View-Specific Coding of Face Shape

Face masks are less effective than sunglasses in masking face identity

The effect of covering faces on face identification is recently garnering interest amid the COVID-19 pandemic. Here, we investigated how face identification performance was affected by two types of face disguise: sunglasses and face masks. Observers studied a series of faces; then judged whether a series of test faces, comprising studied and novel faces, had been studied before or not. Face stimuli were presented either without coverings (full faces), wearing sunglasses covering the upper region (eyes, eyebrows), or wearing surgical masks covering the lower region (nose, mouth, chin). We found that sunglasses led to larger reductions in sensitivity (d') to face identity than face masks did, while both disguises increased the tendency to report faces as studied before, a bias that was absent for full faces. In addition, faces disguised during either study or test only (i.e. study disguised faces, test with full faces; and vice versa) led to further reductions in sensitivity from both studying and testing with disguised faces, suggesting that congruence between study and test is crucial for memory retrieval. These findings implied that the upper region of the face, including the eye-region features, is more diagnostic for holistic face-identity processing than the lower face region.

Perceptual similarity modulates effects of learning from variability on face recognition

Face recognition is a challenging classification task that humans perform effortlessly for familiar faces. Recent studies have emphasized the importance of exposure to high variability appearances of the same identity to perform this task. However, these studies did not explicitly measure the perceptual similarity between the learned images and the images presented at test, which may account for the advantage of learning from high variability. Particularly, randomly selected test images are more likely to be perceptually similar to learned high variability images, and dissimilar to learned low variability images. Here we dissociated effects of learning from variability and study-test perceptual similarity, by collecting human similarity ratings for the study and test images. Using these measures, we independently manipulated the variability between the learning images and their perceptual similarity to the test images. Different groups of participants learned face identities from a low or high variability set of images. The learning phase was followed by a face matching test (Experiment 1) or a face recognition task (Experiment 2) that presented novel images of the learned identities that were perceptually dissimilar or similar to the learned images. Results of both experiments show that perceptual similarity between study and test, rather than image variability at learning per se, predicts face recognition. We conclude that learning from high variability improves face recognition for perceptually similar but not for perceptually dissimilar images. These findings may not be specific to faces and should be similarly evaluated for other domains.

Incongruence in Lighting Impairs Face Identification

The effect of uniform lighting on face identity processing is little understood, despite its potential influence on our ability to recognize faces. Here, we investigated how changes in uniform lighting level affected face identification performance during face memory tests. Observers were tasked with learning a series of faces, followed by a memory test where observers judged whether the faces presented were studied before or novel. Face stimuli were presented under uniform bright or dim illuminations, and lighting across the face learning and the memory test sessions could be the same (“congruent”) or different (“incongruent”). This led to four experimental conditions: (1) Bright/Dim (learning bright faces, testing on dim faces); (2) Bright/Bright; (3) Dim/Bright; and (4) Dim/Dim. Our results revealed that incongruent lighting levels across sessions (Bright/Dim and Dim/Bright) significantly reduced sensitivity (d’) to faces and introduced conservative biases compared to congruent lighting levels (Bright/Bright and Dim/Dim). No significant differences in performance were detected between the congruent lighting conditions (Bright/Bright vs. Dim/Dim) and between the incongruent lighting conditions (Bright/Dim vs. Dim/Bright). Thus, incongruent lighting deteriorated performance in face identification. These findings implied that the level of uniform lighting should be considered in an illumination-specific face representation and potential applications such as eyewitness testimony.

