-category pairs and within-category pairs of equal physical distance (20%) were created, in order that the between-categorical pairs straddled the categorical boundary of the continuum from which they are issued. For instance, for the continuum M1-F1, the pair "38%-58%" crossed the boundary (48%) while the pair "8%-28%" was formed with stimuli belonging to the same category. "Same-pairs" (20%-20%) were also generated for methodological reasons in order that subjects have the same chance to respond "same" or "different" (Figure 6). In this way, 4 between-gender pairs (in this example, "36%-56%", "38%-58%", "42%-62%", "44%64%"), 4 within-gender pairs (in this example, "8%-28%", "12%-32%", "68%-88%", "72%-92%") and 8 same pairs ("10%-10%", "20%-20%", "30%-30%", "40%-40%", "60%-60%", "70%-70%", "80%-80%", "90%-90%") were created for the continuum M1-F1. Each pair was repeated two

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Study 1- Categorical perception of gender information times, once in order A-B and once in the order B-A. The same procedure was applied to the other 8 continua by considering their respective categorical boundary. Subjects were thus confronted with 288 pairs (144 "same" and 144 "different" with 72 "between" and 72 "within-pairs").

Between-pairs

Within-pairs

Same-pairs

Figure 6-

Pairs of stimuli which crossed (between) or not (within) the boundary of the

continuum M1-F1 were generated. Pairs of identical stimuli (same) were also created for methodological purpose. 18

Study 1- Categorical perception of gender information 5.3. Design and Procedure These 288 pairs constituted 12 blocks of 24 pairs and were used in a delayed "samedifferent" matching task. Each trial was formed with a central fixation cross (300ms), a blank interval (800ms), the first morphed face (400ms), a blank interval (800ms), the second morphed face (400ms) and finally a blank interval of 1200ms. The subject's task was to make a button-press response (right hand) to indicate whether the second image was exactly the same as the first or not. Subjects had 1600 ms from the onset of the second morphed face to respond. No feedback was given. Responses and latencies were recorded. The 12 blocks of 24 pairs were presented in random order. The assignement of buttons (same/different) was counterbalanced across subjects. Stimuli were presented at a distance of 1 meter on a 256 scale 15" Macintosh colour monitor using Superlab software. All stimuli were the same size (60 X 78 mm). Before starting the task, subjects were confronted with 15 pratice trials to be familiarized with the procedure.

6.Results Both accuracy and correct latencies were analyzed. Experiment 2 was aimed at showing that judgments of difference are easier for between-category pairs than for within-category pairs, even if the physical difference between each pair was kept constant. Table 3 illustrates the mean correct latencies and the mean percentages of correct responses. These data were computed from 72 between-category pairs, 72 within-category pairs and 144 same-pairs submitted to each of the 14 subjects. Same-pairs generated 77% correct responses with a mean correct latency of 860ms. Only different pairs were submitted to analyses. A clear pattern of categorical perception emerged, i.e. there was an enhanced discrimination for between- versus within-category pairs.

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Study 1- Categorical perception of gender information

Between

Within

Same

% Correct

Correct latencies

70

867

(8,615)

(141)

59

904

(13,385)

(138)

77

860

(9,95)

(144)

Table 3- Mean percentages of correct responses (SD) and mean correct latencies (SD) for between-, within- and same-category pairs (Exp.2).

Indeed, subjects made fewer errors ( t 13 = -3,944; p = .002) and had shorter correct latencies (t 13 = -2,360; p = .035) for between- than for within-category pairs. Moreover, subjects's performance for within- (t 13 = -2,996; p = .027) and between- (t 13 = -8,655; p < .0001) category pairs was significantly above chance.

7. Discussion The data from Experiment 2 showed a clear pattern of categorical perception. In fact, subjects discriminated more easily (less errors and faster correct latencies) between-gender pairs than within-gender pairs, even if the physical difference between the pairs is identical. Nevertheless, we note that, to create a continuum varying from one gender to the other one, we had to use a male face, that represents a particular individual (for instance, M1) and a female face, representing another particular individual (for instance, F1). That is, we could not avoid that the transition 20

