Visual representation of eye gaze is coded by a nonopponent multichannel system

Viewpoint adaptation revealed potential representational differences between 2D images and 3D objects

For convenience and experimental control, cognitive science has relied largely on images as stimuli rather than the real, tangible objects encountered in the real world. Recent evidence suggests that the cognitive processing of images may differ from real objects, especially in the processing of spatial locations and actions, thought to be mediated by the dorsal visual stream. Perceptual and semantic processing in the ventral visual stream, however, has been assumed to be largely unaffected by the realism of objects. Several studies have found that one key difference accounting for differences between real objects and images is actability; however, less research has investigated another potential difference - the three-dimensional nature of real objects as conveyed by cues like binocular disparity. To investigate the extent to which perception is affected by the realism of a stimulus, we compared viewpoint adaptation when stimuli (a face or a kettle) were 2D (flat images without binocular disparity) vs. 3D (i.e., real, tangible objects or stereoscopic images with binocular disparity). For both faces and kettles, adaptation to 3D stimuli induced stronger viewpoint aftereffects than adaptation to 2D images when the adapting orientation was rightward. A computational model suggested that the difference in aftereffects could be explained by broader viewpoint tuning for 3D compared to 2D stimuli. Overall, our finding narrowed the gap between understanding the neural processing of visual images and real-world objects by suggesting that compared to 2D images, real and simulated 3D objects evoke more broadly tuned neural representations, which may result in stronger viewpoint invariance.

Decreased frontal lobe complexity in left-behind children during joint attention: a fNIRS study with multivariable and multiscale sample entropy analysis

Human brain development is shaped by experiences, especially during preschool, the critical period for cognitive and socioemotional development. This study employed the functional Near-Infrared Spectroscopy technique to explore the neural differences between left-behind children (LBC) and non-left-behind children (NLBC) on joint attention. Through collecting brain image data of 50 children (26 boys, aged 65.08 ± 6.28 months) and conducting multivariable and multiscale sample entropy (MMSE) analysis, the present study found that: (i) LBC showed lower brain complexity than NLBC in right prefrontal cortex; (ii) all participants demonstrated higher brain complexity in responding to joint attention conditions, compared to initiating joint attention ones; (iii) their brain complexity during joint attention was negatively associated with their emotional abilities. The findings advance our understanding of early brain development in LBC by providing evidence for the neural process characteristics of joint attention. Implications for early intervention to promote their brain development are also addressed.

Visual adaptation reveals multichannel coding for numerosity

Visual numerosity is represented automatically and rapidly, but much remains unknown about the computations underlying this perceptual experience. For example, it is unclear whether numerosity is represented with an opponent channel or multichannel coding system. Within an opponent channel system, all numerical values are represented via the relative activity of two pools of neurons (i.e., one pool with a preference for small numerical values and one pool with a preference for large numerical values). However, within a multichannel coding system, all numerical values are represented directly, with separate pools of neurons for each (discriminable) numerical value. Using an adaptation paradigm, we assessed whether the visual perception of number is better characterized by an opponent channel or multichannel system. Critically, these systems make distinct predictions regarding the pattern of aftereffects exhibited when an observer is adapted to an intermediate numerical value. Opponent channel coding predicts no aftereffects because both pools of neurons adapt equally. By contrast, multichannel coding predicts repulsive aftereffects, wherein numerical values smaller than the adapter are underestimated and those larger than the adapter are overestimated. Consistent with multichannel coding, visual adaptation to an intermediate value (50 dots) yielded repulsive aftereffects, such that participants underestimated stimuli ranging from 10-50 dots, but overestimated stimuli ranging from 50-250 dots. These findings provide novel evidence that the visual perception of number is supported by a multichannel, not opponent channel, coding system, and raise important questions regarding the contributions of different cortical regions, such as the ventral and lateral intraparietal areas, to the representation of number.

Ramp-shaped neural tuning supports graded population-level representation of the object-to-scene continuum

We can easily perceive the spatial scale depicted in a picture, regardless of whether it is a small space (e.g., a close-up view of a chair) or a much larger space (e.g., an entire class room). How does the human visual system encode this continuous dimension? Here, we investigated the underlying neural coding of depicted spatial scale, by examining the voxel tuning and topographic organization of brain responses. We created naturalistic yet carefully-controlled stimuli by constructing virtual indoor environments, and rendered a series of snapshots to smoothly sample between a close-up view of the central object and far-scale view of the full environment (object-to-scene continuum). Human brain responses were measured to each position using functional magnetic resonance imaging. We did not find evidence for a smooth topographic mapping for the object-to-scene continuum on the cortex. Instead, we observed large swaths of cortex with opposing ramp-shaped profiles, with highest responses to one end of the object-to-scene continuum or the other, and a small region showing a weak tuning to intermediate scale views. However, when we considered the population code of the entire ventral occipito-temporal cortex, we found smooth and linear representation of the object-to-scene continuum. Our results together suggest that depicted spatial scale information is encoded parametrically in large-scale population codes across the entire ventral occipito-temporal cortex.

Spatial selectivity in adaptation to gaze direction

A person's focus of attention is conveyed by the direction of their eyes and face, providing a simple visual cue fundamental to social interaction. A growing body of research examines the visual mechanisms that encode the direction of another person's gaze as we observe them. Here we investigate the spatial receptive field properties of these mechanisms, by testing the spatial selectivity of sensory adaptation to gaze direction. Human observers were adapted to faces with averted gaze presented in one visual hemifield, then tested in their perception of gaze direction for faces presented in the same or opposite hemifield. Adaptation caused strong, repulsive perceptual aftereffects, but only for faces presented in the same hemifield as the adapter. This occurred even though adapting and test stimuli were in the same external location across saccades. Hence, there was clear evidence for retinotopic adaptation and a relative lack of either spatiotopic or spatially invariant adaptation. These results indicate that adaptable representations of gaze direction in the human visual system have retinotopic spatial receptive fields. This strategy of coding others' direction of gaze with positional specificity relative to one's own eye position may facilitate key functions of gaze perception, such as socially cued shifts in visual attention.

