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The human brain has dedicated mechanisms for processing other people’s movements. Previous research has revealed how these mechanisms contribute to perceiving the movements of individuals but has left open how we perceive groups of people moving together. Across three experiments, we test whether movement perception depends on the spatiotemporal relationships among the movements of multiple agents. In Experiment 1, we combine EEG frequency tagging with apparent human motion and show that posture and movement perception can be dissociated at harmonically related frequencies of stimulus presentation. We then show that movement but not posture processing is enhanced when observing multiple agents move in synchrony. Movement processing was strongest for fluently moving synchronous groups (Experiment 2) and was perturbed by inversion (Experiment 3). Our findings suggest that processing group movement relies on binding body postures into movements and individual movements into groups. Enhanced perceptual processing of movement synchrony may form the basis for higher order social phenomena such as group alignment and its social consequences.

Previous research suggests that belief in free will correlates with intentionality attribution. However, whether belief in free will is also related to more basic social processes is unknown. Based on evidence that biological motion contains intentionality cues that observers spontaneously extract, we investigate whether people who believe more in free will, or in related constructs, such as dualism and determinism, would be better at picking up such cues and therefore at detecting biological agents hidden in noise, or would be more inclined to detect intentionality cues and therefore to detect biological agents even when there are none. Signal detection theory was used to measure participants' ability to detect biological motion from scrambled background noise (d ') and their response bias (c) in doing so. In two experiments, we found that belief in determinism and belief in dualism, but not belief in free will, were associated with biological motion perception. However, no causal effect was found when experimentally manipulating free will-related beliefs. In sum, our results show that biological motion perception, a low-level social process, is related to high-level beliefs about dualism and determinism.

If neurotypical people rely on specialized perceptual mechanisms when perceiving biological motion, then one would not expect an association between task performance and IQ. However, if those with ASD recruit higher order cognitive skills when solving biological motion tasks, performance may be predicted by IQ. In a meta-analysis that included 19 articles, we found an association between biological motion perception and IQ among observers with ASD but no significant relationship among typical observers. If the task required emotion perception, then there was an even stronger association with IQ in the ASD group.

We tested the relationship between biological motion perception and the Autism-Spectrum Quotient. In three experiments, we indexed observers’ performance on a classic left-right discrimination task in which participants were asked to report the facing direction of walkers containing solely structural or kinematics information, a motion discrimination task in which participants were asked to indicate the apparent motion of a (non-biological) random-dot stimulus, and a novel naturalness discrimination task. In the naturalness discrimination task, we systematically manipulated the degree of natural acceleration contained in the stimulus by parametrically morphing between a fully veridical stimulus and one where acceleration was removed. Participants were asked to discriminate the more natural stimulus (i.e., acceleration-containing stimulus) from the constant velocity stimulus. Although we found no reliable associations between overall AQ scores nor subdomain scores with performance on the direction-related tasks, we found a robust association between performance on the biological motion naturalness task and attention switching domain scores. Our findings suggest that understanding the relationship between the Autism Spectrum and perception is a far more intricate problem than previously suggested. While it has been shown that the AQ can be used as a proxy to tap into perceptual endophenotypes in Autism, the eventual diagnostic value of the perceptual task depends on the task’s consideration of biological content and demands.

The body of most creatures is composed of interconnected joints. During motion, the spatial location of these joints changes, but they must maintain their distances to one another, effectively moving semirigidly. This pattern, termed “biological motion” in the literature, can be used as a visual cue, enabling many animals (including humans) to distinguish animate from inanimate objects. Crucially, even artificially created scrambled stimuli, with no recognizable structure but that maintains semirigid movement patterns, are perceived as animated. However, to date, biological motion perception has only been reported in vertebrates. Due to their highly developed visual system and complex visual behaviors, we investigated the capability of jumping spiders to discriminate biological from nonbiological motion using point-light display stimuli. These kinds of stimuli maintain motion information while being devoid of structure. By constraining spiders on a spherical treadmill, we simultaneously presented 2 point-light displays with specific dynamic traits and registered their preference by observing which pattern they turned toward. Spiders clearly demonstrated the ability to discriminate between biological motion and random stimuli, but curiously turned preferentially toward the latter. However, they showed no preference between biological and scrambled displays, results that match responses produced by vertebrates. Crucially, spiders turned toward the stimuli when these were only visible by the lateral eyes, evidence that this task may be eye specific. This represents the first demonstration of biological motion recognition in an invertebrate, posing crucial questions about the evolutionary history of this ability and complex visual processing in nonvertebrate systems., The body of most creatures is composed of interconnected joints, constraining their motion to a pattern known as “biological motion.” This study shows that jumping spiders can distinguish animate from inanimate objects based solely on visual motion characteristics — an ability previously known only in vertebrates.