Study 1- Categorical perception of gender information between one gender and the other one be confounded with the transition between one identiy and another one. However, even if this possible confound has to be stressed, we suggest that, in the present study, the facilitation to discriminate between-gender pairs as compared to withingender pairs would only be due to gender information and not to identity information1. We argue this for two reasons. First, the stimuli used in our experiments are generated from faces that are totally unfamiliar to the subjects. By using familiar faces, Beale and Keil (1995) showed that subjects discriminated more easily two morphed faces belonging to different identities than two morphed faces belonging to the same one. More importantly, they showed that this categorical perception effect was correlated with face familiarity. Indeed, a strong categorical perception effect emerged for pairs of faces rated as highly familiar (for instance, Kennedy/Clinton), while this effect disappeared for pairs of faces rated as relatively unfamiliar (Burns/Harris). Tanaka, Giles, Kremen and Simon (1998) explained these results by the fact that for between-categorical pairs, the morphed faces are "attracted" by two different stored representations while for the within-categorical pairs, the two faces are "attracted" by a single representation. Then, on the basis of the literature (Beale & Keil, 1995; Stevenage, 1998; Goldstone, 1998), no categorical perception effect should be obtained by using unfamiliar faces, due to the fact that, by definition, unfamiliar faces are not represented in memory. Second, we suggest that if subjects cannot rely on stored information about faces' identity in order to discriminate more easily between-categorical than within-categorical pairs, they will focus on the available gender information, which is the same in within-categorical pairs but differs in between-categorical pairs. This postulate is based on the diagnosticity principle (Schyns, 1998) which can be considered as one of the key components for successful categorisation performance. According to this principle, subjects attend to the features of a stimulus that have classificatory significance (Nosofsky, 1986). Thus, in the present study, the information available from the faces and the nature of the discrimination task demands influence the diagnosticity of specific cues, in such a way that gender cues become particularly useful (i.e., diagnostic) for the task at hand as compared to identity cues. 1

Note that all stimuli were matched for other cues, such as size, skin tone and colouration, age, emotional expression (neutral) so that only gender and identity could vary in the presented pairs.

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Study 1- Categorical perception of gender information In keeping with these considerations, we report a third experiment in which we compared the performance of subjects confronted with pairs of faces (1) moving from one gender (unfamiliar male) to the other one (unfamiliar female), and (2) moving from one unfamiliar face to another one, but with the same gender (two males or females). Indeed, if the transition between an unknown identity and another one plays a role in the good discrimination performance obtained for betweengender pairs in Experiment 2, we should also observe a good discrimination performance if subjects are confronted with pairs of morphed faces varying only on the identity dimension (and not on gender). Conversely, if the discrimination performance was bad for these between-identity pairs (i.e. at chance level), this would mean that the identity dimension is not reliable to perform the discrimination task and that the gender represents the useful « diagnostic » (Schyns, 1998) cues.

EXPERIMENT 3 As mentioned above, a possible confound could be advanced due to the fact that the passage through one gender to the other one is necessarily correlated to the passage from one identity to another one. Nevertheless, faces are unknown to subjects so that no stored representations are available : thus, we suggest that the categorical perception effect found in the present study is only due to gender information. The goal of Experiment 3 was to show, by means of a delayed same-different matching task, a sharper discrimination when subjects are confronted with pairs of unfamiliar faces showing an identity and a gender changes as compared to pairs of unfamiliar faces showing only an identity change.

8. Method 8.1. Subjects Twelve new volunteers (6 females) of the Department of Psychology took part in the experiment. All were aged between 23 and 28 years, reported no neurological disease and had normal or corrected-to-normal vision.

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Study 1- Categorical perception of gender information 8.2. Stimuli Six continua moving from one unfamiliar identity to another one, without gender change (M1/M2, M2/M3, M1/M3, F1/F2, F2/F3 and F1/F3), were created (see Figure 7 for examples). Six continua -already used in Experiment 1- (M1/F1, M2/F2, M3/F3, M1/F2, M2/F3, M3/F1) of male/female pairs were also used. By this way, we had 6 continua of faces moving from one unfamiliar face to another one, the two faces being of the same gender (IDENTITY condition), and 6 continua of faces moving from one unfamiliar male (or female) face to another female (or male) one (SEX condition).

10%

Figure 7-

30%

50%

70%

90%

Illustration for the continua F2-F3 and M2-M1, with, from left, the morphed faces

10%/90%, 30%/70%, 50%/50%, 70%/30% and 90%/10%.

