Perception of head orientation

Specialized Networks for Social Cognition in the Primate Brain

Primates have evolved diverse cognitive capabilities to navigate their complex social world. To understand how the brain implements critical social cognitive abilities, we describe functional specialization in the domains of face processing, social interaction understanding, and mental state attribution. Systems for face processing are specialized from the level of single cells to populations of neurons within brain regions to hierarchically organized networks that extract and represent abstract social information. Such functional specialization is not confined to the sensorimotor periphery but appears to be a pervasive theme of primate brain organization all the way to the apex regions of cortical hierarchies. Circuits processing social information are juxtaposed with parallel systems involved in processing nonsocial information, suggesting common computations applied to different domains. The emerging picture of the neural basis of social cognition is a set of distinct but interacting subnetworks involved in component processes such as face perception and social reasoning, traversing large parts of the primate brain.

How facial masks alter the interaction of gaze direction, head orientation, and emotion recognition

The COVID-19 pandemic has altered the way we interact with each other: mandatory mask-wearing obscures facial information that is crucial for emotion recognition. Whereas the influence of wearing a mask on emotion recognition has been repeatedly investigated, little is known about the impact on interaction effects among emotional signals and other social signals. Therefore, the current study sought to explore how gaze direction, head orientation, and emotional expression interact with respect to emotion perception, and how these interactions are altered by wearing a face mask. In two online experiments, we presented face stimuli from the Radboud Faces Database displaying different facial expressions (anger, fear, happiness, neutral, and sadness), gaze directions (−13°, 0°, and 13°), and head orientations (−45°, 0°, and 45°) – either without (Experiment 1) or with mask (Experiment 2). Participants categorized the displayed emotional expressions. Not surprisingly, masks impaired emotion recognition. Surprisingly, without the mask, emotion recognition was unaffected by averted head orientations and only slightly affected by gaze direction. The mask strongly interfered with this ability. The mask increased the influence of head orientation and gaze direction, in particular for the emotions that were poorly recognized with mask. The results suggest that in case of uncertainty due to ambiguity or absence of signals, we seem to unconsciously factor in extraneous information.

Turning Heads: The Effects of Face View and Eye Gaze Direction on the Perceived Attractiveness of Expressive Faces

We investigated whether the perceived attractiveness of expressive faces was influenced by head turn and eye gaze towards or away from the observer. In all experiments, happy faces were consistently rated as more attractive than angry faces. A head turn towards the observer, whereby a full-face view was shown, was associated with relatively higher attractiveness ratings when gaze direction was aligned with face view (Experiment 1). However, preference for full-face views of happy faces was not affected by gaze shifts towards or away from the observer (Experiment 2a). In Experiment 3, the relative duration of each face view (front-facing or averted at 15°) during a head turn away or towards the observer was manipulated. There was benefit on attractiveness ratings for happy faces shown for a longer duration from the front view, regardless of the direction of head turn. Our findings support previous studies indicating a preference for positive expressions on attractiveness judgements, which is further enhanced by the front views of faces, whether presented during a head turn or shown statically. In sum, our findings imply a complex interaction between cues of social attention, indicated by the view of the face shown, and reward on attractiveness judgements of unfamiliar faces.

Ecological cocktail party listening reveals the utility of extended high-frequency hearing

A fundamental principle of neuroscience is that each species' and individual's sensory systems are tailored to meet the demands placed upon them by their environments and experiences. What has driven the upper limit of the human frequency range of hearing? The traditional view is that sensitivity to the highest frequencies (i.e., beyond 8 kHz) facilitates localization of sounds in the environment. However, this has yet to be demonstrated for naturally occurring non-speech sounds. An alternative view is that, for social species such as humans, the biological relevance of conspecific vocalizations has driven the development and retention of auditory system features. Here, we provide evidence for the latter theory. We evaluated the contribution of extended high-frequency (EHF) hearing to common ecological speech perception tasks. We found that restricting access to EHFs reduced listeners' discrimination of talker head orientation by approximately 34%. Furthermore, access to EHFs significantly improved speech recognition under listening conditions in which the target talker's head was facing the listener while co-located background talkers faced away from the listener. Our findings raise the possibility that sensitivity to the highest audio frequencies fosters communication and socialization of the human species. These findings suggest that loss of sensitivity to the highest frequencies may lead to deficits in speech perception. Such EHF hearing loss typically goes undiagnosed, but is widespread among the middle-aged population.

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.

Evaluating spatiotemporal interactions between shapes

Spatiotemporal interactions between stimuli can alter the perceived curvature along the outline of a shape (Habak, Wilkinson, Zakher, & Wilson, 2004; Habak, Wilkinson, & Wilson, 2006). To better understand these interactions, we used a forward and backward masking paradigm with radial frequency (RF) contours while measuring RF detection thresholds. In Experiment 1, we presented a mask alongside a target contour and altered the stimulus onset asynchrony between this target-mask pair and a temporal mask. We found that a temporal mask increased thresholds when it preceded the target-mask stimulus by 130-180 ms but decreased thresholds when it followed the target-stimulus mask by 180 ms. Furthermore, Experiment 2 demonstrated that the effects of temporal and spatial masks are approximately additive. We discuss these findings in relation to theories of transient and sustained channels in vision.