Representing multiple agents and their mutual relations is a prerequisite to understand social events. Using functional MRI on human adults, we show that visual areas dedicated to body-form and body-motion perception contribute to processing social events, by holding the representation of multiple moving bodies and encoding the spatial relations between them. In particular, seeing animations of human bodies facing and moving toward (vs. away from) each other, increased neural activity in the body-selective cortex (extrastriate body area -EBA) and posterior superior temporal sulcus for biological-motion perception (bm-pSTS). In those areas, representation of body postures and movements, as well as of the overall scene, was more accurate for facing-body (vs. non-facing body) stimuli. Effective connectivity analysis with Dynamic Causal Modeling revealed increased coupling between EBA and bm-pSTS during perception of facing-body stimuli. The attunement of human vision to multiple-body scenes involving perceptual cues of interaction such as face-to-face positioning and approaching behaviour, was further supported by the participants’ better performance in a match-to-sample task with facing-body vs. non-facing body stimuli. Thus, visuo-spatial cues of interaction in multiple-person scenarios affect the perceptual representation of body and body motion and, by promoting functional integration, streamline the process from body perception to action representation.

Contains fulltext : 236627.pdf (Publisher’s version ) (Open Access) Linguistic labels exert a particularly strong top-down influence on perception. The potency of this influence has been ascribed to their ability to evoke category-diagnostic features of concepts. In doing this, they facilitate the formation of a perceptual template concordant with those features, effectively biasing perceptual activation towards the labelled category. In this study, we employ a cueing paradigm with moving, point-light stimuli across three experiments, in order to examine how the number of biological motion features (form and kinematics) encoded in lexical cues modulates the efficacy of lexical top-down influence on perception. We find that the magnitude of lexical influence on biological motion perception rises as a function of the number of biological motion-relevant features carried by both cue and target. When lexical cues encode multiple biological motion features, this influence is robust enough to mislead participants into reporting erroneous percepts, even when a masking level yielding high performance is used. 14 p.

Few studies have investigated biological motion perception in dogs and it remains unknown whether dogs recognise the biological identity of two-dimensional animations of human motion cues. To test this, we assessed the dogs’ (N = 32) responses to point-light displays of a human performing a pointing gesture towards one of two pots. At the start of the experiment the demonstrator was a real-life person, but over the course of the test dogs were presented with two-dimensional figurative representations of pointing gestures in which visual information was progressively removed until only the isolated motion cues remained. Dogs’ accuracy was above chance level only with real-life and black-and-white videos, but not with the silhouette or the point-light figure. Dogs’ accuracy during these conditions was significantly lower than in the real-life condition. This result could not be explained by trial order since dogs’ performance was still not higher than chance when only the point-light figure condition was presented after the initial demonstration. The results imply that dogs are unable to recognise humans in two-dimensional depictions of human motion cues only. In spite of extensive exposure to human movement, dogs need more perceptual cues to detect equivalence between human two-dimensional animations and the represented living entity.

Humans and other animals have evolved to act in groups, but how does the brain distinguish multiple people moving in group from multiple people moving independently? Across three experiments, we test whether biological motion perception depends on the spatiotemporal relationships among people moving together. In Experiment 1, we apply EEG frequency tagging to apparent biological motion and show that fluently ordered sequences of body postures drive brain activity at three hierarchical levels of biological motion processing: image, body sequence, and movement. We then show that movement-, but not body- or image-related brain responses are enhanced when observing four agents moving in synchrony. Neural entrainment was strongest for fluently moving synchronous groups (Experiment 2), displayed in upright orientation (Experiment 3). Our findings show that the brain preferentially entrains to the collective movement of human agents, deploying perceptual organization principles of synchrony and common fate for the purpose of social perception.