Within-person variability can improve the identification of unfamiliar faces across changes in viewpoint

Unfamiliar face identification is concerningly error prone, especially across changes in viewing conditions. Within-person variability has been shown to improve matching performance for unfamiliar faces, but this has only been demonstrated using images of a front view. In this study, we test whether the advantage of within-person variability from front views extends to matching to target images of a face rotated in view. Participants completed either a simultaneous matching task (Experiment 1) or a sequential matching task (Experiment 2) in which they were tested on their ability to match the identity of a face shown in an array of either one or three ambient front-view images, with a target image shown in front, three-quarter, or profile view. While the effect was stronger in Experiment 2, we found a consistent pattern in match trials across both experiments in that there was a multiple image matching benefit for front, three-quarter, and profile-view targets. We found multiple image effects for match trials only, indicating that providing observers with multiple ambient images confers an advantage for recognising different images of the same identity but not for discriminating between images of different identities. Signal detection measures also indicate a multiple image advantage despite a more liberal response bias for multiple image trials. Our results show that within-person variability information for unfamiliar faces can be generalised across views and can provide insights into the initial processes involved in the representation of familiar faces.

Does automatic human face categorization depend on head orientation?

Whether human categorization of visual stimuli as faces is optimal for full-front views, best revealing diagnostic features but lacking depth cues, remains largely unknown. To address this question, we presented 16 human observers with unsegmented natural images of different living and non-living objects at a fast rate (f = 12 Hz), with natural face images appearing at f/9 = 1.33 Hz. Faces posing all full-front or at ¾ side view angles appeared in separate sequences. Robust frequency-tagged 1.33 Hz (and harmonic) occipito-temporal electroencephalographic (EEG) responses reflecting face-selective neural activity did not differ in overall amplitude between full-front and ¾ side views. Despite this, alternating between full-front and ¾ side views within a sequence led to significant responses at specific harmonics of .67 Hz (f/18), objectively isolating view-dependent face-selective responses over occipito-temporal regions. Critically, a time-domain analysis showed that these view-dependent face-selective responses reflected only an earlier response to full-front than ¾ side views by 8-13 ms. Overall, these findings indicate that the face-selective neural representation is as robust for ¾ side faces as for full-front faces in the human brain, but full-front views provide a slightly earlier processing-time advantage as compared to rotated face views.

Serial dependence of facial identity reflects high-level face coding

Serial dependence of facial identity is a type of bias where the perceived identity of a face is biased towards a previously presented face. There are individual differences in serial dependence strength and tuning (how the strength varies depending on stimuli similarity), and previous research has shown that both stronger and more narrowly tuned serial dependence of facial identity is associated with better face recognition abilities. These results are consistent with the idea that this bias plays a functional role in face perception. It is important, therefore, to determine whether serial dependence of facial identity reflects a high-level face-coding mechanism acting on the identity of a face or instead predominantly reflects a bias in low-level features, which are also subject to serial dependence. We first sought evidence that serial dependence of facial identity survived changes in low-level visual features, by varying face viewpoint between successive stimuli. We found that serial dependence persisted across changes in viewpoint, arguing against an entirely low-level locus for this bias. We next tested whether the bias was affected by inversion, as sensitivity to inversion is argued to be a characteristic of high-level face-selective processing. Serial dependence was stronger and more narrowly tuned for upright than inverted faces. Taken together, our results are consistent with the view that serial dependence of facial identity affects high-level visual representations and may reflect a face-coding mechanism that is operating at the level of facial identity.

Are expression aftereffects fully explained by tilt adaptation?

Facial expressions are used as critical social cues in everyday life. Adaptation to expressions causes expression aftereffects. These aftereffects are thought to reflect the operation of face-selective neural mechanisms, and are used by researchers to investigate the nature of those mechanisms. However, recent evidence suggests that expression aftereffects could be at least partially explained by the inheritance of lower-level tilt adaptation through the visual hierarchy. We investigated whether expression aftereffects could be entirely explained by tilt adaptation. Participants completed an expression adaptation task in which we controlled for the influence of tilt by changing the orientation of the adaptor relative to the test stimuli. Although tilt adaptation appeared to make some contribution to the expression aftereffect, robust expression aftereffects still remained after minimizing tilt inheritance, indicating that expression aftereffects cannot be fully explained by tilt adaptation. There was also significant reduction in the expression aftereffects after inverting the adapting face, providing evidence that face-selective processing is involved in these aftereffects.