Radial frequency patterns describe a small and perceptually distinct subset of all possible planar shapes

The visual system is exposed to a vast number of shapes and objects. Yet, human object recognition is effortless, fast and largely independent of naturally occurring transformations such as position and scale. The precise mechanisms of shape encoding are still largely unknown. Radial frequency (RF) patterns are a special class of closed contours defined by modulation of a circle's radius. These patterns have been frequently and successfully used as stimuli in vision science to investigate aspects of shape processing. Given their mathematical properties, RF patterns can not represent any arbitrary shape, but the ability to generate more complex, biologically relevant, shapes depicting the outlines of objects such as fruits or human heads raises the possibility that RF patterns span a representative subset of possible shapes. However, this assumption has not been tested before. Here we show that only a small fraction of all possible shapes can be represented by RF patterns and that this small fraction is perceptually distinct from the general class of all possible shapes. Specifically, we derive a general measure for the distance of a given shape's outline from the set of RF patterns, allowing us to scan large numbers of object outlines automatically. We find that only between 1% and 6% of naturally smooth outlines can be exactly represented by RF patterns. We present results from a visual search experiment, which revealed that searching an RF pattern among non-radial frequency patterns is efficient, whereas searching an RF pattern among other RF patterns is inefficient (and vice versa). These results suggest that RF patterns represent only a restricted subset of possible planar shapes and that results obtained with this special class of stimuli can not simply be expected to generalise to any arbitrary planar shape.

Both the Observer's Expertise and the Subject's Facial Symmetry Can Affect Anatomical Position of the Head

PURPOSE

It is easier to judge facial deformity when the patient's head is in anatomic position. The purposes of this study were to determine 1) whether a group of expert observers would agree more than a group of nonexperts on what is the correct anatomic position of the head, 2) whether there would be more variation in the alignment of an asymmetrical face compared with a symmetrical one, and 3) whether the alignments of experts would be more repeatable than those of nonexperts.

MATERIALS AND METHODS

Thirty-one orthodontists (experts) and 31 dental students (nonexperts) were recruited for this mixed-model study. They were shown randomly oriented 3-dimensional head photographs of an adult with a symmetrical face and an adolescent with an asymmetrical face. In viewing software, the observers oriented the images into anatomic position. They repeated the orientations 4 weeks later. Data were analyzed using a generalized linear model and Bland-Altman plots. The primary predictor variables were experience and symmetry status. The outcome variable was the anatomic position of the head. The other variables of interest included time and orientation direction.

RESULTS

There was a statistically significant difference between measurements completed by experts and nonexperts (F_1,60 = 14.83; P < .01). The interaction between expertise and symmetrical status showed a statistically significant difference between symmetrical and asymmetrical faces in the expert and nonexpert groups (F_1,60 = 9.93; P = .003). The interaction between expertise and time showed a statistically significant difference in measurement over time in the expert and nonexpert groups (F_1,60 = 4.66; P = .03).

CONCLUSIONS

The study shows that experts can set a head into anatomic position better than nonexperts. In addition, facial asymmetry has a profound effect on the ability of an observer to align a head in the correct anatomic position. Moreover, observer-guided alignment is not reproducible.

Phase-selective masking with radial frequency contours

Sensitivity to changes in the shape of a closed-contour figure is affected by surrounding figures (Vision Research 44 (2004) 2815-2823). We examined how between-contour masking depends on radial frequency. Experiment 1 replicated previous studies that found that masking between adjacent radial frequency (RF) patterns was greatest when the two shapes were phase aligned, and that the magnitude of masking declined approximately linearly with increasing phase offsets. In addition, we found that the effect of phase offset on masking was very similar for RFs ranging from 3 to 8, a result that suggests that sensitivity to phase decreases with increasing radial frequency. Experiment 2 tested this idea and found that phase discrimination threshold for single cycles of curvature was approximately proportional to radial frequency. Experiment 3 showed that both curvature maxima and minima contribute to phase dependent masking between RF contours. Together, Experiments 1-3 demonstrate that the strength of phase-dependent masking does not depend on RF, but is related to sensitivity for phase shifts in isolated contours, and is affected by both positive and negative curvature extrema. We discuss these results in relation to properties of curvature sensitive neurons.

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.