Subsequently, between-category pairs of equal physical distance (20%) were created for the SEX and IDENTITY conditions. The pairs “ 35%-55% ”, “ 40%-60% ” and “ 45%-65% ” were used. “ Same-pairs ” (35%-35%) were also generated for methodological reasons in order that subjects have the same chance to respond “ same ” or “ different ” (Figure 8). Thus, 3 SEX between-pairs (“ 35%-55% ”, “ 40%-60% ” and “ 45%-65% ”), 3 IDENTITY between-pairs (“ 35%-55% ”, “ 40%-60% ” and “ 45%-65% ”), and 6 same-pairs (“ 35%-35% ”, “ 40%-40% ”, “ 45%-45% ”, “ 55%-55% ”, “ 60%-60% ” and “ 65%-65% ”) were created for each of the 12

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Study 1- Categorical perception of gender information continua. Each pair was repeated two times, once in order A-B and once in order B-A. Subjects were thus confronted with 144 pairs (72 “ same ” and 72 “ different ” with 36 belonging to the SEX condition and 36 belonging to the IDENTITY condition).

SEX CONDITION

IDENTITY CONDITION - FEMALE

IDENTITY CONDITION - MALE Figure 8-

24

Pairs of different morphed faces used in the SEX and IDENTITY conditions.

Study 1- Categorical perception of gender information 8.3. Design and Procedure These 144 pairs constituted 6 blocks of 24 pairs and were used in a delayed “ samedifferent ” matching task. For details, see Experiment 2.

9. Results Both accuracy and correct latencies were analyzed. Experiment 3 was aimed at showing a sharper discrimination when subjects are confronted with pairs of unknown faces presenting a gender change as compared to pairs of unknown faces of the same gender (even if the physical difference between each pair is identical). Figure 9 illustrates the mean correct latencies and the mean percentages of correct responses. A clear pattern of results emerged, i.e. a better discrimination in the SEX condition as compared to the IDENTITY condition.

Comparison across gender and identity boundaries 80

760 740

70 720 60

700 680

50 660 40

640 620

30 600 20

580

Pairs 25

% ms

Figure 9-

Study 1- Categorical perception of gender information Mean correct latencies and mean performance (N=12) for the SEX and IDENTITY

conditions.

Indeed, subjects displayed fewer errors (t 11 = 3,182 ; p = .009) and shorter correct latencies (t 11 = 2, 93 ; p = .014) for “ different ” pairs of the SEX condition as compared to the ones of IDENTITY condition. Subjects’s results did not vary for the “ same ” pairs of the SEX and IDENTITY conditions, neither for accuracy (t 11 = 0,953 ; NS) nor for correct latencies ( t 11 = 0,695 ; NS). Moreover, subjects’s performance was significantly above chance for the SEX condition (t 11 = -3,453; p = .005), but not for the IDENTITY condition (t 11 = -0,547, NS).

10. Discussion The results of Experiment 2 suggest that people discriminated more easily unfamiliar morphed faces belonging to two different gender categories than morphed faces belonging to the same gender category. However, a possible confound could be due to the fact that moving from one gender to the other one necessarily implies the passage through one identity to another one. Thus, the enhanced discriminability of between-gender pairs as compared to within-gender pairs could be accounted for as by the passage through one gender to the other one as well as by the passage through one unfamiliar identity to another one. We argued that, as unfamiliar faces have not available stored representations in memory, subjects rely on diagnostic cues (Schyns, 1998), i.e. the gender cues, to operate the discrimination task. The results of Experiment 3 show that when subjects were confronted with pairs of faces belonging to two different unknown identities but sharing the same gender, their discrimination performance was not significantly different from chance. However, when subjects were confronted with pairs of faces belonging to two unknown identities but presenting a gender change, their performance was enhanced and significantly above chance (mean of 68%). These empirical data suggest (1) that the boundaries across different-sex faces are sharper than those across unfamiliar facial identities, and (2) that the results of Experiment 2 were really illustrative of a categorical perception effect due to gender change and not to the transition from one unknown identity to another one. 26

Study 1- Categorical perception of gender information The question is to determine whether these data can provide new information about how faces are encoded. This will be discussed in the General Discussion where it is suggested that the multidimensional space (MDS) framework (Valentine, 1991) offers an adequate conceptual background to interpret these results (see Burton & Vokey, 1998; and Cabeza, Bruce, Kato and Oda, 1999, for discussions of the limitations of the approach).