Normal gaze processing in developmental prosopagnosia

Two key functions in human face perception are gaze discrimination and identity recognition. Here we examine whether gaze discrimination can be intact when identity recognition is impaired in developmental prosopagnosia (DP). We ran a large sample of developmental prosopagnosics (DPs) with a series of gaze discrimination tasks that assess various mechanisms in gaze processing. Experiment 1 (N = 101 DP participants) investigates spatial processing using an abnormal eye gaze detection task and a Wollaston illusion task that measures perceptual integration of eye and head direction. Experiment 2 (N = 45 DP participants) investigates temporal processing using an adaptation task and a serial dependence task. Despite their deficits with identity recognition, DPs performed in the normal range across both experiments. These results demonstrate that gaze discrimination can be normal in DP, and that various mechanisms of gaze processing can be spared when identity recognition is impaired. Our findings clarify the highly selective nature of impairments in DP and provide support for neurocognitive models of face perception with distinct mechanisms for gaze and identity processing.

Contrast Adaptation in Face Perception Revealed Through EEG and Behavior

Exposure to a face can produce biases in the perception of subsequent faces. Typically, these face aftereffects are studied by adapting to an individual face or category (e.g., faces of a given gender) and can result in renormalization of perceptions such that the adapting face appears more neutral. These shifts are analogous to chromatic adaptation, where a renormalization for the average adapting color occurs. However, in color vision, adaptation can also adjust to the variance or range of colors in the distribution. We examined whether this variance or contrast adaptation also occurs for faces, using an objective EEG measure to assess response changes following adaptation. An average female face was contracted or expanded along the horizontal or vertical axis to form four images. Observers viewed a 20 s sequence of the four images presented in a fixed order at a rate of 6 Hz, while responses to the faces were recorded with EEG. A 6 Hz signal was observed over right occipito-temporal channels, indicating symmetric responses to the four images. This test sequence was repeated after 20 s adaptation to alternations between two of the faces (e.g., horizontal contracted and expanded). This adaptation resulted in an additional signal at 3 Hz, consistent with asymmetric responses to adapted and non-adapted test faces. Adapting pairs have the same mean (undistorted) as the test sequence and thus should not bias responses driven only by the mean. Instead, the results are consistent with selective adaptation to the distortion axis. A 3 Hz signal was also observed after adapting to face pairs selected to induce a mean bias (e.g., expanded vertical and expanded horizontal), and this signal was not significantly different from that observed following adaption to a single image that did not form part of the test sequence (e.g., a single image expanded both vertically and horizontally). In a further experiment, we found that this variance adaptation can also be observed behaviorally. Our results suggest that adaptation calibrates face perception not only for the average characteristics of the faces we experience but also for the gamut of faces to which we are exposed.

Fear-specific leftward bias in gaze direction judgment

Previous studies have shown that humans have a left spatial attention bias in cognition and behaviour. However, whether there exists a leftward perception bias of gaze direction has not been investigated. To address this gap, we conducted three behavioural experiments using a forced-choice gaze direction judgment task. The point of subjective equality (PSE) was employed to measure whether there was a leftward perception bias of gaze direction, and if there was, whether this bias was modulated by face emotion. The results of experiment 1 showed that the PSE of fearful faces was significantly positive as compared to zero and this effect was not found in angry, happy, and neutral faces, indicating that participants were more likely to judge the gaze direction of fearful faces as directed to their left-side space, namely a leftward perception bias. With the response keys counterbalanced between participants, experiment 2a replicated the findings in experiment 1. To further investigate whether the gaze direction perception variation was contributed by emotional or low-level features of faces, experiment 2b and 3 used inverted faces and inverted eyes, respectively. The results revealed similar leftward perception biases of gaze direction in all types of faces, indicating that gaze direction perception was biased by emotional information in faces rather than low-level facial features. Overall, our study demonstrates that there a fear-specific leftward perception bias in processing gaze direction. These findings shed new light on the cerebral lateralization in humans.

From Gaze Perception to Social Cognition: The Shared-Attention System

When two people look at the same object in the environment and are aware of each other's attentional state, they find themselves in a shared-attention episode. This can occur through intentional or incidental signaling and, in either case, causes an exchange of information between the two parties about the environment and each other's mental states. In this article, we give an overview of what is known about the building blocks of shared attention (gaze perception and joint attention) and focus on bringing to bear new findings on the initiation of shared attention that complement knowledge about gaze following and incorporate new insights from research into the sense of agency. We also present a neurocognitive model, incorporating first-, second-, and third-order social cognitive processes (the shared-attention system, or SAS), building on previous models and approaches. The SAS model aims to encompass perceptual, cognitive, and affective processes that contribute to and follow on from the establishment of shared attention. These processes include fundamental components of social cognition such as reward, affective evaluation, agency, empathy, and theory of mind.