There is variability in our perception of the standard head orientation

The purposes of this study were to determine: (1) whether an observer's perception of the correct anatomical alignment of the head changes with time, and (2) whether different observers agree on the correct anatomical alignment. To determine whether the perception of the correct anatomical alignment changes with time (intra-observer comparison), a group of 30 observers were asked to orient, into anatomical alignment, the three-dimensional (3D) head photograph of a normal man, on two separate occasions. To determine whether different observers agree on the correct anatomical alignment (inter-observer comparison), the observed orientations were compared. The results of intra-observer comparisons showed substantial variability between the first and second anatomical alignments. Bland-Altman coefficients of repeatability for pitch, yaw, and roll, were 6.9°, 4.4°, and 2.4°, respectively. The results of inter-observer comparisons showed that the agreement for roll was good (sample variance 0.4, standard deviation (SD) 0.7°), the agreement for yaw was moderate (sample variance 2.0, SD 1.4°), and the agreement for pitch was poor (sample variance 15.5, SD 3.9°). In conclusion, the perception of correct anatomical alignment changes considerably with time. Different observers disagree on the correct anatomical alignment. Agreement among multiple observers was bad for pitch, moderate for yaw, and good for roll.

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.

The Impact of Symmetry on the Efficiency of Human Face Perception

The presence of symmetric properties in a stimulus has been shown to often exert an influence on perception and information processing. Investigations into symmetry have given rise to the notion that it is processed easily and efficiently by the human visual system. However, only a handful of studies have attempted to actually measure symmetry’s role in the efficiency of information use. We explored the impact of symmetry on the perception of human faces, a domain where it has been thought to play a particularly important role. Specifically, we measured information processing efficiency, defined as human performance relative to that of an ideal observer, for the detection, discrimination, and identification of symmetric and asymmetric faces. Surprisingly, we found no evidence for significant differences in efficiency between these two classes of stimuli. Training yielded significant improvements in overall efficiency, but had no significant effect on the relative efficiency of asymmetric and symmetric face identification. Our results indicate that although symmetry may be important to other aspects of face perception (e.g., perceived beauty), it has no discernible impact upon the efficiency with which information is used when detecting, discriminating, and identifying faces.

Configural Processing in the Perception of Eye-Gaze Direction

Traditional accounts of gaze perception emphasise the geometric or configural cues present in the eye; the position of the iris in relation to the corner of the eye, for example. This kind of geometric account has been supported, in part, by findings that gaze judgments are impaired in faces rotated through 180 degrees, a manipulation known to disrupt the processing of relations between facial elements. However, studies involving this manipulation have confounded inversion of the face context with inversion of the eye region. The effects of inversion might therefore have been caused by a disruption of the computation of gaze direction from the eye region itself and/or a disruption of the influence that face context might exert on gaze processing. In the experiment reported here we independently manipulated eye orientation and the orientation of the face context, and measured participants' sensitivity to gaze direction. Performance was severely affected by inversion of the eyes, regardless of the orientation of the face, whereas face inversion had no significant effect on gaze sensitivity. Previous reports of a face-inversion effect on gaze perception can therefore be attributed to inversion of the eye region itself which, we suggest, disrupts some form of configural or relational processing that is normally involved in the computation of eye-gaze direction.

Spatial limitations in averaging social cues

The direction of social attention from groups provides stronger cueing than from an individual. It has previously been shown that both basic visual features such as size or orientation and more complex features such as face emotion and identity can be averaged across multiple elements. Here we used an equivalent noise procedure to compare observers' ability to average social cues with their averaging of a non-social cue. Estimates of observers' internal noise (uncertainty associated with processing any individual) and sample-size (the effective number of gaze-directions pooled) were derived by fitting equivalent noise functions to discrimination thresholds. We also used reverse correlation analysis to estimate the spatial distribution of samples used by participants. Averaging of head-rotation and cone-rotation was less noisy and more efficient than averaging of gaze direction, though presenting only the eye region of faces at a larger size improved gaze averaging performance. The reverse correlation analysis revealed greater sampling areas for head rotation compared to gaze. We attribute these differences in averaging between gaze and head cues to poorer visual processing of faces in the periphery. The similarity between head and cone averaging are examined within the framework of a general mechanism for averaging of object rotation.

Accuracy of Head Orientation Perception in Triadic Situations: Experiment in a Virtual Environment

Research has revealed high accuracy in the perception of gaze in dyadic (sender–receiver) situations. Triadic situations differ from these in that an observer has to report where a sender is looking, not relative to himself. This is more difficult owing to the less favourable position of the observer. The effect of the position of the observer on the accuracy of the identification of the sender's looking direction is relatively unexplored. Here, we investigate this, focusing exclusively on head orientation. We used a virtual environment to ensure good stimulus control. We found a mean angular error close to 5°. A higher observer viewpoint results in more accurate identification. Similarly, a viewpoint with a smaller angle to the sender's midsagittal plane leads to an improvement in identification performance. Also, we found an effect of underestimation of the error in horizontal direction, similar to findings for dyadic situations.