Heading estimation from optic flow relies on the assumption that the visual world is rigid. This assumption is violated when one moves through a crowd of people, a common and socially important situation. The motion of people in the crowd contains cues to their translation in the form of the articulation of their limbs, known as biological motion. We investigated how translation and articulation of biological motion influence heading estimation from optic flow for self-motion in a crowd. Participants had to estimate their heading during simulated self-motion toward a group of walkers who collectively walked in a single direction. We found that the natural combination of translation and articulation produces surprisingly small heading errors. In contrast, experimental conditions that either present only translation or only articulation produced strong idiosyncratic biases. The individual biases explained well the variance in the natural combination. A second experiment showed that the benefit of articulation and the bias produced by articulation were specific to biological motion. An analysis of the differences in biases between conditions and participants showed that different perceptual mechanisms contribute to heading perception in crowds. We suggest that coherent group motion affects the reference frame of heading perception from optic flow.

This paper explores in parallel the underlying mechanisms in human perception of biological motion and the best approaches for automatic classification of gait. The experiments tested three different learning paradigms, namely, biological, biomimetic, and non-biomimetic models for gender identification from human gait. Psychophysical experiments with twenty-one observers were conducted along with computational experiments without applying any gender specific modifications to the models or the stimuli. Results demonstrate the utilization of a generic memory based learning system in humans for gait perception, thus reducing ambiguity between two opposing learning systems proposed for biological motion perception. Results also support the biomimetic nature of memory based artificial neural networks (ANN) in their ability to emulate biological neural networks, as opposed to non-biomimetic models. In addition, the comparison between biological and computational learning approaches establishes a memory based biomimetic model as the best candidate for a generic artificial gait classifier (83% accuracy, p < 0.001), compared to human observers (66%, p < 0.005) or non-biomimetic models (83%, p < 0.001) while adhering to human-like sensitivity to gender identification, promising potential for application of the model in any given non-gender based gait perception objective with superhuman performance.

We examined mechanisms underlying infants’ ability to categorize human biological motion stimuli from sex-typed walk motions, focusing on how visual attention to dynamic information in point-light displays (PLDs) contributes to infants' social category formation. We tested for categorization of PLDs produced by women and men by habituating infants to a series of female or male walk motions and then recording posthabituation preferences for new PLDs from the familiar or novel category (Experiment 1). We also tested for intrinsic preferences for female or male walk motions (Experiment 2). We found that infant boys were better able to categorize PLDs than were girls and that male PLDs were preferred overall. Neither of these effects was found to change with development across the observed age range (∼4–18 months). We conclude that infants’ categorization of walk motions in PLDs is constrained by intrinsic preferences for higher motion speeds and higher spans of motion and, relatedly, by differences in walk motions produced by men and women.

Many domains of human behavior are based on multisensory representations. Knowledge about the principles of multisensory integration is useful to configure real‐time movement information for the online support of perceptuomotor processes (motor perception, control, and learning). A powerful method for generating real‐time information is movement sonification. Remarkable evidence exists on movement‐acoustic real‐time information being effective in behavioral domains (music training, motor rehabilitation, sports). Here, we investigate whether and how biological motion perception can be enhanced, substituted, or modulated by kinematic sonification, with a focus on pitch coding. We work with gross motor cyclic movements and investigate the effectiveness of pitch scaling and consistent transposition on audiovisual motor perception accuracy (Experiment A). Beyond that, a new kind of audiovisual stimulus with inconsistent pitch transposition is used to produce a directed modulation of the integrated audiovisual percept (Experiment B). Results from Experiment A indicate pitch being powerful for mediating kinematic information to enhance motor perception and substituting information between perceptual modalities, even exceeding visual performance. Beyond these findings, results from Experiment B indicate that visual estimations of movement velocity can be enhanced or reduced auditorily. Movement sonification used for reshaping intermodal adjustments should be a powerful new tool for subconsciously shaping human movement patterns in the future. European Commission/H2020- FETPROACT-2014/641321/EU