Face Pareidolia Recruits Mechanisms for Detecting Human Social Attention

Face pareidolia is the phenomenon of seeing facelike structures in everyday objects. Here, we tested the hypothesis that face pareidolia, rather than being limited to a cognitive or mnemonic association, reflects the activation of visual mechanisms that typically process human faces. We focused on sensory cues to social attention, which engage cell populations in temporal cortex that are susceptible to habituation effects. Repeated exposure to “pareidolia faces” that appear to have a specific direction of attention causes a systematic bias in the perception of where human faces are looking, indicating that overlapping sensory mechanisms are recruited when we view human faces and when we experience face pareidolia. These cross-adaptation effects are significantly reduced when pareidolia is abolished by removing facelike features from the objects. These results indicate that face pareidolia is essentially a perceptual phenomenon, occurring when sensory input is processed by visual mechanisms that have evolved to extract specific social content from human faces.

Perceiving facial affective ambiguity: A behavioral and neural comparison of adolescents and adults

The current study examined perceptual differences between adults and youth in perceiving ambiguous facial expressions. We estimated individuals' internal representation for facial expressions and compared it between age groups (adolescents: N = 108, Mage = 13.04 years, 43.52% female; adults: N = 81, Mage = 31.54, 65.43% female). We found that adolescents' perceptual representation for facial emotion is broader than that of adults', such that adolescents experience more difficulty in identifying subtle configurational differences of facial expressions. At the neural level, perceptual uncertainty in face-selective regions (e.g., fusiform face area, occipital face area) were significantly higher for adolescents than for adults, suggesting that adolescents' brains more similarly represent lower intensity emotional faces than do adults'. Our results provide evidence for age-related differences concerning psychophysical differences in perceptual representation of emotional faces at the neural and behavioral level. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Perceiving gaze from head and eye rotations: An integrative challenge for children and adults

Gaze is an emergent visual feature. A person's gaze direction is perceived not just based on the rotation of their eyes, but also their head. At least among adults, this integrative process appears to be flexible such that one feature can be weighted more heavily than the other depending on the circumstances. Yet it is unclear how this weighting might vary across individuals or across development. When children engage emergent gaze, do they prioritize cues from the head and eyes similarly to adults? Is the perception of gaze among individuals with autism spectrum disorder (ASD) emergent, or is it reliant on a single feature? Sixty adults (M = 29.86 years-of-age), thirty-seven typically developing children and adolescents (M = 9.3 years-of-age; range = 7-15), and eighteen children with ASD (M = 9.72 years-of-age; range = 7-15) viewed faces with leftward, rightward, or direct head rotations in conjunction with leftward or rightward pupil rotations, and then indicated whether the face was looking leftward or rightward. All individuals, across development and ASD status, used head rotation to infer gaze direction, albeit with some individual differences. However, the use of pupil rotation was heavily dependent on age. Finally, children with ASD used pupil rotation significantly less than typically developing (TD) children when inferring gaze direction, even after accounting for age. Our approach provides a novel framework for understanding individual and group differences in gaze as it is actually perceived-as an emergent feature. Furthermore, this study begins to address an important gap in ASD literature, taking the first look at emergent gaze perception in this population.

Seeing eye-to-eye: Social gaze interactions influence gaze direction identification

We tested whether gaze direction identification of individual faces can be modulated by prior social gaze encounters. In two experiments, participants first completed a joint-gaze learning task using a saccade/antisaccade paradigm. Participants would encounter some ‘joint-gaze faces’ that would consistently look at the participants saccade goal before participants looked there (Experiment 1) or would follow the participants gaze to the target (Experiment 2). ‘Non-joint-gaze faces’ would consistently look in the opposite direction. Participants then completed a second task in which they judged the gaze direction of the faces they had previously encountered. Participants were less likely to erroneously report faces with slightly deviated gaze as looking directly at them if the face had previously never engaged in joint gaze with them. However, this bias was only present when those faces had looked first (Experiment 1) and not when the faces looked after participants (Experiment 2). Comparing these data with gaze identification responses of a control group that did not complete any joint-gaze learning phase revealed that the difference in gaze identification in Experiment 1 is likely driven by a lowering of direct gaze bias in response to non-joint-gaze faces. Thus, previous joint-gaze experiences can affect gaze direction judgements at an identity-specific level. However, this modulation may rely on the socio-cognitive information available from viewing other’s initiation behaviours, especially when they fail to engage in social contact.

The Role of Emotional Expression and Eccentricity on Gaze Perception

The perception of another’s gaze direction and facial expression complements verbal communication and modulates how we interact with other people. However, our perception of these two cues is not always accurate, even when we are looking directly at the person. In addition, in many cases social communication occurs within groups of people where we can’t always look directly at every person in the group. Here, we sought to examine how the presence of other people influences our perception of a target face. We asked participants to judge the direction of gaze of the target face as either looking to their left, to their right or directly at them, when the face was viewed on its own or viewed within a group of other identity faces. The target face either had an angry or a neutral expression and was viewed directly (foveal experiment), or within peripheral vision (peripheral experiment). When the target was viewed within a group, the flanking faces also had either neutral or angry expressions and their gaze was in one of five different directions (from averted leftwards to averted rightwards in steps of 10°). When the target face was viewed foveally there was no effect of target emotion on participants’ judgments of its gaze direction. There was also no effect of the presence of flankers (regardless of expression) on the perception of the target gaze. When the target face was viewed peripherally, participants judged its direction of gaze to be direct over a wider range of gaze deviations than when viewed foveally, and more so for angry faces than neutral faces. We also find that flankers (regardless of emotional expression) did not influence performance. This suggests that observers judge that angry faces were looking at them over a broad range of gaze deviations in the periphery only, possibly resulting from increased uncertainty about the stimulus.