Adaptation to dynamic faces produces face identity aftereffects

Face aftereffects are well established for static stimuli and have been used extensively as a tool for understanding the neural mechanisms underlying face recognition. It has also been argued that adaptive coding, as demonstrated by face aftereffects, plays a functional role in face recognition by calibrating our face norms to reflect current experience. If aftereffects tap high-level perceptual mechanisms that are critically involved in everyday face recognition then they should also occur for moving faces. Here we asked whether face identity aftereffects can be induced using dynamic adaptors. The face identity aftereffect occurs when adaptation to a particular identity (e.g., Dan) biases subsequent perception toward the opposite identity (e.g., antiDan). We adapted participants to video of real faces that displayed either rigid, non-rigid, or no motion and tested for aftereffects in static antifaces. Adapt and test stimuli differed in size, to minimize low-level adaptation. Aftereffects were found in all conditions, suggesting that face identity aftereffects tap high-level mechanisms important for face recognition. Aftereffects were not significantly reduced in the motion conditions relative to the static condition. Overall, our results support the view that face aftereffects reflect adaptation of high-level mechanisms important for real-world face recognition in which faces are moving.

Orientation Encoding and Viewpoint Invariance in Face Recognition: Inferring Neural Properties from Large-Scale Signals

Viewpoint-invariant face recognition is thought to be subserved by a distributed network of occipitotemporal face-selective areas that, except for the human anterior temporal lobe, have been shown to also contain face-orientation information. This review begins by highlighting the importance of bilateral symmetry for viewpoint-invariant recognition and face-orientation perception. Then, monkey electrophysiological evidence is surveyed describing key tuning properties of face-selective neurons-including neurons bimodally tuned to mirror-symmetric face-views-followed by studies combining functional magnetic resonance imaging (fMRI) and multivariate pattern analyses to probe the representation of face-orientation and identity information in humans. Altogether, neuroimaging studies suggest that face-identity is gradually disentangled from face-orientation information along the ventral visual processing stream. The evidence seems to diverge, however, regarding the prevalent form of tuning of neural populations in human face-selective areas. In this context, caveats possibly leading to erroneous inferences regarding mirror-symmetric coding are exposed, including the need to distinguish angular from Euclidean distances when interpreting multivariate pattern analyses. On this basis, this review argues that evidence from the fusiform face area is best explained by a view-sensitive code reflecting head angular disparity, consistent with a role of this area in face-orientation perception. Finally, the importance is stressed of explicit models relating neural properties to large-scale signals.

About Face: Matching Unfamiliar Faces Across Rotations of View and Lighting

Matching the identities of unfamiliar faces is heavily influenced by variations in their images. Changes to viewpoint and lighting direction during face perception are commonplace across yaw and pitch axes and can result in dramatic image differences. We report two experiments that, for the first time, factorially investigate the combined effects of lighting and view angle on matching performance for unfamiliar faces. The use of three-dimensional head models allowed control of both lighting and viewpoint. We found viewpoint effects in the yaw axis with little to no effect of lighting. However, for rotations about the pitch axis, there were both viewpoint and lighting effects and these interacted where lighting effects were found only for front views and views from below. The pattern of effects was similar regardless of whether view variation occurred as a result of head (Experiment 1) or camera (Experiment 2) suggesting that face matching is not purely image based. Along with face inversion effects in Experiment 1, the results of this study suggest that face perception is based on shape and surface information and draws on implicit knowledge of upright faces and ecological (top) lighting conditions.