Differences in the Judged Direction of Gaze From Heads Imaged in 3-D versus 2-D

Observers viewed a model imaged on an LCD monitor in 3-D or 2-D as she gazed at points along a horizontally oriented meter stick from a distance of 80 cm. Her head was either straight or turned 20 degrees to the side, and for each head orientation, her gaze was straight, 10 degrees, or 20 degrees to the side, with her eyes individually open, both open, or both closed. For images in which both eyes were closed, the observers pointed along the same meter stick to where they judged her head to be pointed. When one or both eyes were open they judged where she appeared to be gazing. Gaze from the 2-D images agreed with previous studies. A comparison of the results from the 2-D versus 3-D images showed that the judgments were virtually identical for gaze from the straight versus turned head in all combinations of straight versus averted gaze, eyes individually open versus both open. The judgments were also virtually identical for head point. This suggests that 2-D studies that have used procedures similar to those described in this study may be applied to real world 3-D gaze perception.

Perceiving where another person is looking: the integration of head and body information in estimating another person's gaze

The process through which an observer allocates his/her attention based on the attention of another person is known as joint attention. To be able to do this, the observer effectively has to compute where the other person is looking. It has been shown that observers integrate information from the head and the eyes to determine the gaze of another person. Most studies have documented that observers show a bias called the overshoot effect when eyes and head are misaligned. That is, when the head is not oriented straight to the observer, perceived gaze direction is sometimes shifted in the direction opposite to the head turn. The present study addresses whether body information is also used as a cue to compute perceived gaze direction. In Experiment 1, we observed a similar overshoot effect in both behavioral and saccadic responses when manipulating body orientation. In Experiment 2, we explored whether the overshoot effect was due to observers assuming that the eyes are oriented further than the head when head and body orientation are misaligned. We removed horizontal eye information by presenting the stimulus from a side view. Head orientation was now manipulated in a vertical direction and the overshoot effect was replicated. In summary, this study shows that body orientation is indeed used as a cue to determine where another person is looking.

The effect of central vision loss on perception of mutual gaze

PURPOSE

To evaluate the effects of central vision loss (CVL) on mutual gaze perception (knowing whether somebody else is looking at you), an important nonverbal visual cue in social interactions.

METHODS

Twenty-three persons with CVL (visual acuity 20/50 to 20/200), 16 with a bilateral central scotoma and 7 without, and 23 age-matched control subjects completed a gaze perception task and a brief questionnaire. They adjusted the eyes of a life-size virtual head on a monitor at a 1-m distance until they either appeared to be looking straight at them or were at the extreme left/right or up/down positions at which the eyes still appeared to be looking toward them (defining the range of mutual gaze in the horizontal and vertical planes).

RESULTS

The nonscotoma group did not differ from the control subjects in any gaze task measure. However, the gaze direction judgments of the scotoma group had significantly greater variability than those of the nonscotoma and control groups (p < 0.001). In addition, their mutual gaze range tended to be wider (p = 0.15), suggesting a more liberal judgment criterion. Contrast sensitivity was the strongest predictor of variability in gaze direction judgments followed by self-reported difficulties.

CONCLUSIONS

Our results suggest that mutual gaze perception is relatively robust to CVL. However, a follow-up study that simulates less-than-optimal viewing conditions of everyday social interactions is needed. The gaze perception task holds promise as a research tool for investigating the effects of vision impairment on mutual gaze judgments. Self-reported difficulty and contrast sensitivity were both independent predictors of gaze perception performance, suggesting that the task captured higher-order as well as low-level visual abilities.

The processing of coherent global form and motion patterns without visual awareness

In the present study we addressed whether the processing of global form and motion was dependent on visual awareness. Continuous flash suppression (CFS) was used to suppress from awareness global dot motion (GDM) and Glass pattern stimuli. We quantified the minimum time taken for both pattern types to break suppression with the signal coherence of the pattern (0, 25, 50, and 100% signal) and the type of global structure (rotational, and radial) as independent variables. For both form and motion patterns increasing signal coherence decreased the time required to break suppression. This was the same for both rotational and radial global patterns. However, GDM patterns broke suppression faster than Glass patterns. In a supplementary experiment, we confirmed that this difference in break times is not because of the temporal nature of GDM patterns in attracting attention. In Experiment 2, we examined whether the processing of dynamic Glass patterns were similarly dependent on visual awareness. The processing of dynamic Glass patterns is involves both motion and form systems, and we questioned whether the interaction of these two systems was dependent on visual awareness. The suppression of dynamic Glass patterns was also dependent on signal coherence and the time course of suppression break resembled the detection of global motion and not global form. In Experiment 3 we ruled out the possibility that faster suppression break times was because the visual system is more sensitive to highly coherent form and motion patterns. Here contrast changing GDM and Glass patterns were superimposed on the dynamic CFS mask, and the minimum time required for them to be detected was measured. We showed that there was no difference in detection times for patterns of 0 and 100% coherence. The advantage of highly coherent global motion and form patterns in breaking suppression indicated that the processing and interaction of global motion and form systems occur without visual awareness.