Face perception: A brief journey through recent discoveries and current directions

Faces are a rich source of information about the people around us. Identity, state of mind, emotions, intentions, age, gender, ethnic background, attractiveness and a host of other attributes about an individual can be gleaned from a face. When face perception fails, dramatic psycho-social consequences can follow at the individual level, as in the case of prosopagnosic parents who are unable to recognize their children at school pick-up. At the species level, social interaction patterns are shaped by human face perception abilities. The computational feat of recognizing faces and facial attributes, and the challenges overcome by the human brain to achieve this feat, have fascinated generations of vision researchers. In this paper, we present a brief overview of some of the milestones of discovery as well as outline a selected set of current directions and open questions on this topic.

Fixed or flexible? Orientation preference in identity and gaze processing in humans

Vision begins with the encoding of contrast at specific orientations. Several works showed that humans identify their conspecifics best based on the horizontally-oriented information contained in the face image; this range conveys the main morphological features of the face. In contrast, the vertical structure of the eye region seems to deliver optimal cues to gaze direction. The present work investigates whether the human face processing system flexibly tunes to vertical information contained in the eye region when processing gaze direction. Alternatively, face processing may invariantly rely on the horizontal range, supporting the domain specificity of orientation tuning for faces and the gateway role of horizontal content to access any type of facial information. Participants judged the gaze direction of faces staring at a range of lateral positions. They additionally performed an identification task with upright and inverted face stimuli. Across tasks, stimuli were filtered to selectively reveal horizontal (H), vertical (V), or combined (HV) information. Most participants identified faces better based on horizontal than vertical information confirming the horizontal tuning of face identification. In contrast, they showed a vertically-tuned sensitivity to gaze direction. The logistic functions fitting the “left” and “right” response proportion as a function of gaze direction were indeed steeper when based on vertical than on horizontal information. The finding of a vertically-tuned processing of gaze direction favours the hypothesis that visual encoding of face information flexibly switches to the orientation channel carrying the cues most relevant to the task at hand. It suggests that horizontal structure, though predominant in the face stimulus, is not a mandatory gateway for efficient face processing. The present evidence may help better understand how visual signals travel the visual system to enable rich and complex representations of naturalistic stimuli such as faces.

Adaptive sensory coding of gaze direction in schizophrenia

Schizophrenia has been associated with differences in how the visual system processes sensory input. A fundamental mechanism that regulates sensory processing in the brain is gain control, whereby the responses of sensory neurons to a given stimulus are modulated in accordance with the spatial and temporal context. Some studies indicate an impairment of certain cortical gain control mechanisms in schizophrenia in low-level vision, reflected, for instance, in how the visual appearance of a stimulus is affected by the presence of other stimuli around it. In the present study, we investigated higher-level, social vision in schizophrenia, namely the perception of other people's direction of gaze (i.e. a type of face processing). Recent computational modelling work indicates that perceptual aftereffects-changes in perception that occur following repeated exposure to faces that display a specific direction of gaze-are indicative of two distinct forms of gain control involved in the coding of gaze direction across sensory neurons. We find that individuals with schizophrenia display strong perceptual aftereffects following repeated exposure to faces with averted gaze, and a modelling analysis indicates similarly robust gain control in the form of (i) short-term adjustment of channel sensitivities in response to the recent sensory history and (ii) divisive normalization of the encoded gaze direction. Together, this speaks to the typical coding of other people's direction of gaze in the visual system in schizophrenia, including flexible gain control, despite the social-cognitive impairments that can occur in this condition.

Adaptation to the Direction of Others' Gaze: A Review

The direction of another person’s gaze provides us with a strong cue to their intentions and future actions, and, correspondingly, the human visual system has evolved to extract information about others’ gaze from the sensory stream. The perception of gaze is a remarkably plastic process: adaptation to a particular direction of gaze over a matter of seconds or minutes can cause marked aftereffects in our sense of where other people are looking. In this review, we first discuss the measurement, specificity, and neural correlates of gaze aftereffects. We then examine how studies that have explored the perceptual and neural determinants of gaze aftereffects have provided key insights into the nature of how other people’s gaze direction is represented within the visual hierarchy. This includes the level of perceptual representation of gaze direction (e.g., relating to integrated vs. local facial features) and the interaction of this system with higher-level social-cognitive functions, such as theory of mind. Moreover, computational modeling of data from behavioral studies of gaze adaptation allows us to make inferences about the functional principles that govern the neural encoding of gaze direction. This in turn provides a foundation for testing computational theories of neuropsychiatric conditions in which gaze processing is compromised, such as autism.

Autistic adults show preserved normalisation of sensory responses in gaze processing

Progress in our understanding of autism spectrum disorder (ASD) has recently been sought by characterising how systematic differences in canonical neural computations employed across the sensory cortex might contribute to clinical symptoms in diverse sensory, cognitive, and social domains. A key proposal is that ASD is characterised by reduced divisive normalisation of sensory responses. This provides a bridge between genetic and molecular evidence for an increased ratio of cortical excitation to inhibition in ASD and the functional characteristics of sensory coding that are relevant for understanding perception and behaviour. Here we tested this hypothesis in the context of gaze processing (i.e., the perception of other people's direction of gaze), a domain with direct relevance to the core diagnostic features of ASD. We show that reduced divisive normalisation in gaze processing is associated with specific predictions regarding the psychophysical effects of sensory adaptation to gaze direction, and test these predictions in adults with ASD. We report compelling evidence that both divisive normalisation and sensory adaptation occur robustly in adults with ASD in the context of gaze processing. These results have important theoretical implications for defining the types of divisive computations that are likely to be intact or compromised in this condition (e.g., relating to local vs distal control of cortical gain). These results are also a strong testament to the typical sensory coding of gaze direction in ASD, despite the atypical responses to others' gaze that are a hallmark feature of this diagnosis.