The perception of human motion is a vital ability in our daily lives. Human movement recognition is often studied using point-light stimuli in which dots represent the joints of a moving person. Depending on task and stimulus, the local motion of the single dots, and the global form of the stimulus can be used to discriminate point-light stimuli. Previous studies often measured motion coherence for global motion perception and contrasted it with performance in biological motion perception to assess whether difficulties in biological motion processing are related to more general difficulties with motion processing. However, it is so far unknown as to how performance in global motion tasks relates to the ability to use local motion or global form to discriminate point-light stimuli. Here, we investigated this relationship in more detail. In Experiment 1, we measured participants’ ability to discriminate the facing direction of point-light stimuli that contained primarily local motion, global form, or both. In Experiment 2, we embedded point-light stimuli in noise to assess whether previously found relationships in task performance are related to the ability to detect signal in noise. In both experiments, we also assessed motion coherence thresholds from random-dot kinematograms. We found relationships between performances for the different biological motion stimuli, but performance for global and biological motion perception was unrelated. These results are in accordance with previous neuroimaging studies that highlighted distinct areas for global and biological motion perception in the dorsal pathway, and indicate that results regarding the relationship between global motion perception and biological motion perception need to be interpreted with caution.

Social perceptual deficits in schizophrenia are well established. Recent work suggests that the ability to extract social information from bodily cues is reduced in patients. However, little is known about the neurobiological mechanisms underlying this deficit. In the current study, 20 schizophrenia patients and 16 controls completed two tasks using point-light animations during fMRI: a basic biological motion task and an emotion in biological motion task. The basic biological motion task was used to localize activity in posterior superior temporal sulcus (pSTS), a critical region for biological motion perception. During the emotion in biological motion task, participants viewed brief videos depicting happiness, fear, anger, or neutral emotions and were asked to decide which emotion was portrayed. Activity in pSTS and amygdala was interrogated during this task. Results indicated that patients showed overall reduced activation compared to controls in pSTS and at a trend level in amygdala across emotions, despite similar task performance. Further, a functional connectivity analysis revealed that controls, but not patients, showed significant positive connectivity between pSTS and left frontal regions as well as bilateral angular gyrus during the emotion in biological motion task. These findings indicate that schizophrenia patients show aberrant neural activity and functional connectivity when extracting complex social information from simple motion stimuli, which may contribute to social perception deficits in this disorder., Highlights • Perception of social information from bodily cues is impaired in schizophrenia. • We examined neural correlates of perception of emotion from biological motion. • Activity in amygdala and posterior superior temporal sulcus was reduced in patients. • pSTS functional connectivity with frontal and parietal regions was reduced in patients. • Aberrant neural responses may contribute to social perceptual deficits in schizophrenia.

We conducted two studies to test how deaf signed language users perceive biological motions. We created 18 Biological Motion point-light displays (PLDs) depicting everyday human actions, and 18 Scrambled control PLDs. First, we conducted an online behavioral rating survey, in which deaf and hearing raters identified the biological motion PLDs and rated how easy it was for them to identify the actions. Then, we conducted an EEG study in which Deaf Signers and Hearing Non-Signers watched both the Biological Motion PLDs and the Scrambled PLDs, and we computed the time-frequency responses within the theta, alpha, and beta EEG rhythms. From the behavioral rating task, we show that the deaf raters reported significantly less effort required for identifying the Biological motion PLDs, across all stimuli. The EEG results showed that the Deaf Signers showed theta, mu, and beta differentiation between Scrambled and Biological PLDs earlier and more consistently than Hearing Non-Signers. We conclude that native ASL users exhibit experience-dependent neuroplasticity in the domain of biological human motion perception.

The posterior STS (pSTS) is an important brain region for perceptual analysis of social cognitive cues. This study seeks to characterize the pattern of network connectivity emerging from the pSTS in three core social perception localizers: biological motion perception, gaze recognition, and the interpretation of moving geometric shapes as animate. We identified brain regions associated with all three of these localizers and computed the functional connectivity pattern between them and the pSTS using a partial correlations metric that characterizes network connectivity. We find a core pattern of cortical connectivity that supports the hypothesis that the pSTS serves as a hub of the social brain network. The right pSTS was the most highly connected of the brain regions measured, with many long-range connections to pFC. Unlike other highly connected regions, connectivity to the pSTS was distinctly lateralized. We conclude that the functional importance of right pSTS is revealed when considering its role in the large-scale network of brain regions involved in various aspects of social cognition.

