Opposite aftereffects for Chinese and Caucasian faces are selective for social category information and not just physical face differences

Opposite changes in perception (aftereffects) can be simultaneously induced for faces from different social categories—for example, Chinese and Caucasian faces. We investigated whether these aftereffects are generated in high-level face coding that is sensitive to the social category information in faces, or in earlier visual coding sensitive to simple physical differences between faces. We caricatured the race of face stimuli and created face continua ranging from caricatured Caucasian faces (SuperCaucasian) to caricatured Chinese faces (SuperChinese). Participants were adapted to oppositely distorted faces that were a fixed physical distance apart on the morph continua. Larger opposite aftereffects were found following adaptation to faces from different race categories (e.g., contracted Chinese and expanded Caucasian faces), than for faces that were the same physical distance apart on the morph continua, but were within a race category (e.g., contracted SuperChinese and expanded Chinese faces). These results suggest that opposite aftereffects for Chinese and Caucasian faces reflect the recalibration of face neurons tuned to high-level social category information.     

Opposite aftereffects for Chinese and Caucasian faces are selective for social category information and not just physical face differences

Opposite changes in perception (aftereffects) can be simultaneously induced for faces from different social categories—for example, Chinese and Caucasian faces. We investigated whether these aftereffects are generated in high-level face coding that is sensitive to the social category information in faces, or in earlier visual coding sensitive to simple physical differences between faces. We caricatured the race of face stimuli and created face continua ranging from caricatured Caucasian faces (SuperCaucasian) to caricatured Chinese faces (SuperChinese). Participants were adapted to oppositely distorted faces that were a fixed physical distance apart on the morph continua. Larger opposite aftereffects were found following adaptation to faces from different race categories (e.g., contracted Chinese and expanded Caucasian faces), than for faces that were the same physical distance apart on the morph continua, but were within a race category (e.g., contracted SuperChinese and expanded Chinese faces). These results suggest that opposite aftereffects for Chinese and Caucasian faces reflect the recalibration of face neurons tuned to high-level social category information.     

Sex-contingent face aftereffects depend on perceptual category rather than structural encoding

Many studies have used visual adaptation to investigate how recent experience with faces influences perception. While faces similar to those seen during adaptation phases are typically perceived as more 'normal' after adaptation, it is possible to induce aftereffects in one direction for one category (e.g. female) and simultaneously induce aftereffects in the opposite direction for another category (e.g. male). Such aftereffects could reflect 'category-contingent' adaptation of neurons selective for perceptual category (e.g. male or female) or 'structure-contingent' adaptation of lower-level neurons coding the physical characteristics of different face patterns. We compared these explanations by testing for simultaneous opposite after effects following adaptation to (a) two groups of faces from distinct sex categories (male and female) or (b) two groups of faces from the same sex category (female and hyper-female) where the structural differences between the female and hyper-female groups were mathematically identical to those between male and female groups. We were able to induce opposite aftereffects following adaptation between sex categories but not after adaptation within a sex category. These findings indicate the involvement of neurons coding perceptual category in sex-contingent face aftereffects and cannot be explained by neurons coding only the physical aspects of face patterns.     

The effect of familiarity on face adaptation

Face aftereffects can provide information on how faces are stored by the human visual system (eg Leopold et al, 2001 Nature Neuroscience 4 89-94), but few studies have used robustly represented (highly familiar) faces. In this study we investigated the influence of facial familiarity on adaptation effects. Participants were adapted to a series of distorted faces (their own face, a famous face, or an unfamiliar face). In experiment 1, figural aftereffects were significantly smaller when participants were adapted to their own face than when they were adapted to the other faces (ie their own face appeared significantly less distorted than a famous or unfamiliar face). Experiment 2 showed that this 'own-face' effect did not occur when the same faces were used as adaptation stimuli for participants who were unfamiliar with them. Experiment 3 replicated experiment 1, but included a pre-adaptation baseline. The results highlight the importance of considering facial familiarity when conducting research on face aftereffects.     