Bandwidths for the perception of head orientation decrease during childhood

Adults use the orientation of people's heads as a cue to the focus of their attention. We examined developmental changes in mechanisms underlying sensitivity to head orientation during childhood. Eight-, 10-, 12-year-olds, and adults were adapted to a frontal face view or a 20° left or right side view before judging the orientation of a face at or near frontal. After frontal adaptation, there were no age differences in judgments of head orientation. However, after adaptation to a 20° left or right side view, aftereffects were larger and sensitivity to head orientation was lower in 8- and 10-year-olds than in adults, with no difference between 12-year-olds and adults. A computational model indicates that these results can be modeled as a consequence of decreasing neural tuning bandwidths and decreasing additive internal noise during childhood, and/or as a consequence of increasing inhibition during childhood. These results provide the first evidence that neural mechanisms underlying sensitivity to head orientation undergo considerable refinement during childhood.

Contextual effects of scene on the visual perception of object orientation in depth

We investigated the effect of background scene on the human visual perception of depth orientation (i.e., azimuth angle) of three-dimensional common objects. Participants evaluated the depth orientation of objects. The objects were surrounded by scenes with an apparent axis of the global reference frame, such as a sidewalk scene. When a scene axis was slightly misaligned with the gaze line, object orientation perception was biased, as if the gaze line had been assimilated into the scene axis (Experiment 1). When the scene axis was slightly misaligned with the object, evaluated object orientation was biased, as if it had been assimilated into the scene axis (Experiment 2). This assimilation may be due to confusion between the orientation of the scene and object axes (Experiment 3). Thus, the global reference frame may influence object orientation perception when its orientation is similar to that of the gaze-line or object.

The Effect of Head Turn and Illumination on the Perceived Direction of Gaze

Thirty observers judged the direction of monocular and binocular gaze from a model's LCD-imaged head when the head, gaze, and illumination were either straight or turned 20.6° to the side. The judged direction of binocular eye contact from a turned head was judged to go past the direction of the observer, while judged monocular gaze from the eye toward the observer followed that for binocular gaze. Although judged monocular gaze from the eye away from the observer also passed the observer, it did so by a smaller amount. The judged direction of binocular gaze in the same direction as the head turn was judged to pass the direction of the head turn. The judged direction of monocular gaze from the eye away from the observer was nearly true, but the judged direction of monocular gaze from the eye toward the observer was judged as closer to the observer than true. Illumination of the model's head from the side made the direction of both head turn and gaze appear to move away from the direction of the light source. These results are discussed in connection with angle k and the perceived direction of head turn.

Increment thresholds for radial frequency trajectories produce a dipper function

Radial frequency (RF) trajectories are a new class of stimuli that have been developed to study the visual perception of periodic motion (Or et al., 2011). These stimuli are described by a moving dot that traces a distorted path through space with periodic radial deformations whose frequency, amplitude, and phase can be independently specified. Here, we extend Or et al.'s findings by investigating how the discrimination of RF amplitude changes as a function of different reference amplitudes in a two-interval forced choice task. Using an RF3 trajectory (a pattern with three cycles of deformation along its trajectory), increment thresholds were measured at six different reference amplitudes: Detection (discriminating a circle from RF3), 1X (discriminating a pair of RF3 patterns, with the amplitude of one member of this pair set to (1X) threshold obtained from the detection condition), 2.5X, 5X, 10X, and 15X. Data show that sensitivity to changes in amplitude improves at 2.5X by a factor of about 2, recovers to detection threshold levels at 5X, and continues to rise at 10X and 15X. These results generalize across both radial frequency and the angular speed of the trajectory, and persist with low contrast trajectories. Our findings point to the existence of a neural mechanism that is sensitive to deviations from circular motion trajectories.

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.

Global shape processing involves a hierarchy of integration stages

Radial Frequency (RF) patterns can be used to study the processing of familiar shapes, e.g. triangles and squares. Opinion is divided over whether the mechanisms that detect these shapes integrate local orientation and position information directly, or whether local orientations and positions are first combined to represent extended features, such as curves, and that it is local curvatures that the shape mechanism integrates. The latter view incorporates an intermediate processing stage, the former does not. To differentiate between these hypotheses we studied the processing of micro-patch sampled RF patterns as a function of the luminance polarity of successive elements on the contour path. Our first study measures shape after effects involving suprathreshold amplitude RF shapes and shows that alternating the luminance polarity of successive micro-patch elements disrupts adaptation of the global shape. Our second study shows that polarity alternations also disrupt sensitivity to threshold-amplitude RF patterns. These results suggest that neighbouring points of the contour shape are integrated into extended features by a polarity selective mechanism, prior to global shape processing, consistent with the view that for both threshold amplitude and suprathreshold amplitude patterns, global processing of RF shapes involves an intermediate stage of processing.