On Response Bias in the Face Congruency Effect for Internal and External Features

Some years ago Cheung et al. (2008) proposed the complete design (CD) for measuring the failure of selective attention in composite objects. Since the CD is a fully balanced design, analysis of response bias may reveal potential effects of the experimental manipulation, the stimulus material, and/or attributes of the observers. Here we used the CD to prove whether external features modulate perception of internal features with the context congruency paradigm (Nachson et al., 1995; Meinhardt-Injac et al., 2010) in a larger sample of N = 303 subjects. We found a large congruency effect (Cohen's d = 1.78), which was attenuated by face inversion (d = 1.32). The congruency relation also strongly modulated response bias. In incongruent trials the proportion of "different" responses was much larger than in congruent trials (d = 0.79), which was again attenuated by face inversion (d = 0.43). Because in incongruent trials the wholes formed by integrating external and internal features are always different, while in congruent trials same and different wholes occur with the same frequency, a congruency related bias effect is expected from holistic integration. Our results suggest two behavioral markers of holistic processing in the context congruency paradigm: a performance advantage in congruent compared to incongruent trials, and a tendency toward more "different" responses in incongruent, compared to congruent trials. Since the results for both markers differed only quantitatively in upright and inverted presentation, our findings indicate no change of the face processing mode by picture plane rotation. A potential transfer to the composite face paradigm is discussed.

Face-Space: A Unifying Concept in Face Recognition Research

The concept of a multidimensional psychological space, in which faces can be represented according to their perceived properties, is fundamental to the modern theorist in face processing. Yet the idea was not clearly expressed until 1991. The background that led to the development of face-space is explained, and its continuing influence on theories of face processing is discussed. Research that has explored the properties of the face-space and sought to understand caricature, including facial adaptation paradigms, is reviewed. Face-space as a theoretical framework for understanding the effect of ethnicity and the development of face recognition is evaluated. Finally, two applications of face-space in the forensic setting are discussed. From initially being presented as a model to explain distinctiveness, inversion, and the effect of ethnicity, face-space has become a central pillar in many aspects of face processing. It is currently being developed to help us understand adaptation effects with faces. While being in principle a simple concept, face-space has shaped, and continues to shape, our understanding of face perception.

The Role of Familiarity on Viewpoint Adaptation for Self-Face and Other-Face Images

An adaptation method was used to investigate whether self-face processing is dissociable from general face processing. We explored the viewpoint aftereffect with face images having different degrees of familiarity (never-before-seen faces, recently familiarized faces, personally familiar faces, and the participant’s own face). A face viewpoint aftereffect occurs after prolonged viewing of a face viewed from one side, with the result that the perceived viewing direction of a subsequently presented face image shown near the frontal view is biased in a direction which is the opposite of the adapting orientation. We found that (1) the magnitude of the viewpoint aftereffect depends on the level of familiarity of the adapting and test faces, (2) a cross-identity transfer of the viewpoint aftereffect is found between all categories of faces, but not between an unfamiliar adaptor face and the self-face test, and (3) learning affects the processing of the self-face in greater measure than any other category of faces. These results highlight the importance of familiarity on the face aftereffects, but they also suggest the possibility of separate representations for the self-face, on the one side, and for highly familiar faces, on the other.

Probing the Visual Representation of Faces With Adaptation

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.

Face adaptation effects: reviewing the impact of adapting information, time, and transfer

The ability to adapt is essential to live and survive in an ever-changing environment such as the human ecosystem. Here we review the literature on adaptation effects of face stimuli to give an overview of existing findings in this area, highlight gaps in its research literature, initiate new directions in face adaptation research, and help to design future adaptation studies. Furthermore, this review should lead to better understanding of the processing characteristics as well as the mental representations of face-relevant information. The review systematizes studies at a behavioral level in respect of a framework which includes three dimensions representing the major characteristics of studies in this field of research. These dimensions comprise (1) the specificity of adapting face information, e.g., identity, gender, or age aspects of the material to be adapted to (2) aspects of timing (e.g., the sustainability of adaptation effects) and (3) transfer relations between face images presented during adaptation and adaptation tests (e.g., images of the same or different identities). The review concludes with options for how to combine findings across different dimensions to demonstrate the relevance of our framework for future studies.