Perception and imagery of faces generate similar gender aftereffects

Visual adaptation is known to bias perception away from the properties of the adapting stimuli, toward opposite properties, resulting in perceptual aftereffects. For example, prolonged exposure to a face has been shown to produce an identity aftereffect, biasing perception of a subsequent face toward the opposite identity. Such repulsive aftereffects have been observed for both visually perceived and visually imagined faces, suggesting that both perception and imagery yield typical aftereffects. However, recent studies have reported opposite patterns of aftereffects for perception and imagery of face gender. In these studies, visually perceived faces produced typical effects in which perception of androgynous faces was biased away from the gender of the adaptor, whereas imagery of the same stimuli produced atypical aftereffects, biasing the perceived gender of androgynous faces toward the gender of the adaptor. These findings are highly unusual and warrant further research. The present study aimed to gather new evidence on the direction of gender aftereffects following perception and imagery of faces. Experiment 1 had participants view and imagine female and male faces of famous and non-famous individuals. To determine the effect of concomitant visual stimulation on imagery and adaptation, participants visualized faces both in the presence and in the absence of a visual input. In Experiment 2, participants were adapted to perceived and imagined faces of famous and non-famous actors matched on gender typicality. This manipulation allowed us to determine the effect of face familiarity on the magnitude of gender aftereffects. Contrary to evidence from previous studies, our results demonstrated that both perception and imagery produced typical aftereffects, biasing the perceived gender of androgynous faces in the opposite direction to the gender of the adaptor. Famous faces yielded largest adaptation effects across tasks. Experiment 2 confirmed that these effects depended on familiarity rather than on sexual dimorphism. In both experiments, this effect was greater for perception than imagery. Additionally, imagery of famous faces produced strongest aftereffects when it was performed in the absence of visual stimulation. The implications of these findings are discussed.     

Opposite effects of visual versus imagined presentation of faces on subsequent sex perception

Viewing faces of one sex changes the perception of subsequently seen ambiguous faces. Here we investigate if the mechanisms responsible for this sex aftereffect are also activated during mental imagery of faces. Participants categorized the sex of ambiguous faces after either viewing images of male or female actors' faces or imagining these same faces. As in previous studies, the ambiguous images were categorized as female more often after viewing male faces than after viewing female faces. The opposite effect was found for imagined faces, however; the ambiguous images were categorized as female more often after imagining female faces than after imagining male faces. Although our results are inconsistent with findings that imagined faces cause either no aftereffects or similar aftereffects to visually presented faces, our results are consistent with recent evidence that visual and imagined presentation of faces cause opposite adaptation effects on an early electrophysiological response associated with face processing.     

Representing young and older adult faces: Shared or age-specific prototypes?

ABSTRACT

Young adults recognize young adult faces more accurately than older adult faces and are more sensitive to how individual young faces deviate from a norm/prototype. Here we used an adaptation paradigm to examine whether young and older adult faces are represented by separable norms and the extent to which the coding dimensions for these two categories overlap. In Experiment 1, following adaptation to oppositely distorted young and older faces (e.g., expanded young and compressed older faces), adults’ normality judgments simultaneously shifted in opposite directions for the two face categories, providing evidence for separable norms. In Experiment 2, participants were adapted to distorted faces from a single age category (e.g., compressed young); aftereffects transferred across face age but were larger for the face age that matched adaptation. Collectively, these results provide evidence that young and older faces are processed with regard to separable norms that share some underlying coding dimensions.     

View-specific coding of face shape

Monkey and human cortex contain view-specific face neurons, but it remains unclear whether they code face shape. We tested the view specificity of face-shape coding by inducing figural face aftereffects at one viewpoint (3/4 left) and testing generalization to different viewpoints (front view and 3/4 right). The aftereffects were induced by adaptation to consistent figural distortions (contracted or expanded), which shifts the distortion perceived as most normal toward the adapting distortion. The strong aftereffect that was observed at the adapting view was significantly and substantially reduced for both front-view test faces and mirror-image (3/4 right) test faces, indicating view specificity. The limited transfer across mirror views is strong evidence of view specificity, given their figural similarity. The aftereffects survived a size change between adaptation and test faces (Experiment 2), a result that rules out low-level adaptation as an explanation. These results provide strong evidence that face-shape coding is view-specific.     