Functional Mechanisms Encoding Others' Direction of Gaze in the Human Nervous System

The direction of others' gaze is a strong social signal to their intentions and future behavior. Pioneering electrophysiological research identified cell populations in the primate visual cortex that are tuned to specific directions of observed gaze, but the functional architecture of this system is yet to be precisely specified. Here, we develop a computational model of how others' gaze direction is flexibly encoded across sensory channels within the gaze system. We incorporate the divisive normalization of sensory responses—a computational mechanism that is thought to be widespread in sensory systems but has not been examined in the context of social vision. We demonstrate that the operation of divisive normalization in the gaze system predicts a surprising and distinctive pattern of perceptual changes after sensory adaptation to gaze stimuli and find that these predictions closely match the psychophysical effects of adaptation in human observers. We also find that opponent coding, broadband multichannel, and narrowband multichannel models of sensory coding make distinct predictions regarding the effects of adaptation in a normalization framework and find evidence in favor of broadband multichannel coding of gaze. These results reveal the functional principles that govern the neural encoding of gaze direction and support the notion that divisive normalization is a canonical feature of nervous system function. Moreover, this research provides a strong foundation for testing recent computational theories of neuropsychiatric conditions in which gaze processing is compromised, such as autism and schizophrenia.

Detecting and identifying offset gaze

A number of experiments have demonstrated that observers can accurately identify stimuli that they fail to detect (Rollman and Nachmias, 1972; Harris and Fahle, 1995; Allik et al. 1982, 2014). Using a 2x2AFC double judgements procedure, we demonstrated an analogous pattern of performance in making judgements about the direction of eye gaze. Participants were shown two faces in succession: one with direct gaze and one with gaze offset to the left or right. We found that they could identify the direction of gaze offset (left/right) better than they could detect which face contained the offset gaze. A simple Thurstonian model, under which the detection judgement is shown to be more computationally complex, was found to explain the empirical data. A further experiment incorporated metacognitive ratings into the double judgements procedure to measure observers' metacognitive awareness (Meta-d') across the two judgements and to assess whether observers were aware of the evidence for offset gaze when detection performance was at and below threshold. Results suggest that metacognitive awareness is tied to performance, with approximately equal Meta-d' across the two judgements, when sensitivity is taken into account. These results show that both performance and metacognitive awareness rely not only on the strength of sensory evidence but also on the computational complexity of the decision, which determines the relative distance of that evidence from the decision axes.

Adaptation of social and non-social cues to direction in adults with autism spectrum disorder and neurotypical adults with autistic traits

Perceptual constancy strongly relies on adaptive gain control mechanisms, which shift perception as a function of recent sensory history. Here we examined the extent to which individual differences in magnitude of adaptation aftereffects for social and non-social directional cues are related to autistic traits and sensory sensitivity in healthy participants (Experiment 1); and also whether adaptation for social and non-social directional cues is differentially impacted in adults with Autism Spectrum Disorder (ASD) relative to neurotypical (NT) controls (Experiment 2). In Experiment 1, individuals with lower susceptibility to adaptation aftereffects, i.e. more 'veridical' perception, showed higher levels of autistic traits across social and non-social stimuli. Furthermore, adaptation aftereffects were predictive of sensory sensitivity. In Experiment 2, only adaptation to eye-gaze was diminished in adults with ASD, and this was related to difficulties categorizing eye-gaze direction at baseline. Autism Diagnostic Observation Schedule (ADOS) scores negatively predicted lower adaptation for social (head and eye-gaze direction) but not non-social (chair) stimuli. These results suggest that the relationship between adaptation and the broad socio-cognitive processing style captured by 'autistic traits' may be relatively domain-general, but in adults with ASD diminished adaptation is only apparent where processing is most severely impacted, such as the perception of social attention cues.

The center of attention: Metamers, sensitivity, and bias in the emergent perception of gaze

A person's gaze reveals much about their focus of attention and intentions. Sensitive perception of gaze is thus highly relevant for social interaction, especially when it is directed toward the viewer. Yet observers also tend to overestimate the likelihood that gaze is directed toward them. How might the visual system balance these competing goals, maximizing sensitivity for discriminating gazes that are relatively direct, while at the same time allowing many gazes to appear as if they look toward the viewer? Perceiving gaze is an emergent visual process that involves integrating information from the eyes with the rotation of the head. Here, we examined whether the visual system leverages emergent representation to balance these competing goals. We measured perceived gaze for a large range of pupil and head combinations and found that head rotation has a nonlinear influence on a person's apparent direction of looking, especially when pupil rotations are relatively direct. These perceptual distortions could serve to expand representational space and thereby enhance discriminability of gazes that are relatively direct. We also found that the emergent perception of gaze supports an abundance of direct gaze metamers-different combinations of head and pupil rotations that combine to generate the appearance of gaze directed toward the observer. Our results thus demonstrate a way in which the visual system flexibly integrates information from facial features to optimize social perception. Many gazes can be made to look toward you, yet similar gazes need not appear alike.