Supplementary material related to this article can be found, in the online version, at doi: https://doi.org/10.1016/j.infbeh.2020.101450., Biological motion perception is a key component of action perception contributing to social cognition in crucial ways. Contemporary neuroimaging studies show that biological motion is processed differently in the human brain from other types of motion. In particular, the right posterior Superior Temporal Sulcus (rpSTS), an area known for its central role in social perception, has been consistently associated with the perception of biological motion in the mature brain. By contrast, most findings investigating the development of biological motion perception in infancy come from behavioral studies, and far less is known regarding the right STS' role in processing biological motion. The current study used fNIRS to measure brain activation to biological motion in the rSTS region of 7-8-month-old infants. Infants were presented with two conditions: an approaching coherent motion of a person walking (coherent point-light-walker, PLW); and a spatially scrambled version of this display, where the global configuration of a person walking was disrupted (scrambled PLW). We found a functional activation, i.e., a significant increase in HbO2 concentration in relation to baseline, in the right middle-posterior temporal cortex only when infants viewed the coherent point-light-walker. This activation statistically differed from the scrambled point-light-walker, and no significant activations were found for viewing the scrambled motion. Our study adds evidence pointing to rSTS' sensitivity to the global human configuration in biological motion processing during infancy. The rSTS seems thus to become functionally specialized in biological motion configuration as early as at 7-8 months of age.

BackgroundBiological motion, namely the movement of others, conveys information that allows the identification of affective states and intentions. This makes it an important avenue of research in autism spectrum disorder where social functioning is one of the main areas of difficulty. We aimed to create a quantitative summary of previous findings and investigate potential factors, which could explain the variable results found in the literature investigating biological motion perception in autism.MethodsA search from five electronic databases yielded 52 papers eligible for a quantitative summarisation, including behavioural, eye-tracking, electroencephalography and functional magnetic resonance imaging studies.ResultsUsing a three-level random effects meta-analytic approach, we found that individuals with autism generally showed decreased performance in perception and interpretation of biological motion. Results additionally suggest decreased performance when higher order information, such as emotion, is required. Moreover, with the increase of age, the difference between autistic and neurotypical individuals decreases, with children showing the largest effect size overall.ConclusionWe highlight the need for methodological standards and clear distinctions between the age groups and paradigms utilised when trying to interpret differences between the two populations.

International audience; Social cognition is dependent on the ability to extract information from human stimuli. Of those, patterns of biological motion (BM) and in particular walking patterns of other humans, are prime examples. Although most often tested in isolation, BM outside the laboratory is often associated with multisensory cues (i.e. we often hear and see someone walking) and there is evidence that vision-based judgments of BM stimuli are systematically influenced by motor signals. Furthermore, cross-modal visuo-tactile mechanisms have been shown to influence perception of bodily stimuli. Based on these observations, we here investigated if somatosensory inputs would affect visual BM perception. In two experiments, we asked healthy participants to perform a speed discrimination task on two point light walkers (PLW) presented one after the other. In the first experiment, we quantified somatosensory-visual interactions by presenting PLW together with tactile stimuli either on the participants' forearms or feet soles. In the second experiment, we assessed the specificity of these interactions by presenting tactile stimuli either synchronously or asynchronously with upright or inverted PLW. Our results confirm that somatosensory input in the form of tactile foot stimulation influences visual BM perception. When presented with a seen walker's footsteps, additional tactile cues enhanced sensitivity on a speed discrimination task, but only if the tactile stimuli were presented on the relevant body-part (under the feet) and when the tactile stimuli were presented synchronously with the seen footsteps of the PLW, whether upright or inverted. Based on these findings we discuss potential mechanisms of somatosensory-visual interactions in BM perception.