Race-contingent aftereffects suggest distinct perceptual norms for different race faces

Faces of one's own race and of other races are thought to be located in different regions of face space (Valentine, 1991 Valentine, T. 1991. A unified account of the effects of distinctiveness, inversion, and race in face recognition. Quarterly Journal of Experimental Psychology, 43A: 161–204. [Taylor & Francis Online] , [Google Scholar]). Here we investigated whether faces of different races (Caucasian and Chinese faces) have dissociable neural coding and distinct norms. We used adaptation techniques to alter perceptions of normality (aftereffects) for faces. Caucasian and Chinese participants adapted to distorted faces of one race (e.g., Chinese contracted faces—Experiment 1), or oppositely distorted faces of both races (e.g., Chinese contracted and Caucasian expanded faces—Experiment 2). We then tested for aftereffects in Chinese and Caucasian test faces. In Experiment 1 aftereffects were reduced when a change in race between the adapt faces and test faces occurred. In Experiment 2 aftereffects occurred in opposite directions for the two races. These results demonstrate that dissociable neural populations code faces of different races and suggest the existence of race-specific face norms.     

Reduced adaptability,but no fundamental disruption,of norm‐based face coding following early visual deprivation from congenital cataracts

Faces are adaptively coded relative to visual norms that are updated by experience, and this adaptive coding is linked to face recognition ability. Here we investigated whether adaptive coding of faces is disrupted in individuals (adolescents and adults) who experience face recognition difficulties following visual deprivation from congenital cataracts in infancy. We measured adaptive coding using face identity aftereffects, where smaller aftereffects indicate less adaptive updating of face‐coding mechanisms by experience. We also examined whether the aftereffects increase with adaptor identity strength, consistent with norm‐based coding of identity, as in typical populations, or whether they show a different pattern indicating some more fundamental disruption of face‐coding mechanisms. Cataract‐reversal patients showed significantly smaller face identity aftereffects than did controls (Experiments 1 and 2). However, their aftereffects increased significantly with adaptor strength, consistent with norm‐based coding (Experiment 2). Thus we found reduced adaptability but no fundamental disruption of norm‐based face‐coding mechanisms in cataract‐reversal patients. Our results suggest that early visual experience is important for the normal development of adaptive face‐coding mechanisms.     

The development of norm-based coding and race-specific face prototypes: An examination of 5- and 8-year-olds’ face space

The current research investigated the organization of children’s face space by examining whether 5- and 8-year-olds show race-contingent aftereffects. Participants read a storybook in which Caucasian and Chinese children’s faces were distorted in opposite directions. Before and after adaptation, participants judged the normality/attractiveness of expanded, compressed, and undistorted Caucasian and Chinese faces. The method was validated with adults and then refined to test 8- and 5-year-olds. The 5-year-olds were also tested in a simple aftereffects paradigm. The current research provides the first evidence for simple attractiveness aftereffects in 5-year-olds and for race-contingent aftereffects in both 5- and 8-year-olds. Evidence that adults and 5-year-olds may possess only a weak prototype for Chinese children’s faces suggests that Caucasian adults’ prototype for Chinese adult faces does not generalize to child faces and that children’s face space undergoes a period of increasing differentiation between 5 and 8 years of age.     

Face aftereffects suggest interdependent processing of expression and sex and of expression and race

Bruce and Young (1986) proposed that functionally different aspects of faces (e.g., sex, identity, and expression) are processed independently. Although interdependent processing of identity and expression and of identity and sex have been demonstrated previously, evidence for interdependent processing of sex and expression is equivocal. Using a visual adaptation paradigm, we show that expression aftereffects can be simultaneously induced in different directions along anger–fear continua for male and female faces (Experiment 1) and for East Asian and Black African faces (Experiment 2). These findings for sex- and race-contingent expression aftereffects suggest that processing of expression is interdependent with processing of sex and race and are therefore problematic for models of face perception that have emphasized independent processing of functionally different aspects of faces. By contrast, our findings are consistent with models of face processing that propose that invariant physical aspects of faces and changeable social cues can be processed interdependently.     