Bidirectional Gender Face Aftereffects: Evidence Against Normative Facial Coding

Facial appearance can be altered, not just by restyling but also by sensory processes. Exposure to a female face can, for instance, make subsequent faces look more masculine than they would otherwise. Two explanations exist. According to one, exposure to a female face renormalizes face perception, making that female and all other faces look more masculine as a consequence—a unidirectional effect. According to that explanation, exposure to a male face would have the opposite unidirectional effect. Another suggestion is that face gender is subject to contrastive aftereffects. These should make some faces look more masculine than the adaptor and other faces more feminine—a bidirectional effect. Here, we show that face gender aftereffects are bidirectional, as predicted by the latter hypothesis. Images of real faces rated as more and less masculine than adaptors at baseline tended to look even more and less masculine than adaptors post adaptation. This suggests that, rather than mental representations of all faces being recalibrated to better reflect the prevailing statistics of the environment, mental operations exaggerate differences between successive faces, and this can impact facial gender perception.

The "where" of social attention: Head and body direction aftereffects arise from representations specific to cue type and not direction alone

Human beings have remarkable social attention skills. From the initial processing of cues, such as eye gaze, head direction, and body orientation, we perceive where other people are attending, allowing us to draw inferences about the intentions, desires, and dispositions of others. But before we can infer why someone is attending to something in the world we must first accurately represent where they are attending. Here we investigate the "where" of social attention perception, and employ adaptation paradigms to ascertain how head and body orientation are visually represented in the human brain. Across two experiments we show that the representation of two cues to social attention (head and body orientation) exists at the category-specific level. This suggests that aftereffects do not arise from "social attention cells" discovered in macaques or from abstract representations of "leftness" or "rightness."

Absence of Sex-Contingent Gaze Direction Aftereffects Suggests a Limit to Contingencies in Face Aftereffects

Face aftereffects (e.g., expression aftereffects) can be simultaneously induced in opposite directions for different face categories (e.g., male and female faces). Such aftereffects are typically interpreted as indicating that distinct neural populations code the categories on which adaptation is contingent, e.g., male and female faces. Moreover, they suggest that these distinct populations selectively respond to variations in the secondary stimulus dimension, e.g., emotional expression. However, contingent aftereffects have now been reported for so many different combinations of face characteristics, that one might question this interpretation. Instead, the selectivity might be generated during the adaptation procedure, for instance as a result of associative learning, and not indicate pre-existing response selectivity in the face perception system. To alleviate this concern, one would need to demonstrate some limit to contingent aftereffects. Here, we report a clear limit, showing that gaze direction aftereffects are not contingent on face sex. We tested 36 young Caucasian adults in a gaze adaptation paradigm. We initially established their ability to discriminate the gaze direction of male and female test faces in a pre-adaptation phase. Afterwards, half of the participants adapted to female faces looking left and male faces looking right, and half adapted to the reverse pairing. We established the effects of this adaptation on the perception of gaze direction in subsequently presented male and female test faces. We found that adaptation induced pronounced gaze direction aftereffects, i.e., participants were biased to perceive small gaze deviations to both the left and right as direct. Importantly, however, aftereffects were identical for male and female test faces, showing that the contingency of face sex and gaze direction participants experienced during the adaptation procedure had no effect.

A Bayesian approach to person perception

Here we propose a Bayesian approach to person perception, outlining the theoretical position and a methodological framework for testing the predictions experimentally. We use the term person perception to refer not only to the perception of others' personal attributes such as age and sex but also to the perception of social signals such as direction of gaze and emotional expression. The Bayesian approach provides a formal description of the way in which our perception combines current sensory evidence with prior expectations about the structure of the environment. Such expectations can lead to unconscious biases in our perception that are particularly evident when sensory evidence is uncertain. We illustrate the ideas with reference to our recent studies on gaze perception which show that people have a bias to perceive the gaze of others as directed towards themselves. We also describe a potential application to the study of the perception of a person's sex, in which a bias towards perceiving males is typically observed.

Atypical integration of social cues for orienting to gaze direction in adults with autism

BACKGROUND

Gaze direction provides important information about social attention, and people tend to reflexively orient in the direction others are gazing. Perceiving the gaze of others relies on the integration of multiple social cues, which include perceptual information related to the eyes, gaze direction, head position, and body orientation of others. Autism spectrum conditions (ASC) are characterised by social and emotional deficits, including atypical gaze behaviour. The social-emotional deficits may emerge from a reliance on perceptual information involving details and features, at the expense of more holistic processing, which includes the integration of features. While people with ASC are often able to physically compute gaze direction and show intact reflexive orienting to others' gaze, they show deficits in reading mental states from the eyes.

METHODS

The present study recruited 23 adult males with a diagnosis of ASC and 23 adult males without ASC as a control group. They were tested using a spatial cuing paradigm involving head and body cues in a photograph of a person followed by a laterally presented target. The task manipulated the orientation of head with respect to body orientation to test subsequent shifts of attention in observers.

RESULTS

The results replicated previous findings showing facilitated shifts of attention by the healthy control participants toward laterally presented targets cued by a congruently rotated head combined with a front view of a body. In contrast, the ASC group showed facilitated orienting to targets when both the head and body were rotated towards the target.

CONCLUSIONS

The findings reveal atypical integration of social cues in ASC for orienting of attention. This is suggested to reflect abnormalities in cognitive and neural mechanisms specialized for processing of social cues for attention orienting in ASC.