Visual bandwidths for face orientation increase during healthy aging

Perception of visual motion declines during healthy aging, and evidence suggests that this reflects decreases in cortical GABA inhibition that increase neural noise and motion bandwidths. This is supported by neurophysiological data on motion perception in senescent monkeys. Much less is known about deficits in higher level form vision. For example, face perception of frontal views remains relatively constant from adolescence through age 70 with a modest decline thereafter. However, we have shown recently that the elderly have a specific deficit in face matching when a transformation must be made between frontal and left or right side views. Here we use face view adaptation to demonstrate that this deficit results from significant broadening of cortical bandwidths for face orientation along with increased internal noise. A neural model shows that these bandwidths increase by a factor of 1.74 between age 26 and age 67 years. This is similar to the increase reported for motion bandwidths in senescent monkeys. Furthermore, the neural model demonstrates that head orientation bandwidth increases can arise from decreased cortical inhibition. Thus, high levels of form vision degrade in parallel with higher levels of motion perception and likely result from similar causes.

Global shape processing: which parts form the whole?

Research suggests that detection of low-frequency radial frequency (RF) patterns involves global shape processing and that points of maximum curvature (corners) contribute more than points of minimum curvature (sides). However, this has only been tested with stimuli presented at the threshold of discriminability from a circle. We used RF pattern adaptation to (a) examine whether a supra-threshold RF pattern is processed as a global shape and (b) determine what the critical features are for representing its shape. We measured the perceived amplitude shift of an RF test pattern after prolonged exposure either to a higher amplitude pattern or to various combinations of its parts (concave maxima, convex maxima, inflections). We found greater shifts in perceived amplitude after adaptation to a "whole" pattern than after adaptation to its component parts, which alternated to produce equal net contrast. Furthermore, when adapting to specific parts of the shape in isolation, we found that each part generated a similar magnitude aftereffect. Although the whole is clearly greater than the sum of the parts, we find that concave maxima, convex maxima, and inflections contribute equally to global shape processing, a fact that is only apparent when using a supra-threshold appearance-based task.

Neural correlates to seen gaze-direction and head orientation in the macaque monkey amygdala

Human neuropsychological studies suggest that the amygdala is implicated in social cognition, in which cognition of seen gaze-direction, especially the direct gaze, is essential, and that the perception of gaze direction is modulated by the head orientation of the facial stimuli. However, neural correlates to these issues remain unknown. In the present study, neuronal activity was recorded from the macaque monkey amygdala during performance of a sequential delayed non-matching-to-sample task based on gaze direction. The facial stimuli consisted of two head orientations (frontal; straight to the monkey, profile; 30 degrees rightwards from the front) with different gaze directions (directed toward and averted to the left or right of the monkey). Of the 1091 neurons recorded, 61 responded to more than one facial stimulus. Of these face-responsive neurons, 44 displayed responses selective to the facial stimuli (face neurons). Most amygdalar face neurons discriminated both gaze direction and head orientation, and exhibited a significant interaction between the two types about information. Furthermore, factor analysis on the response magnitudes of the face neurons to the facial stimuli revealed that two factors derived from these facial stimuli were correlated with two head orientations. The overall responses of the face neurons to direct gazes in the profile and frontal faces were significantly larger than that to averted gazes. The results suggest that information of both gaze and head direction is integrated in the amygdala, and that the amygdala is implicated in detection of direct gaze.

Visual search for human gaze direction by a Chimpanzee (Pan troglodytes)

Background

Humans detect faces with direct gazes among those with averted gazes more efficiently than they detect faces with averted gazes among those with direct gazes. We examined whether this “stare-in-the-crowd” effect occurs in chimpanzees (Pan troglodytes), whose eye morphology differs from that of humans (i.e., low-contrast eyes, dark sclera).

Methodology/Principal Findings

An adult female chimpanzee was trained to search for an odd-item target (front view of a human face) among distractors that differed from the target only with respect to the direction of the eye gaze. During visual-search testing, she performed more efficiently when the target was a direct-gaze face than when it was an averted-gaze face. This direct-gaze superiority was maintained when the faces were inverted and when parts of the face were scrambled. Subsequent tests revealed that gaze perception in the chimpanzee was controlled by the contrast between iris and sclera, as in humans, but that the chimpanzee attended only to the position of the iris in the eye, irrespective of head direction.

Conclusion/Significance

These results suggest that the chimpanzee can discriminate among human gaze directions and are more sensitive to direct gazes. However, limitations in the perception of human gaze by the chimpanzee are suggested by her inability to completely transfer her performance to faces showing a three-quarter view.