Viewpoint and Pose in Body-Form Adaptation

Faces and bodies are complex structures, perception of which can play important roles in person identification and inference of emotional state. Face representations have been explored using behavioural adaptation: in particular, studies have shown that face aftereffects show relatively broad tuning for viewpoint, consistent with origin in a high-level structural descriptor far removed from the retinal image. Our goals were to determine first, if body aftereffects also showed a degree of viewpoint invariance, and second if they also showed pose invariance, given that changes in pose create even more dramatic changes in the 2-D retinal image. We used a 3-D model of the human body to generate headless body images, whose parameters could be varied to generate different body forms, viewpoints, and poses. In the first experiment, subjects adapted to varying viewpoints of either slim or heavy bodies in a neutral stance, followed by test stimuli that were all front-facing. In the second experiment, we used the same front-facing bodies in neutral stance as test stimuli, but compared adaptation from bodies in the same neutral stance to adaptation with the same bodies in different poses. We found that body aftereffects were obtained over substantial viewpoint changes, with no significant decline in aftereffect magnitude with increasing viewpoint difference between adapting and test images. Aftereffects also showed transfer across one change in pose but not across another. We conclude that body representations may have more viewpoint invariance than faces, and demonstrate at least some transfer across pose, consistent with a high-level structural description.

Ignored faces produce figural face aftereffects

Following adaptation to faces with contracted (or expanded) internal features, faces previously perceived as normal appear distorted in the opposite direction. This figural face aftereffect suggests face-coding mechanisms adapt to changes in the spatial relations of features and/or the global structure of faces. Here, we investigated whether the figural aftereffect requires spatial attention. Participants ignored a distorted adapting face and performed a highly demanding letter-count task. Before and after adaptation, participants rated the normality of morphed distorted faces ranging from 50% contracted through undistorted to 50% expanded. A robust aftereffect was observed. These results suggest that the figural face aftereffect can occur in the absence of spatial attention, even when the attentional demands of the relevant task are high.

View-adaptation reveals coding of face pose along image, not object, axes

High-level adaptation effects reveal important features of the neural coding of objects and faces. View-adaptation in particular is a highly useful means of characterizing how depth rotation of the face is represented and therefore, how view-invariant recognition of the face may be achieved. In the present study, we used view adaptation to determine the extent to which depth rotations of a face are represented in an image-based or object-based manner. Specifically, we dissociated object-based axes from image-based axes via a 90° planar rotation of the adapting face and observed that participants' responses pre- and post-adaptation are most consistent with an image-based representation of depth rotations of the face. We discuss our data in the context of previous results describing the impact of planar rotation on related aspects of face perception.

The influence of combined orthodontic-orthognathic surgical treatment on perceptions of attractiveness: a longitudinal study

The purpose of this investigation was to quantitatively evaluate the influence of completing the orthognathic treatment process on patients' perceptions of attractiveness and their desire for surgical correction. The mandibular prominence of an idealized profile image was altered in 2 mm increments from -16 to 12 mm, in order to represent retrusion and protrusion of the mandible, respectively. These images were rated on a seven-point Likert scale by 50 patients at T1 (pre-treatment) and T2 (6 months following orthodontic appliance removal). At T1, mandibular retrusion became noticeable at -4 mm and protrusion at 2 mm. The results remained unchanged at T2. Surgery was desired from -9 mm at T1 and -10 mm at T2. For mandibular protrusion, surgery was desired from 3 mm at T1 and 4 mm at T2. The odds of desire for surgery were reduced by 85 per cent for those patients who had undergone bimaxillary surgery in relation to those with single jaw surgery. The lowest rated images demonstrated severe degrees of mandibular protrusion and retrusion. The highest rated images represented the idealized facial profile and minor variations thereof; there was little change in perception between T1 and T2. Going through the process of orthognathic treatment does not appear to have any significant effect on patients' perceptions of facial profile attractiveness or the limits of mandibular sagittal deviation at which they would desire surgery. The clinician's information provision during treatment does not seem to unduly influence orthognathic patients and does not make them more critical of jaw deformities.