Adding years to your life (or at least looking like it): a simple normalization underlies adaptation to facial age

Adaptation has been widely used to probe how experience shapes the visual encoding of faces, but the pattern of perceptual changes produced by adaptation and the neural mechanisms these imply remain poorly characterized. We explored how adaptation alters the perceived age of faces, a fundamental facial attribute which can uniquely and reliably be scaled by observers. This allowed us to measure how adaptation to one age level affected the full continuum of perceived ages. Participants guessed the ages of faces ranging from 18-89, before or after adapting to a different set of faces composed of younger, older, or middle-aged adults. Adapting to young or old faces induced opposite linear shifts in perceived age that were independent of the model's age. Specifically, after adapting to younger or older faces, faces of all ages appeared 2 to 3 years older or younger, respectively. In contrast, middle-aged adaptors induced no aftereffects. This pattern suggests that adaptation leads to a simple and uniform renormalization of age perception, and is consistent with a norm-based neural code for the mechanisms mediating the perception of facial age.

The influences of face inversion and facial expression on sensitivity to eye contact in high-functioning adults with autism spectrum disorders

We examined the influences of face inversion and facial expression on sensitivity to eye contact in high-functioning adults with and without an autism spectrum disorder (ASD). Participants judged the direction of gaze of angry, fearful, and neutral faces. In the typical group only, the range of directions of gaze leading to the perception of eye contact (the cone of gaze) was narrower for upright than inverted faces. In both groups, the cone of gaze was wider for angry faces than for fearful or neutral faces. These results suggest that in high-functioning adults with ASD, the perception of eye contact is not tuned to be finer for upright than inverted faces, but that information is nevertheless integrated across expression and gaze direction.

Repetition Suppression in Ventral Visual Cortex Is Diminished as a Function of Increasing Autistic Traits

Repeated viewing of a stimulus causes a change in perceptual sensitivity, known as a visual aftereffect. Similarly, in neuroimaging, repetitions of the same stimulus result in a reduction in the neural response, known as repetition suppression (RS). Previous research shows that aftereffects for faces are reduced in both children with autism and in first-degree relatives. With functional magnetic resonance imaging, we found that the magnitude of RS to faces in neurotypical participants was negatively correlated with individual differences in autistic traits. We replicated this finding in a second experiment, while additional experiments showed that autistic traits also negatively predicted RS to images of scenes and simple geometric shapes. These findings suggest that a core aspect of neural function--the brain's response to repetition--is modulated by autistic traits.

Face identity aftereffects increase monotonically with adaptor extremity over, but not beyond, the range of natural faces

Face identity aftereffects have been used to test theories of the neural coding underlying expert face recognition. Previous studies reported larger aftereffects for adaptors that are morphed further from the average face than for adaptors closer to the average, which appeared to support opponent coding along face-identity dimensions. However, only two levels were tested and it is not clear where they were located relative to the range of naturally occurring faces. This range is of interest given the functional need of the visual system both to produce good discrimination of real everyday faces and to process novel kinds of faces that we may encounter. Here, Experiment 1 establishes the boundary of faces judged as being able to occur in everyday life. Experiment 2 then shows that aftereffects increase with adaptor extremity up to this natural-range boundary, drop significantly immediately outside the boundary, and then remain stable with no drop towards zero even for highly distorted adaptors far beyond the boundary. Computational modelling shows that this unexpected pattern cannot be explained either by a simple opponent or by a classic multichannel model. However, its qualitative features can be captured either by a combination of opponent and multichannel coding (raising the possibility that not all identity-related face dimensions are opponent coded), or by a 3-pool model containing two S-shaped-response channels and a central bell-shaped channel around the average face (raising the possibility of unexpected similarities with coding of eye and head direction).

Cone of direct gaze as a marker of social anxiety in males

The fear of being scrutinised is a core feature of social anxiety disorder and socially anxious individuals overestimate being 'looked at'. A recent development in the vision sciences is a reliable psychophysical index of the range of eye gaze angles judged as being directed at oneself (Cone of Direct Gaze: CoDG). We tested the CoDG as a measure of "being looked at" in social anxiety. Participants were stratified into high/low social anxiety groups and asked to judge whether they were being 'looked at' by computerised male faces varying in eye gaze deviation and facial emotion. High socially anxious males had a wider CoDG than low socially anxious males; high and low socially anxious females did not differ. Fearful faces elicited narrower cones than neutral or angry faces; however, the effect size was small and not evident for the high socially anxious males. Measures of subjective reactions to the study, and to being looked at in general, indicated that the results may be in part due to males suffering more discomfort when being looked at. The results show that measures derived from psychophysics, in this case, the CoDG, have potential as clinical and research tools for measuring anxiety about being scrutinised.

Eye gaze is not coded by cardinal mechanisms alone

Gaze is an important social cue in regulating human and non-human interactions. In this study, we employed an adaptation paradigm to examine the mechanisms underlying the perception of another's gaze. Previous research has shown that the interleaved presentation of leftwards and rightwards gazing adaptor stimuli results in observers judging a wider range of gaze deviations as being direct. We applied a similar paradigm to examine how human observers encode oblique (e.g. upwards and to the left) directions of gaze. We presented observers with interleaved gaze adaptors and examined whether adaptation differed between congruent (adaptor and test along same axis) and incongruent conditions. We find greater adaptation in congruent conditions along cardinal (horizontal and vertical) and non-cardinal (oblique) directions suggesting gaze is not coded alone by cardinal mechanisms. Our results suggest that the functional aspects of gaze processing might parallel that of basic visual features such as orientation.