11. General Discussion and conclusions 11.1. The MDS (Valentine, 1991). A useful framework for considering face recognition is the face-space metaphor. Valentine (1991) suggested that faces are stored as points in a n-dimensional Euclidean space where (1) dimensions are not explicitly specified but correspond to physical properties of faces and (2) the origin of the dimensions determines the most typical or average face in the population (norm face) in such a way that typical faces are close to the central tendency while distinctive faces are far from it2. In such a framework, the density of points in the multidimensional space decreases with the distance from the origin (prototype). The MDS framework does not, however, necessarily require the existence of a prototype and as a matter of fact, two different models have been proposed to account for recognition of individual faces : one requiring the existence of a general prototype, and the other not. In the former, the norm-based coding model , Rhodes et al. (1987) and Valentine (1991) assume that faces are encoded in terms of their deviation from a single general face norm (prototype) representing the central tendency. To recognize a face, subjects have (1) to encode the stimulus as a dimensional vector (with the central tendency point as the origin) and (2) to proceed to a similarity decision for knowing whether the stimulus matched a previously stored vector. In the latter, the exemplar-based coding model , Valentine (1991) assumes that faces are encoded as points rather than vectors and thus, according to Valentine (1991, p.168), "...the origin of the multidimensional space plays no part in encoding stimuli, it merely indicates the point of maximum

2

In Valentine's framework (1991), the density of face representations are normally distributed around the norm face such that (1) the density of points is high near the norm and (2) the more the distance between the norm and a target face is high, the more this face is atypical (distinctive). We have to note that Burton and Vokey (1998) discussed some of these points, showing that it is not the case that most faces will cluster around the norm and suggesting to (1) discriminate the concepts of local and global densities of points in a space, (2) specify that typicality has not to be equated with local points density and (3) specify the dimensions of the Euclidean space in order that distance from the norm can function as a definition of typicality.

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Study 1- Categorical perception of gender information exemplar density...". Therefore, the similarity between two faces is judged by the distance separating representations of the faces (points) in the MDS. This framework has given rise to several recent investigations (e.g., Johnston, Kanazawa, Kato and Oda, 1997) and it provides a good account of "effects of distinctiveness, inversion and race" (Valentine, 1991). In such a model, the recognition of familiar faces is optimal when a face stimulus matches a stored exemplar or a stored vector. Nevertheless, the recognition system is able to identify a familiar face accross deviations in face input, as for example changes due to age (Bruck, Cavanagh and Ceci, 1991), orientation (Hill, Schyns and Ahamatsu, 1998), viewpoint (Perrett, Oram and Ashbridge, 1998; Newell, Chioro and Valentine, 1999) and caricatures (Rhodes, Brennan and Carey, 1987; Lewis and Johnston, 1999). These behavioural data lead to the proposal of an attractor field model (Tanaka et al., 1998). According to this approach, a face representation is activated by any stimulus falling within the boundaries of its attractor field3. It is suggested that this conceptual framework can help to interpret the results obtained in the present experiments.

11.2. The MDS and the results of Experiment 1 By considering the norm-based coding model of the MDS (Valentine, 1991), one can imagine memory representations of two distinct "male" and "female" prototypes, which respectively average all the male and female faces encountered across life. In the present Experiment 1, subjects saw unfamiliar male or female faces and these faces could be encoded in relation to their respective gender prototypes. Indeed, as suggested by results of Experiment 3, the identity dimension is not pertinent (to achieve the purpose of the discrimination task) due to the fact that all faces are totally unknown to subjects. Then, for example, the presentation of a morphed face "M1-90%" will be encoded in the MDS in such a way that (1) the distance from the prototype will depend on its degree of gender typicality; and (2) the vector distance between this representation and the male prototype was shorter than the vector distance between this point and the female prototype. This will occur if there is enough male information in these faces to be more attracted by the male than the female prototype. However, 50% morphed faces were as close to the female than the male prototype, and consequently, they give rise to ambiguous and slow responses. On such a view, the 3

The problem is to define these boundaries. For example, Tanaka et al. (1998) showed that the attractor field of an atypical face is larger than the one of a typical face. Moreover, Lewis and Johnston (1999b) provide a method for

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Study 1- Categorical perception of gender information categorisation of unfamiliar male and female faces is considered to be grounded on the similarity between a prototype and face inputs4. Conversely, a similar account can be proposed in terms of the exemplar-based coding model (Valentine, 1991). Indeed, we can imagine that subjects have in memory several exemplars of familiar males and females. Then, for example, the presentation of a morphed face "M1-90%" will be encoded in such a way that it will be attracted by a stored familiar male while a morphed face "F1-90%" will be attracted by a stored familiar female. In such a view, categorisation of unfamiliar male and female faces is considered to be grounded on the similarity between stored exemplars and face inputs.