The role of skin texture and facial shape in representations of age and identity

Faces have both shape and skin texture, but the relative importance of the two in face representations is unclear. Our goals were first, to determine the contribution of shape versus texture to aftereffects for facial age and identity and second, to assess whether adaptation transferred between shape and texture, suggesting integration in a single representation. In our first experiment we examined age aftereffects. We obtained young and old images of two celebrities and created hybrid images, one combining the structure of the old face with the skin texture of the young face, the other combining the young structure with the old skin texture. This allowed us to create adaptation contrasts where the two adapting faces had the same facial structure but different skin texture, and vice versa. In the second experiment, we performed a similar study but this time examining identity aftereffects between two people of a similar age. We found that both skin texture and facial shape generated significant age aftereffects, but the contribution was greater from texture than from shape. Both texture and shape also generated significant identity aftereffects, but the contribution was greater from shape than from texture. In the last experiment, we used the normal and hybrid images to determine if adaptation to one property (i.e., texture) could create aftereffects in the perception of age in the other property (i.e., shape). While there was significant within-component adaptation for texture and shape, there was no evidence of cross-component adaptation. We conclude that shape and texture contribute differently to different face representations, with texture dominating for age. The lack of cross-component adaptation transfer suggests independent encoding of shape and texture, at least for age representations.

Visual adaptation and face perception

The appearance of faces can be strongly affected by the characteristics of faces viewed previously. These perceptual after-effects reflect processes of sensory adaptation that are found throughout the visual system, but which have been considered only relatively recently in the context of higher level perceptual judgements. In this review, we explore the consequences of adaptation for human face perception, and the implications of adaptation for understanding the neural-coding schemes underlying the visual representation of faces. The properties of face after-effects suggest that they, in part, reflect response changes at high and possibly face-specific levels of visual processing. Yet, the form of the after-effects and the norm-based codes that they point to show many parallels with the adaptations and functional organization that are thought to underlie the encoding of perceptual attributes like colour. The nature and basis for human colour vision have been studied extensively, and we draw on ideas and principles that have been developed to account for norms and normalization in colour vision to consider potential similarities and differences in the representation and adaptation of faces.

Electrophysiological correlates of face distortion after-effects

When observers are exposed to a distorted face the perceived configuration of a subsequently presented face is altered, a phenomenon called face distortion after-effect (FDAE). We compared the face-related components of the event-related potential (ERP) after adaptation to noise images—veridical and distorted faces. We found large bilateral adaptation effects on the P100 and N170 components that are related to face detection. Moreover, we found smaller adaptation effects on the N170, recorded over the right hemisphere, which can be related to the behavioural distortion after-effect and to face configurations. Our results suggest that the observed ERP adaptation effects are general for various steps of face processing and that the FDAEs similarly to gender after-effects are related to the early face-specific ERP components.

Identity-specific face adaptation effects: evidence for abstractive face representations

The effects of selective adaptation on familiar face perception were examined. After prolonged exposure to photographs of a celebrity, participants saw a series of ambiguous morphs that were varying mixtures between the face of that person and a different celebrity. Participants judged fewer of the morphs to resemble the celebrity to which they had been adapted, implying that they were now less sensitive to that particular face. Similar results were obtained when the adapting faces were highly dissimilar in viewpoint to the test morphs; when they were presented upside-down; or when they were vertically stretched to three times their normal height. These effects rule out explanations of adaptation effects solely in terms of low-level image-based adaptation. Instead they are consistent with the idea that relatively viewpoint-independent, person-specific adaptation occurred, at the level of either the "Face Recognition Units" or "Person Identity Nodes" in Burton, Bruce and Johnston's (1990) model of face recognition.

Adaptation improves discrimination of face identity

Whether face adaptation confers any advantages to perceptual processing remains an open question. We investigated whether face adaptation can enhance the ability to make fine discriminations in the vicinity of the adapted face. We compared face discrimination thresholds in three adapting conditions: (i) same-face: where adapting and test faces were the same, (ii) different-face: where adapting and test faces differed, and (iii) baseline: where the adapting stimulus was a blank. Discrimination thresholds for morphed identity changes involving the adapted face (same-face) improved compared with those from both the baseline (no-adaptation) and different-face conditions. Since adapting to a face did not alter discrimination performance for other faces, this effect is selective for the facial identity that is adapted. These results indicate a form of gain control to heighten perceptual sensitivity in the vicinity of a currently viewed face, analogous to forms of adaptive gain control at lower levels of the visual system.