Not just the norm: Exemplar-based models also predict face aftereffects

The face recognition literature has considered two competing accounts of how faces are represented within the visual system: Exemplar-based models assume that faces are represented via their similarity to exemplars of previously experienced faces, while norm-based models assume that faces are represented with respect to their deviation from an average face, or norm. Face identity aftereffects have been taken as compelling evidence in favor of a norm-based account over an exemplar-based account. After a relatively brief period of adaptation to an adaptor face, the perceived identity of a test face is shifted toward a face with attributes opposite to those of the adaptor, suggesting an explicit psychological representation of the norm. Surprisingly, despite near universal recognition that face identity aftereffects imply norm-based coding, there have been no published attempts to simulate the predictions of norm- and exemplar-based models in face adaptation paradigms. Here, we implemented and tested variations of norm and exemplar models. Contrary to common claims, our simulations revealed that both an exemplar-based model and a version of a two-pool norm-based model, but not a traditional norm-based model, predict face identity aftereffects following face adaptation.     

Angry faces are more resistant to forgetting than are happy faces: directed forgetting effects on the identity of emotional faces

Using the item-method directed forgetting paradigm (i.e. intentionally forgetting specified information), we examined directed forgetting of facial identity as a function of facial expression and the sex of the expresser and perceiver. Participants were presented with happy and angry male and female faces cued for either forgetting or remembering, and were then asked to recognise previously studied faces from among a series of neutral faces. For each recognised test face, participants also recalled the face’s previously displayed emotional expression. We found that angry faces were more resistant to forgetting than were happy faces. Furthermore, angry expressions on male faces and happy expressions on female faces were recognised and recalled better than vice versa. Signal detection analyses revealed that male faces gave rise to a greater sensitivity than female faces did, and male participants, but not female participants, showed greater sensitivity to male faces than to female faces. Several theoretical implications are discussed.     

Figural aftereffects in the perception of faces

We examined figural aftereffects in images of human faces, for which changes in configuration are highly discriminable. Observers either matched or rated faces before or after viewing distorted images of faces. Prior adaptation strongly biases face perception by causing the original face to appear distorted in a direction opposite to the adapting distortion. Aftereffects transferred across different faces and were similar for upright or inverted faces, but were weaker when the adapting and test faces had different orientations (e.g., adapt inverted and test upright). Thus the aftereffects depend on which images are distorted, and not simply on the type of distortion introduced. We further show that the aftereffects are asymmetric, for adapting to the original face has little effect on the perception of a distorted face. This asymmetry suggests that adaptation may play an important normalizing role in face perception. Our results suggest that in normal viewing, figural aftereffects may strongly influence form perception and could provide a novel method for probing properties of human face perception.     

Face adaptation to gender: Does adaptation transfer across age categories?

We examined gender adaptation effects for the faces of children and adults and measured the transfer of these effects across age categories. Face gender adaptation is defined by a bias to classify the gender of a gender-neutral face to be opposite to that of an adapting face. An androgynous face, for example, appears male following adaptation to a female face. Participants adapted to male or female faces from the two age categories and classified the gender of morphed adult and child faces from a male–female morph trajectory. Gender adaptation effects were found for children's and adults’ faces and for the transfer between the age categories. The size of these effects was comparable when participants adapted to adult faces and identified the gender of either adult or child faces, and when participants adapted to child faces and identified the gender of child faces. A smaller adaptation effect was found when participants adapted to a child's face but identified the gender of an adult's face. The results indicate an interconnected and partially shared representation of the gender information for child and adult faces. The lack of symmetry in adaptation transfer between child and adult faces suggests that adaptation to adult faces is more effective than adaptation to child faces in activating a gender representation that generalizes across age categories.     