11.3. The MDS and the results of Experiment 2 and 3 Experiment 2 showed that between-gender differences were more easily discriminated than within-gender differences. On the basis of the literature, this result could be unexpected because the faces used in the present study are totally unfamiliar to subjects. Since no stored representations of these identities are available in memory, subjects cannot rely on the discrimination between two different identity representations when confronted with between-categorical pairs while a unique representation is activated by the within-categorical pairs. This statement was assessed by results of Experiment 3, showing that subjects cannot discriminate two morphed faces belonging to two different unknown identities. The better performance in the discrimination of between-categorical pairs as compared to within-categorical pairs found in Experiment 2 cannot thus be explained by referring to the fact that between-categorical pairs are defined by two morphed faces issued from two different models while within-categorical pairs are defined by two morphed faces issued from the same model. We suggest that the gender information gave rise to the obtained categorical perception effect, i.e. an enhanced discriminability for between-categorical pairs as compared to within-ones. According to the norm-based coding model, one can hold that a within-category pair activated a unique and identical prototype (male or female) while a between-category pair activated defining the boundaries of attractor fields. 4 Goldstone (1994b) investigated the role of similarity on category formation. There are some arguments to consider similarity as too unconstrained and not sufficiently sophisticated to ground most categories. Nevertheless, Goldstone concluded that if similarity is too unconstrained to provide a firm basis for categories, it can provide a useful ground for

29

Study 1- Categorical perception of gender information both prototypes. Subjects may then be led to consider within-category pairs as similar because the two stimuli are attracted by the same prototype. This will make differentiation between these two stimuli more difficult, relative to between-category pairs for which subjects can rely on different prototypes. Conversely, according to the exemplar-based coding model, within-category pairs should activate the same stored male (female) exemplar or the same part of the “ exemplar-space ” model while between-categorical pairs activate both male and female stored exemplars or two different regions of the “ exemplar-space ” model. Subjects discriminated between-category pairs more easily because differential gender information was activated, while the same representations are activated for within-category pairs.

11.4. Conclusions The present results support the idea that there is categorical perception of gender information for unfamiliar faces. By using a morphing procedure, we artificially created faces represented in the MDS by points which are moving linearly from a point situated close to the male prototype or to a male exemplar to another point situated close to the female prototype or to a female exemplar. As already shown for phonemes (Liberman et al., 1957), colours (Bornstein & Korda, 1964), facial expressions (Etcoff & Magee, 1992; Calder et al., 1996; de Gelder et al., 1997; Young et al., 1997) and familiar facial identities (Beale & Keil, 1995), we showed that two unknown faces belonging to different gender are easier to discriminate than two unknown faces belonging to the same one. It would be interesting that further researches tried to understand whether this categorical perception effect was due to acquired similarity or distinctiveness. However, it was already interesting to note that the nature of the phenomenon of categorical perception of unknown facial gender information seems to be different than the nature of classical categorical perception effect described for phonemes or colours. In fact, the phenomenon that we evidenced requires (1) visual analysis of faces, (2) encoding in the MDS, (3) comparison to a prototype or exemplars and (4) decision making, while, for instance, categorical perception of colour does not need (2) and (3). It is suggested that the mechanisms implied in categorical perception of facial gender information are thus different than those implied in the categorical perception of colours or phonemes. Moreover, an 30important subset of categories. We think that this is the case for male/female categories.

Study 1- Categorical perception of gender information results of the present study (Experiment 2) suggested interactions between the information demands of specific categorisation tasks and the perceptual information available from the input stimulus. This stressed the importance of considering the principles governing the formation of categories as tightly anchored with the perceptual aspects of recognition (Goldstone, 1994a; Schyns, 1998).

ACKNOWLEDGMENTS We are grateful to Glyn Humphreys, Michael Lewis and two anonymous reviewers for their helpful comments and suggestions on a previous draft of this manuscript. This study was supported by the grant n°95/00-189 ("Action de Recherche Concertée") from the Government of the French-speaking Community. The first author was supported by the Belgian Fund of the Scientific Research (FNRS).

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