Face Viewpoint Effects about Three Axes: The Role of Configural and Featural Processing

We directly compared recognition for faces following 0° – 75° viewpoint rotation about the yaw, pitch, and roll axes. The aim was to determine the extent to which configural and featural information supported face recognition following rotations about each of these axes. Experiment 1 showed that performance on a sequential-matching task was viewpoint-dependent for all three types of rotation. The best face-recognition accuracy and shortest reaction time was found for roll rotations, then for yaw rotations, and finally the worst accuracy and slowest reaction time was found for pitch rotations. Directional differences in recognition were found for pitch rotations, but not for roll or yaw. Experiment 2 provided evidence that, in all three cases, viewpoint-dependent declines in recognition were primarily driven by the loss of configural information. However, it also appeared that significant featural information was lost following yaw and pitch (but not roll) rotations. Together, these findings show that unfamiliar-face recognition is viewpoint-dependent following rotation about each axis (and in each direction), and that performance is based on the availability of configural and, to a lesser extent, featural information.

Adaptation to different mouth shapes influences visual perception of ambiguous lip speech

We investigated the effects of adaptation to mouth shapes associated with different spoken sounds (sustained /m/ or /u/) on visual perception of lip speech. Participants were significantly more likely to label ambiguous faces on an /m/-to-/u/ continuum as saying /u/ following adaptation to /m/ mouth shapes than they were in a preadaptation test. By contrast, participants were significantly less likely to label the ambiguous faces as saying /u/ following adaptation to /u/ mouth shapes than they were in a preadaptation test. The magnitude of these aftereffects was equivalent when the same individual was shown in the adaptation and test phases of the experiment and when different individuals were presented in the adaptation and test phases. These findings present novel evidence that adaptation to natural variations in facial appearance influences face perception, and they extend previous research on face aftereffects to visual perception of lip speech.

Probing the face-space of individuals with prosopagnosia

A useful framework for understanding the mental representation of facial identity is face-space (Valentine, 1991), a multi-dimensional cognitive map in which individual faces are coded relative to the average of previously encountered faces, and in which the distance among faces represents their perceived similarity. We examined whether individuals with prosopagnosia, a disorder characterized by an inability to recognize familiar faces despite normal visual acuity and intellectual abilities, evince behavior consistent with this underlying representational schema. To do so, we compared the performance of 6 individuals with congenital prosopagnosia (CP), with a group of age- and gender-matched control participants in a series of experiments involving judgments of facial identity. We used digital images of male and female faces and morphed them to varying degrees relative to an average face, to create caricatures, anti-caricatures, and anti-faces (i.e. faces of the opposite identity). Across 5 behavioral tasks, CP individuals' performance was similar to that of the control group and consistent with the face-space framework. As a test of the sensitivity of our measures in revealing face processing abnormalities, we also tested a single acquired prosopagnosic (AP) individual, whose performance on the same tasks deviated significantly from the control and CP groups. The findings suggest that, despite an inability to recognize individual identities, CPs perceive faces in a manner consistent with norm-based coding of facial identity, although their representation is likely supported by a feature-based strategy. We suggest that the apparently normal posterior cortical regions, including the fusiform face area, serve as the neural substrate for at least a coarse, feature-based face-space map in CP and that their face recognition impairment arises from the disconnection between these regions and more anterior cortical sites.

What shape are the neural response functions underlying opponent coding in face space? A psychophysical investigation

Recent evidence has shown that face space represents facial identity information using two-pool opponent coding. Here we ask whether the shape of the monotonic neural response functions underlying such coding is linear (i.e. face space codes all equal-sized physical changes with equal sensitivity) or nonlinear (e.g. face space shows greater coding sensitivity around the average face). Using adaptation aftereffects and pairwise discrimination tasks, our results for face attributes of eye height and mouth height demonstrate linear shape; including for bizarre faces far outside the normal range. We discuss how linear coding explains some results in the previous literature, including failures to find that adaptation enhances face discrimination, and suggest possible reasons why face space can maintain detailed coding of values far outside the normal range. We also discuss specific nonlinear coding models needed to explain other findings, and conclude face space appears to use a mixture of linear and nonlinear representations.