BACKGROUND The ability of healthy individuals to detect biological motion by using a small number of moving points is well established in animals and humans. Perception of human movements may depend on internal models that drive self-generated movements and influence motion discrimination (Reed CL et al. 1995 and 2007). As a person's motor repertoire deteriorates, the accuracy of these models may also decrease. OBJECTIVE Determine if people with symptomatic Huntington's disease (HD) have difficulty perceiving movements. METHODS In this study point-light displays were created with a Vicon Motion Capture System by recording one individual with (impaired) and one individual without (healthy) Parkinson's disease using a 13 joint marker set. Participants were asked to distinguish between three movements and determine if the movement was impaired or healthy. The ability of participants with and without HD to distinguish movement patterns and the time to perception were recorded. RESULTS Analyses found participants with HD had a decreased ability to correctly detect movements and point-light image type. The stair climbing motion showed the largest effect as participants with HD had more difficulty correctly identifying both the movement and whether it was impaired or healthy. In addition, the participants without HD showed an improvement as trials progressed which could not be observed in the HD cohort. CONCLUSIONS As people with symptomatic HD have difficulty perceiving movements further investigations using point-light displays should be done to determine if these impairments might serve as an easily administered, non-invasive marker of disease state.

Two cornerstones of social development—social perception and theory of mind—undergo brain and behavioral changes during middle childhood, but the link between these developing domains is unclear. One theoretical perspective argues that these skills represent domain-specific areas of social development, whereas other perspectives suggest that both skills may reflect a more integrated social system. Given recent evidence from adults that these superficially different domains may be related, the current study examined the developmental relation between these social processes in 52 children aged 7 to 12 years. Controlling for age and IQ, social perception (perception of biological motion in noise) was significantly correlated with two measures of theory of mind: one in which children made mental state inferences based on photographs of the eye region of the face and another in which children made mental state inferences based on stories. Social perception, however, was not correlated with children’s ability to make physical inferences from stories about people. Furthermore, the mental state inference tasks were not correlated with each other, suggesting a role for social perception in linking various facets of theory of mind.

OBJECTIVE We examined biological motion perception in Parkinson's disease (PD). Biological motion perception is related to one's own motor function and depends on the integrity of brain areas affected in PD, including posterior superior temporal sulcus. If deficits in biological motion perception exist, they may be specific to perceiving natural/fast walking patterns that individuals with PD can no longer perform, and may correlate with disease-related motor dysfunction. METHOD Twenty-six nondemented individuals with PD and 24 control participants viewed videos of point-light walkers and scrambled versions that served as foils, and indicated whether each video depicted a human walking. Point-light walkers varied by gait type (natural, parkinsonian) and speed (0.5, 1.0, 1.5 m/s). Participants also completed control tasks (object motion, coherent motion perception), a contrast sensitivity assessment, and a walking assessment. RESULTS The PD group demonstrated significantly less sensitivity to biological motion than the control group (p < .001, Cohen's d = 1.22), regardless of stimulus gait type or speed, with a less substantial deficit in object motion perception (p = .02, Cohen's d = .68). There was no group difference in coherent motion perception. Although individuals with PD had slower walking speed and shorter stride length than control participants, gait parameters did not correlate with biological motion perception. Contrast sensitivity and coherent motion perception also did not correlate with biological motion perception. CONCLUSION PD leads to a deficit in perceiving biological motion, which is independent of gait dysfunction and low-level vision changes, and may therefore arise from difficulty perceptually integrating form and motion cues in posterior superior temporal sulcus. (PsycINFO Database Record

The perception of human movement is a key component of daily social interactions. Although extrastriate area MT+/V5 is closely associated with motion processing, its role in the processing of sparse ‘biological motion’ displays is still unclear. We developed two closed matched psychophysical tasks to assess simple coherent motion perception and biological motion perception, and measured changes in performance caused by application of TMS over MT+/V5. Performance of the simple motion discrimination task was significantly depressed by TMS stimulation, and highly correlated within observers in TMS conditions, but there was no significant decrement in performance of the biological motion task, despite low intra-observer correlations across TMS conditions. We conclude that extrastriate area MT+/V5 is an obligatory waypoint in the neural processing of simple coherent motion, but is not obligatory for the processing of biological motion. Results are consistent with a dual neural processing route for biological motion processing.