Reduced gaze aftereffects are related to difficulties categorising gaze direction in children with autism

Perceptual mechanisms are generally flexible or "adaptive", as evidenced by perceptual aftereffects: distortions that arise following exposure to a stimulus. We examined whether adaptive mechanisms for coding gaze direction are atypical in children diagnosed with an autism spectrum condition. Twenty-four typical children and 24 children with autism, of similar age and ability, were administered a developmentally sensitive eye-gaze adaptation task. In the pre-adaptation phase, children judged whether target faces showing subtle deviations in eye-gaze direction were looking leftwards, rightwards or straight-ahead. Next, children were adapted to faces gazing in one consistent direction (25° leftwards/rightwards) before categorising the direction of the target faces again. Children with autism showed difficulties in judging whether subtle deviations in gaze were directed to the left, right or straight-ahead relative to typical children. Although adaptation to leftward or rightward gaze resulted in reduced sensitivity to gaze on the adapted side for both groups, the aftereffect was significantly reduced in children with autism. Furthermore, the magnitude of children's gaze aftereffects was positively related to their ability to categorise gaze direction. These results show that the mechanisms coding gaze are less flexible in autism and offer a potential new explanation for these children's difficulties discriminating subtle deviations in gaze direction.

The role of the eyes in processing an intact face and its scrambled image: a dense array ERP and low-resolution electromagnetic tomography (sLORETA) study

The aim of the present study was to test whether the eyes of an intact face produced a specific brain response compared to the mouth, nose, or hair and whether their specificity was also maintained in a scrambled face. Fifteen subjects were asked to focus visual attention on global and single elements in intact faces and in their scrambled image. EEG data were recorded from 256-Hydrocel Geodesic Sensor-Net200. Event-related potentials (ERPs) analyses showed a difference between the intact face and the scrambled face from N170 component until 600 ms on the occipito-temporal montage and at 400-600 ms on the frontal montage. Only the eyes showed a difference between conditions (intact/scrambled face) at 500 ms. The most activated source detected by sLORETA was the right middle temporal gyrus (BA21) for both conditions and for all elements. Left BA21 resulted in significantly more activation in response to eyes in the intact face compared to the eyes in the scrambled face at 500 ms. The left BA21 has a central role in high-level visual processing and in understanding others' intentions. These findings suggest a specificity of the eyes and indicate that the eyes play the social and communicative role of comprehending the nonverbal intentions of others only when inserted in an intact face.

What is social about social perception research?

A growing consensus in social cognitive neuroscience holds that large portions of the primate visual brain are dedicated to the processing of social information, i.e., to those aspects of stimuli that are usually encountered in social interactions such as others' facial expressions, actions, and symbols. Yet, studies of social perception have mostly employed simple pictorial representations of conspecifics. These stimuli are social only in the restricted sense that they physically resemble objects with which the observer would typically interact. In an equally important sense, however, these stimuli might be regarded as "non-social": the observer knows that they are viewing pictures and might therefore not attribute current mental states to the stimuli or might do so in a qualitatively different way than in a real social interaction. Recent studies have demonstrated the importance of such higher-order conceptualization of the stimulus for social perceptual processing. Here, we assess the similarity between the various types of stimuli used in the laboratory and object classes encountered in real social interactions. We distinguish two different levels at which experimental stimuli can match social stimuli as encountered in everyday social settings: (1) the extent to which a stimulus' physical properties resemble those typically encountered in social interactions and (2) the higher-level conceptualization of the stimulus as indicating another person's mental states. We illustrate the significance of this distinction for social perception research and report new empirical evidence further highlighting the importance of mental state attribution for perceptual processing. Finally, we discuss the potential of this approach to inform studies of clinical conditions such as autism.

Electrophysiological responses in mothers to their own and unfamiliar child's gaze information

An attachment bond between a mother and her child is one of the most intimate human relationships. It is important for a mother to be sensitive to her child's gaze direction because exchanging gaze information plays a vital role in their relationship. Furthermore, recent studies have revealed differential neural activation patterns in mothers when presented the faces of their own children or the unfamiliar child of other people. Based on these findings, in the present study, we investigated whether mothers show differential neural responses to gaze information of their own child compared to that of an unfamiliar child. To this end, event-related-potentials elicited by the faces of one's own or an unfamiliar child with straight or averted gaze directions were measured using an oddball-paradigm. The results showed that peak amplitudes of the N170 component were enlarged by viewing the straight gazes compared to the averted gazes of one's own child, but not of an unfamiliar child. When the gaze was directed straight, the P3 amplitude elicited by one's own child's face is smaller than that elicited by an unfamiliar child's face. P3s elicited in viewing one's own child's face with averted gaze and in viewing an unfamiliar child's face with straight gaze were positively correlated with state-anxiety. These results bolster the hypothesis that processing the gaze information of one's own child elicits differential neural activation compared to the gaze information of an other person's unfamiliar child at both perceptual and evaluative stages of face processing.

Not all face aftereffects are equal

After prolonged exposure to a female face, faces that had previously seemed androgynous are more likely to be judged as male. Similarly, after prolonged exposure to a face with expanded features, faces that had previously seemed normal are more likely to be judged as having contracted features. These facial aftereffects have both been attributed to the impact of adaptation upon a norm-based opponent code, akin to low-level analyses of colour. While a good deal of evidence is consistent with this, some recent data is contradictory, motivating a more rigorous test. In behaviourally matched tasks we compared the characteristics of aftereffects generated by adapting to colour, to expanded or contracted faces, and to male or female faces. In our experiments opponent coding predicted that the appearance of the adapting image should change and that adaptation should induce symmetrical shifts of two category boundaries. This combination of predictions was firmly supported for colour adaptation, somewhat supported for facial distortion aftereffects, but not supported for facial gender aftereffects. Interestingly, the two face aftereffects we tested generated discrepant patterns of response shifts. Our data suggest that superficially similar aftereffects can ensue from mechanisms that differ qualitatively, and therefore that not all high-level categorical face aftereffects can be attributed to a common coding strategy.