Pulling faces: an investigation of the face-distortion aftereffect

After adaptation to a face distorted to look unnaturally thin or fat, a normal face appears distorted in the opposite direction (Webster and MacLin 1999 Psychonomic Bulletin & Review 6 647-653). When the adapting face is oriented 45 degrees from vertically upright and the test face 45 degrees in the opposite direction, the axis of perceived distortion changes with the orientation of the face. The magnitude of this aftereffect shows a reduction of approximately 40% from that found when both adapting and test faces are tilted identically. This finding suggests that to a large degree the aftereffect is mediated not by low-level retinotopic (image-based) visual mechanisms but at a higher level of object-based processing. Aftereffects of a similar magnitude are obtained when adapting and test images are both either upright or inverted, or for an upright adapter and an inverted test; but aftereffects are smaller when the adapter is inverted and the test upright. This pattern of results suggests that the face-distortion aftereffect is mediated by object-processing mechanisms including, but not restricted to, configurational face-processing mechanisms.     

Atypical updating of face representations with experience in children with autism

Face identity aftereffects are significantly diminished in children with autism relative to typical children, which may reflect reduced perceptual updating with experience. Here, we investigated whether this atypicality also extends to non‐face stimulus categories, which might signal a pervasive visual processing difference in individuals with autism. We used a figural aftereffect task to measure directly perceptual updating following exposure to distorted upright faces, inverted faces and cars, in typical children and children with autism. A size‐change between study and test stimuli limited the likelihood that any processing atypicalities reflected group differences in adaptation to low‐level features of the stimuli. Results indicated that, relative to typical children, figural aftereffects for upright faces, but not inverted faces or cars, were significantly attenuated in children with autism. Moreover, the group difference was amplified when we isolated the ‘face‐selective’ component of the aftereffect, by partialling out the mid‐level shape adaptation common to upright and inverted face stimuli. Notably, the aftereffects of typical children were disproportionately larger for upright faces than for inverted faces and cars, but the magnitude of aftereffects of autistic children was not similarly modulated according to stimulus category. These findings are inconsistent with a pervasive adaptive coding atypicality relative to typical children, and suggest that reduced perceptual updating may constitute a high‐level, and possibly face‐selective, visual processing difference in children with autism.     

Insights into the development of face recognition mechanisms revealed by face aftereffects

An important question in person perception is how we acquire the perceptual/cognitive mechanisms that characterize adult expertise. Children's performance on face recognition tests improves dramatically between age 4 and adolescence suggesting that our face recognition system may change during childhood. Yet, the source of this improvement is controversial. In this review, we consider whether changes in the way identity is represented/coded in face space could contribute to this age-related improvement. Face aftereffects have been extensively applied to studying face coding in adults and more recently they have been applied to studying the mechanisms of face coding in children. Face aftereffects are temporary distortions of perception induced by exposure to faces and are thought to reflect the mechanisms underlying face perception. Face aftereffect techniques have revealed that children as young as 4 years of age show evidence of adult-like face space organization, with opponent coding of face dimensions. These findings are consistent with an emerging picture that the key mechanisms of face perception are present early in childhood.     

Facial expression coding in children and adolescents with autism: Reduced adaptability but intact norm-based coding

Individuals with autism spectrum disorder (ASD) can have difficulty recognizing emotional expressions. Here, we asked whether the underlying perceptual coding of expression is disrupted. Typical individuals code expression relative to a perceptual (average) norm that is continuously updated by experience. This adaptability of face-coding mechanisms has been linked to performance on various face tasks. We used an adaptation aftereffect paradigm to characterize expression coding in children and adolescents with autism. We asked whether face expression coding is less adaptable in autism and whether there is any fundamental disruption of norm-based coding. If expression coding is norm-based, then the face aftereffects should increase with adaptor expression strength (distance from the average expression). We observed this pattern in both autistic and typically developing participants, suggesting that norm-based coding is fundamentally intact in autism. Critically, however, expression aftereffects were reduced in the autism group, indicating that expression-coding mechanisms are less readily tuned by experience. Reduced adaptability has also been reported for coding of face identity and gaze direction. Thus, there appears to be a pervasive lack of adaptability in face-coding mechanisms in autism, which could contribute to face processing and broader social difficulties in the disorder.