The effect of face eccentricity on the perception of gaze direction

The perception of a looker's gaze direction depends not only on iris eccentricity (the position of the looker's irises within the sclera) but also on the orientation of the lookers' head. One among several potential cues of head orientation is face eccentricity, the position of the inner features of the face (eyes, nose, mouth) within the head contour, as viewed by the observer. For natural faces this cue is confounded with many other head-orientation cues, but in schematic faces it can be studied in isolation. Salient novel illustrations of the effectiveness of face eccentricity are 'Necker faces', which involve equal iris eccentricities but multiple perceived gaze directions. In four experiments, iris and face eccentricity in schematic faces were manipulated, revealing strong and consistent effects of face eccentricity on perceived gaze direction, with different types of tasks. An additional experiment confirmed the 'Mona Lisa' effect with this type of stimuli. Face eccentricity most likely acted as a simple but robust cue of head turn. A simple computational account of combined effects of cues of eye and head turn on perceived gaze direction is presented, including a formal condition for the perception of direct gaze. An account of the 'Mona Lisa' effect is presented.

A new step towards understanding Embedded Figures Test performance in the autism spectrum: the radial frequency search task

The Embedded Figure Test (EFT) requires locating a simple shape embedded within a background of overlapping target-irrelevant scene elements. Observers with autism, or those with high levels of autistic-like traits, typically outperform matched comparison groups on the EFT. This research investigated the critical visual properties which give rise to this improved performance. The EFT is a search task and so here a radial frequency (RF) search task was created to directly explore efficacy of visual search and also the influence of element overlap on performance. In all conditions, the task was to detect whether the target RF3 (a triangular shape chosen for its visual properties) was present among a number of distracter RF4 (a square shape) patterns. The conditions employed were: 'singles', where all the patterns were spatially discrete, 'pairs', where two overlapping elements formed each cluster, and 'quads', comprising four overlapping elements per cluster. Compared to students scoring low on the Autism Spectrum Quotient (AQ; n=27), those scoring high on the AQ (n=23) were faster on the EFT and also significantly less influenced by increasing set size of the stimulus array in all RF search task conditions. However, the group difference in RF search performance was unaffected by the amount of stimulus overlap. Thus a simple search task is sufficient to detect a performance advantage associated with higher levels of autistic traits and has the advantages of a solid footing in visual theory and being readily repeatable for the purpose of assessing performance variability and change with interventions.

Relational property between head and eye regions is the primary determinant of the efficiency in search for a deviant gaze

The human visual system is extremely sensitive to the directions of the gazes of others. However, the mechanism underlying gaze direction perception has yet to be clarified. The primary aim of the present study is to investigate whether the relational property between the local eye region and other facial regions serves as the primary visual system cue in detecting a direct gaze. Our results showed that search efficiency was determined primarily by the gaze direction indicated by the relational property regardless of the direction indicated by the local feature information of the eye region; this was true even when the gaze directions indicated by these two types of information were conflicting. These results bolster the hypothesis that the human visual system primarily accesses socially meaningful information in searching for a deviant gaze.

Three cases of developmental prosopagnosia from one family: detailed neuropsychological and psychophysical investigation of face processing

A number of reports have documented that developmental prosopagnosia (DP) can run in families, but the locus of the deficits in those cases remains unclear. We investigated the perceptual basis of three cases of DP from one family (67 year-old father FA, and two daughters, 39 year-old D1 and 34 year-old D2) by combining neuropsychological and psychophysical methods. Neuropsychological tests involving natural facial images demonstrated significant face recognition deficits in the three family members. All three members showed normal facial expression recognition and face detection, and two of them (D2, FA) performed well on within-class object recognition tasks. These individuals were then examined in a series of psychophysical experiments. Intermediate form vision preceding face perception was assessed with radial frequency (RF) patterns. Normal discrimination of RF patterns in these individuals indicates that their face recognition difficulties are higher in the cortical form vision hierarchy than the locus of contour shape processing. Psychophysical experiments requiring discrimination and memory for synthetic faces aimed to quantify their face processing abilities and systematically examine the representation of facial geometry across viewpoints. D1 showed deficits in perceiving geometric information from the face at a given view. D2's impairments seem to arise in later face processing stages involving transferring view-dependent descriptions into a view-invariant representation. FA performed poorly on face learning and recognition relative to the age-appropriate controls. These cases provide evidence for familial transmission of high-level visual recognition deficits with normal intermediate-level form vision.

Head pose estimation in computer vision: a survey

The capacity to estimate the head pose of another person is a common human ability that presents a unique challenge for computer vision systems. Compared to face detection and recognition, which have been the primary foci of face-related vision research, identity-invariant head pose estimation has fewer rigorously evaluated systems or generic solutions. In this paper, we discuss the inherent difficulties in head pose estimation and present an organized survey describing the evolution of the field. Our discussion focuses on the advantages and disadvantages of each approach and spans 90 of the most innovative and characteristic papers that have been published on this topic. We compare these systems by focusing on their ability to estimate coarse and fine head pose, highlighting approaches that are well suited for unconstrained environments.

