Adaptation and the perception of facial age

We examined how the perceived age of adult faces is affected by adaptation to younger or older adult faces. Observers viewed images of a synthetic male face simulating ageing over a modelled range from 15 to 65 years. Age was varied by changing shape cues or textural cues. Age level was varied in a staircase to find the observer's subjective category boundary between “old” and “young”. These boundaries were strongly biased by adaptation to the young or old face, with significant aftereffects induced by either shape or textural cues. A further experiment demonstrated comparable aftereffects for photorealistic images of average older or younger adult faces, and found that aftereffects showed some selectivity for a change in gender but also strongly transferred across gender. This transfer shows that adaptation can adjust to the attribute of age somewhat independently of other facial attributes. These findings suggest that perceived age, like many other natural facial dimensions, is highly susceptible to adaptation, and that this adaptation can be carried by both the structural and textural changes that normally accompany facial ageing.     

Adaptation and the perception of facial age

We examined how the perceived age of adult faces is affected by adaptation to younger or older adult faces. Observers viewed images of a synthetic male face simulating ageing over a modelled range from 15 to 65 years. Age was varied by changing shape cues or textural cues. Age level was varied in a staircase to find the observer's subjective category boundary between "old" and "young". These boundaries were strongly biased by adaptation to the young or old face, with significant aftereffects induced by either shape or textural cues. A further experiment demonstrated comparable aftereffects for photorealistic images of average older or younger adult faces, and found that aftereffects showed some selectivity for a change in gender but also strongly transferred across gender. This transfer shows that adaptation can adjust to the attribute of age somewhat independently of other facial attributes. These findings suggest that perceived age, like many other natural facial dimensions, is highly susceptible to adaptation, and that this adaptation can be carried by both the structural and textural changes that normally accompany facial ageing.     

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.     

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.     

Color—luminance relationships and the McCollough effect

The McCollough effect is an orientation-specific color aftereffect induced by adapting to colored gratings. We examined how the McCollough effect depends on the relationships between color and luminance within the inducing and test gratings and compared the aftereffects to the color changes predicted from selective adaptation to different color—luminance combinations. Our results suggest that the important contingency underlying the McCollough effect is between orientation and color—luminance direction and are consistent with sensitivity changes within mechanisms tuned to specific color—luminance directions. Aftereffects are similar in magnitude for adapting color pairs that differ only in S cone excitation or L and M cone excitation, and they have a similar dependence on spatial frequency. In particular, orientation-specific aftereffects are induced for S cone colors even when the grating frequencies are above the S cone resolution limit. Thus, the McCollough effect persists even when different cone classes encode the orientation and color of the gratings.     

Color-luminance relationships and the McCollough effect

The McCollough effect is an orientation-specific color aftereffect induced by adapting to colored gratings. We examined how the McCollough effect depends on the relationships between color and luminance within the inducing and test gratings and compared the aftereffects to the color changes predicted from selective adaptation to different color-luminance combinations. Our results suggest that the important contingency underlying the McCollough effect is between orientation and color-luminance direction and are consistent with sensitivity changes within mechanisms tuned to specific color-luminance directions. Aftereffects are similar in magnitude for adapting color pairs that differ only in S cone excitation or L and M cone excitation, and they have a similar dependence on spatial frequency. In particular, orientation-specific aftereffects are induced for S cone colors even when the grating frequencies are above the S cone resolution limit. Thus, the McCollough effect persists even when different cone classes encode the orientation and color of the gratings.     

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.     

Visual perception and visual mental imagery of emotional faces generate similar expression aftereffects

What is the relationship between visual perception and visual mental imagery of emotional faces? We investigated this question using a within-emotion perceptual adaptation paradigm in which adaptation to a strong version of an expression was paired with a test face displaying a weak version of the same emotion category. We predicted that within-emotion adaptation to perception and imagery of expressions would generate similar aftereffects, biasing perception of weak emotional test faces toward a more neutral value. Our findings confirmed this prediction. Adaptation to mental images yielded aftereffects that inhibited emotion recognition of test expressions, as participants were less accurate at recognising these stimuli compared to baseline. While the same inhibitory effect was observed when expressions were visually perceived, the size of the aftereffects was greater for perception than imagery. These findings suggest the existence of expression-selective neural mechanisms that subserve both visual perception and visual mental imagery of emotional faces.     

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.     