BACKGROUND. Prior studies have documented biological motion perception deficits in schizophrenia, but it remains unclear whether the impairments arise from poor social cognition, perceptual organization, basic motion processing, or sustained attention/motivation. To address the issue, we had 24 chronic schizophrenia patients and 27 healthy controls perform three tasks: coherent motion, where subjects indicated whether a cloud of dots drifted leftward or rightward; dynamic rigid form, where subjects determined the tilt direction of a translating, point-light rectangle; and biological motion, where subjects judged whether a human point-light figure walked leftward or rightward. Task difficulty was staircase controlled and depended on the directional variability of background dot motion. Catch trials were added to verify task attentiveness and engagement. RESULTS. Patients and controls demonstrated similar performance thresholds and near-ceiling catch trial accuracy for each task (uncorrected ps>.1; ds< .35). In all but the coherent motion task, higher IQ correlated with better performance (ps< .001). CONCLUSION. Schizophrenia patients have intact perception of motion coherence, dynamic rigid form, and biological motion at least for our sample and set-up. We speculate that previously documented biological motion perception deficits arose from task or stimulus differences or from group differences in IQ, attention, or motivation.

Communication through body gestures permeates our daily life. Efficient perception of the message therein reflects one’s social cognitive competency. Here we report that such competency is manifested temporally as shortened subjective duration of social interactions: motion sequences showing agents acting communicatively are perceived to be significantly shorter in duration as compared with those acting noncommunicatively. The strength of this effect is negatively correlated with one’s autistic-like tendency. Critically, intranasal oxytocin administration restores the temporal compression effect in socially less proficient individuals, whereas the administration of atosiban, a competitive antagonist of oxytocin, diminishes the effect in socially proficient individuals. These findings indicate that perceived time, rather than being a faithful representation of physical time, is highly idiosyncratic and ingrained with one’s personality trait. Moreover, they suggest that oxytocin is involved in mediating time perception of social interaction, further supporting the role of oxytocin in human social cognition., eLife digest Einstein once joked: “Put your hand on a hot stove for a minute, and it seems like an hour. Sit with a pretty girl for an hour, and it seems like a minute. That’s relativity.” While it may not have helped explain the space-time continuum, his joke neatly captures how time can appear to pass at different rates. This perception depends in part on the sensory properties of the stimuli we are experiencing. Intense stimuli, such as bright and fast-moving objects, trigger stronger responses in the brain than less intense stimuli, and so we perceive them as longer lasting. But what role do we, as the experiencers, play in how we perceive time? To find out, Liu, Yuan, Chen et al. showed volunteers pairs of movie clips, each featuring two human figures outlined by dots. In one clip, the two figures interacted socially, for example by passing an object between them. In the other, the two figures moved independently of each other. The volunteers had to decide which clip lasted longer. The volunteers generally judged clips containing social interactions to be shorter than those without such interactions, even when this was not the case. Moreover, volunteers with better social skills tended to underestimate the length of the social interaction clips to a greater extent. Previous studies have shown that people who are more social tend to have higher levels of a hormone called oxytocin in their blood. Oxytocin is sometimes referred to as the ‘love hormone’ because it promotes social behavior and bonding. Applying an oxytocin nasal spray to the volunteers who were less socially proficient caused them to perceive the social interaction clips as shorter than before. By contrast, socially proficient volunteers who used a nasal spray that blocks the effects of oxytocin perceived these clips as longer than they had done previously (although they still judged the clips to be shorter than videos that did not show people interacting). The perception of time thus varies between people and may depend at least in part on personality. These results open up a new avenue for studying and manipulating how we process social situations. This could eventually benefit people who struggle with social interactions, such as those with autism spectrum disorders.

Preferential attention to living creatures is believed to be an intrinsic capacity of the visual system of several species, with perception of biological motion often studied and, in humans, it correlates with social cognitive performance. Although domestic dogs are exceptionally attentive to human social cues, it is unknown whether their sociability is associated with sensitivity to conspecific and heterospecific biological motion cues of different social relevance. We recorded video clips of point-light displays depicting a human or dog walking in either frontal or lateral view. In a preferential looking paradigm, dogs spontaneously viewed 16 paired point-light displays showing combinations of normal/inverted (control condition), human/dog and frontal/lateral views. Overall, dogs looked significantly longer at frontal human point-light display versus the inverted control, probably due to its clearer social/biological relevance. Dogs’ sociability, assessed through owner-completed questionnaires, further revealed that low-sociability dogs preferred the lateral point-light display view, whereas high-sociability dogs preferred the frontal view. Clearly, dogs can recognize biological motion, but their preference is influenced by their sociability and the stimulus salience, implying biological motion perception may reflect aspects of dogs’ social cognition.