Radial frequency adaptation suggests polar-based coding of local shape cues

The study of shape processing in the human visual system has frequently employed radial frequency (RF) patterns as conveniently manipulable stimuli. This study uses an adaptation paradigm to investigate how local shape information is sampled in the processing of RF contour shapes. Experiment 1 measured thresholds for detecting a fixed mean radius RF contour following adaptation to RF patterns which, in separate conditions, varied in mean radius and radial frequency. Results reveal that, adaptation is strongly tuned for RF over a range of pattern radii, but is not tuned for the number of cycles of radial modulation per visual degree of contour length; a characteristic that changes with both radius and radial frequency. Experiment 2 manipulated the polar angle separation on the fronto-parallel plane between curvature features on a fixed RF by foreshortening the pattern appearance (consistent with a rotation in depth) and shows that RF shape processing is tuned for fronto-parallel separation angles between curvature features. Results were near identical when a stereo rotation cue was added to the perspective modified RF. In the second part of Experiment 2 we showed that RF shape adaptation is also tuned for the polar angular extent of the curvature represented by the lobe at that angle. Collectively, our results indicate that the polar angle at which local curvature features appear, in addition to the angular extent of the curvature feature at that location, are both critical parameters for coding specific RF shapes.

Effects of head orientation on gaze perception: how positive congruency effects can be reversed

Several past studies have considered how perceived head orientation may be combined with perceived gaze direction in judging where someone else is attending. In three experiments we tested the impact of different sources of information by examining the role of head orientation in gaze-direction judgements when presenting: (a) the whole face; (b) the face with the nose masked; (c) just the eye region, removing all other head-orientation cues apart from some visible part of the nose; or (d) just the eyes, with all parts of the nose masked and no head orientation cues present other than those within the eyes themselves. We also varied time pressure on gaze direction judgements. The results showed that gaze judgements were not solely driven by the eye region. Gaze perception can also be affected by parts of the head and face, but in a manner that depends on the time constraints for gaze direction judgements. While "positive" congruency effects were found with time pressure (i.e., faster left/right judgements of seen gaze when the seen head deviated towards the same side as that gaze), the opposite applied without time pressure.

Specialised higher-level mechanisms for facial-symmetry perception: evidence from orientation-tuning functions

Bilateral symmetry is important in many perceptual analyses from low-level figure-ground segmentation to higher-level face and object perception. Despite the success of low-level, image-based symmetry-detection models, these may not provide a complete account of symmetry perception. Better symmetry detection and stronger preferences for symmetry in upright faces than comparable patterns (e.g. inverted faces) that do not engage specialised face-coding mechanisms suggest a contribution of higher-level mechanisms to symmetry perception. We replicated better symmetry detection and stronger symmetry preferences for upright than inverted faces in experiment 1, and examined their orientation tuning in more detail in experiment 2. Decreasing performance as faces are mis-oriented away from the canonical upright orientation is the signature of specialised face-processing mechanisms, which are engaged less effectively as faces are mis-oriented. Lower-level symmetry-detection mechanisms, which operate better with vertical than horizontal, and horizontal than oblique, axes of symmetry would produce a W-shaped orientation-tuning function. Identical orientation-tuning functions were obtained for symmetry detection and preferences. Both declined with increasing mis-orientation over the 0 degrees-135 degrees range, consistent with a contribution from specialised face-coding mechanisms. Both increased from 135 degrees to 180 degrees, consistent with reliance on lower-level image-based mechanisms for severely misoriented faces. Taken together, the results implicate specialised, higher-level mechanisms in the detection of, and preference for, facial symmetry.

Processing local signals into global patterns

Perceptual organization or grouping is one of the central issues in vision research. Recent reports in the neuroimaging literature suggest that perceptual organization is mediated by distributed visual areas that range from the primary visual cortex (V1) to higher visual areas, depending on the availability of grouping cues and on the weight of contribution of each visual area. Evidence suggests that grouping by proximity and collinearity, and also perhaps filling-in, involve V1, whereas grouping by similarity and symmetry seems to depend on activation of higher visual areas. Further studies should include deliberate controls for confounding factors such as attentional artifacts and radial orientation bias, to clarify how spatiotemporal information in visual areas is integrated to give rise to perceptual organization.

Detection of shape in radial frequency contours: independence of local and global form information

Radial frequency (RF) patterns have been used to study the processes involved in shape perception. The psychophysical literature suggests that there are distinct global and local shape detection processes for low and high radial frequency patterns, but this has not been tested in a combined contour pattern, such as would be needed to describe the contours of most natural objects. Here, we combined frequencies from the local and global range onto a compound RF structure. Observers' ability to detect a single RF component on the compound pattern was measured. Results show that sensitivity to high frequency local deviations in shape was not affected by the presence of a globally perceived low frequency pattern. In the reverse condition, detection of global form was not influenced by adding local deviations onto the structure. This suggests that local and global shape information can be detected independently within the human visual system.