Spatial frequency content of visual imagery

Three experiments employing the McCollough paradigm were conducted to determine the spatial-frequency content of visual imagery. In Experiment 1, large and reliable pattern-contingent color aftereffects were obtained after adaptation to visual imagery. The direction of the aftereffects indicated that subjects were adapting to higher spatial frequencies in their imagery. These results contrast with the data of Experiment 2, which demonstrate that color aftereffects obtained with adaptation to physically present stimuli are mediated by the fundamental spatial frequency components. The magnitude of the imagery-induced aftereffects in Experiment 1 equaled the magnitude of the externally induced aftereffects obtained in Experiment 2 with the same subjects. By blurring the to-be-imaged patterns (Experiment 3), the fundamental Fourier components became the salient perceptual features of the stimuli, and the direction of the imagery-induced aftereffects was reversed from that of Experiment 1, indicating that the spatial frequency content of the imagery had changed from higher to lower frequencies. Under normal viewing conditions, subjects use the higher spatial frequencies associated with the perceptually salient edges of stimuli to construct their images. The results of Experiments 1 and 3 are discussed in light of a current controversy over the nature of information representation in imagery, and it is concluded that support has been obtained for the analog model of visual imagery.     

Lecturers' faces fatigue their students: Face identity aftereffects for dynamic and static faces

Face adaptation has been used as a tool to probe our representations for facial identity. It has also been claimed to play a functional role in face processing, perhaps calibrating the visual system towards encountered faces. However, for this to be so, face aftereffects must be observable following adaptation to ecologically valid moving stimuli, not just after prolonged viewing of static images. We adapted our participants to videos, static image sequences or single images of the faces of lecturers who were personally familiar to them. All three stimulus types produced significant, and equivalent, face identity aftereffects, demonstrating that aftereffects are not confined to static images but can occur after exposure to more naturalistic stimuli. It is also further evidence against explanations of face adaptation effects solely in terms of low-level visual processing.     

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.     

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.     

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.     

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.     

Probing the visual representation of faces with adaptation: A view from the other side of the mean

Sensory adaptation and visual aftereffects have long given insight into the neural codes underlying basic dimensions of visual perception. Recently discovered perceptual adaptation effects for complex shapes like faces can offer similar insight into high-level visual representations. In the experiments reported here, we demonstrated first that face adaptation transfers across a substantial change in viewpoint and that this transfer occurs via processes unlikely to be specific to faces. Next, we probed the visual codes underlying face recognition using face morphs that varied selectively in reflectance or shape. Adaptation to these morphs affected the perception of "opposite" faces both from the same viewpoint and from a different viewpoint. These results are consistent with high-level face representations that pool local shape and reflectance patterns into configurations that specify facial appearance over a range of three-dimensional viewpoints. These findings have implications for computational models of face recognition and for competing neural theories of face and object recognition.     

Effects of spatial filtering and lack of effects of visual imagery on pattern-contingent color aftereffects

Checkerboards contain fundamental two-dimensional Fourier components oriented 45° from the edges of individual checks. Previous studies have shown that contingent color aftereffects following adaptation to chromatic checkerboard stimuli were associated with the fundamental components rather than the edges, In the present experiments, we measured contingent color aftereffects, using the method of constant stimuli, after subjects adapted to unfiltered checkerboards and checkerboards with the fundamental Fourier components removed. The adaptation stimuli were magenta (or green) squares and green (or magenta) diamonds; the test stimuli were vertical or oblique sine-wave gratings with different saturations, After adaptation to unfiltered checkerboards, aftereffects contingent on the fundamental components were obtained. In contrast, after adaptation to filtered stimuli, aftereffects of smaller magnitude were found to be aligned with the edges. The data support the previous findings of spatial-frequency-contingent color after-effects with checkerboard adaptation stimuli and indicate that the aftereffects can be associated with edges if the fundamental components of adaptation stimuli are removed by spatial filtering. We reexamined the possibility of color aftereffects induced by imagery of checkerboards. Contrary to the previous reports, no significant aftereffects were obtained.     

Contrast and frequency competition for orientation-contingent color aftereffects

A “competition” paradigm was developed to examine separately the effects of pattern contrast and spatial frequency characteristics on the strength of orientation-contingent color aftereffects (McCollough effects). After adapting to alternately presented red/black and green/black square-wave gratings (one horizontal, one vertical), 11 subjects viewed seven different kinds of test patterns. Unlike Standard McCollough effect test stimuli, the present patterns had variable luminance profiles running both horizontally and vertically within each test pattern area. Forced choice responses were used to determine which aftereffect color (red or green) appeared, as characteristics of vertical and horizontal luminance profiles were varied separately among test stimulus types. We conclude that pattern contrast and human contrast sensitivity account for aftereffect colors in such stimuli. When contrast is taken into consideration, aftereffects are not predicted by similarity between adaptation and test pattern Fourier characteristics, nor are they predicted by the width, per se, of pattern elements.