Does matching of internal and external facial features depend on orientation and viewpoint?

Although it is recognized that external (hair, head and face outline, ears) and internal (eyes, eyebrows, nose, mouth) features contribute differently to face recognition it is unclear whether both feature classes predominantly stimulate different sensory pathways. We employed a sequential speed-matching task to study face perception with internal and external features in the context of intact faces, and at two levels of contextual congruency. Both internal and external features were matched faster and more accurately in the context of totally congruent/incongruent facial stimuli compared to just featurally congruent/incongruent faces. Matching of totally congruent/incongruent faces was not affected by the matching criteria, but was strongly modulated by orientation and viewpoint. On the contrary, matching of just featurally congruent/incongruent faces was found to depend on the feature class to be attended, with strong effects of orientation and viewpoint only for matching of internal features, but not of external features. The data support the notion that different processing mechanisms are involved for both feature types, with internal features being handled by configuration sensitive mechanisms whereas featural processing modes dominate when external features are the focus.

Psychophysical evidence for a non-linear representation of facial identity

It has been proposed that faces are represented in the visual brain as points within a multi-dimensional “face space”, with the average at its origin. We adapted a psychophysical procedure that measures non-linearities in contrast transduction (by measuring discrimination around different reference/pedestal levels of contrast) to examine the encoding of facial-identity within such a notional space. Specifically we had subjects perform identity discrimination at various pedestal levels of identity (varying from average/0% to caricature/125% identity) to derive “identity dipper functions”. Results indicate that subjects are generally best at spotting identity change in neither average nor full-identity faces, but rather in faces containing an intermediate level of identity (which varies from face-to-face). The overall pattern of results is consistent with the neural encoding of faces involving a single modest non-linear transformation of identity that is consistent across faces and subjects, but that it scaled according to the distinctiveness of the face.

Three-dimensional information in face representations revealed by identity aftereffects

Representations of individual faces evolve with experience to support progressively more robust recognition. Knowledge of three-dimensional face structure is required to predict an image of a face as illumination and viewpoint change. Robust recognition across such transformations can be achieved with representations based on multiple two-dimensional views, three-dimensional structure, or both. We used face-identity adaptation in a familiarization paradigm to address a long-standing controversy concerning the role of two-dimensional versus three-dimensional information in face representations. We reasoned that if three-dimensional information is coded in the representations of familiar faces, then learning a new face using images generated by one three-dimensional transformation should enhance the robustness of the representation to another type of three-dimensional transformation. Familiarization with multiple views of faces enhanced the transfer of face-identity adaptation effects across changes in illumination by compensating for a generalization cost at a novel test viewpoint. This finding demonstrates a role for three-dimensional information in representations of familiar faces.

View-Contingent Aftereffects Suggest Joint Coding of Face Shape and View

While it is well established that different neural populations code different face views, behavioural evidence that these neurons also code other aspects of face shape is equivocal. For example, previous studies have interpreted the partial transfer of face aftereffects across different viewpoints as evidence for either view-specific coding of face shape or that the locus of adaptation is in face-coding mechanisms that are relatively robust to changes in face view. Here we show that it is possible to simultaneously induce aftereffects in opposite directions for 3/4 and front views of upright faces with manipulated mouth position (experiment 1). For example, simultaneous adaptation to 3/4 views with raised mouth position and front views with lowered mouth position caused raised mouth position to appear more normal for 3/4 views of novel faces, but less normal for front views. View-contingent adaptation did not occur for inverted faces, however (experiment 2). Dissociable aftereffects for different views of upright faces, but not for different views of inverted faces, suggest that neurons that code face view can also code other aspects of face shape.