Given the ecological importance of biological motion and its relevance to social cognition, considerable effort has been devoted over the past decade to studying biological motion perception in autism. However, previous studies have asked observers to detect or recognize briefly presented human actions placed in isolation, without spatial or temporal context. Research on typical populations has shown the influence of temporal context in biological motion perception: prolonged exposure to one action gives rise to an aftereffect that biases perception of a subsequently displayed action. Whether people with autism spectrum disorders (ASD) show such adaptation effects for biological motion stimuli remains unknown. To address this question, this study examined how well youth with ASD recognize ambiguous actions and adapt to recently-observed actions. Compared to typically-developing (TD) controls, youth with ASD showed no differences in perceptual boundaries between actions categories, indicating intact ability in recognizing actions. However, children with ASD showed weakened adaptation to biological motion. It is unlikely that the reduced action adaptability in autism was due to delayed developmental trajectory, as older children with ASD showed weaker adaptation to actions than younger children with ASD. Our results further suggest that high-level (i.e., action) processing weakens with age for children with ASD, but this change may be accompanied by a potentially compensatory mechanism based on more involvement of low-level (i.e., motion) processing. Autism Res 2016, 9: 1103-1113. © 2016 International Society for Autism Research, Wiley Periodicals, Inc.

The visual system is able to recognize human motion simply from point lights attached to the major joints of an actor. Moreover, it has been shown that younger adults are able to recognize emotions from such dynamic point-light displays. Previous research has suggested that the ability to perceive emotional stimuli changes with age. For example, it has been shown that older adults are impaired in recognizing emotional expressions from static faces. In addition, it has been shown that older adults have difficulties perceiving visual motion, which might be helpful to recognize emotions from point-light displays. In the current study, 4 experiments were completed in which older and younger adults were asked to identify 3 emotions (happy, sad, and angry) displayed by 4 types of point-light walkers: upright and inverted normal walkers, which contained both local motion and global form information; upright scrambled walkers, which contained only local motion information; and upright random-position walkers, which contained only global form information. Overall, emotion discrimination accuracy was lower in older participants compared with younger participants, specifically when identifying sad and angry point-light walkers. In addition, observers in both age groups were able to recognize emotions from all types of point-light walkers, suggesting that both older and younger adults are able to recognize emotions from point-light walkers on the basis of local motion or global form.

Visual motion, a critical cue in communication, can be manipulated and studied using video playback methods. A primary concern for the video playback researcher is the degree to which objects presented on video appear natural to the non-human subject. Here we argue that the quality of motion cues on video, as determined by the video's image presentation rate (IPR), are of particular importance in determining a subject's social response behaviour. We present an experiment testing the effect of variations in IPR on pigeon (Columbia livia) response behaviour towards video images of courting opposite sex partners. Male and female pigeons were presented with three video playback stimuli, each containing a different social partner. Each stimulus was then modified to appear at one of three IPRs: 15, 30 or 60 progressive (p) frames per second. The results showed that courtship behaviour became significantly longer in duration as IPR increased. This finding implies that the IPR significantly affects the perceived quality of motion cues impacting social behaviour. In males we found that the duration of courtship also depended on the social partner viewed and that this effect interacted with the effects of IPR on behaviour. Specifically, the effect of social partner reached statistical significance only when the stimuli were displayed at 60 p, demonstrating the potential for erroneous results when insufficient IPRs are used. In addition to demonstrating the importance of IPR in video playback experiments, these findings help to highlight and describe the role of visual motion processing in communication behaviour.

The ultimate goal of biological motion perception is to be able to understand actions so as to provide an answer to the question, “Who did what to whom and why?” This inference capacity enables humans to go beyond the surface appearance of behavior in order to successfully interact with others and with the environment. In addition to its functional importance, understanding biological motion bridges several major fields, including perception, reasoning, and social cognition. However, despite its paramount role in human perception and cognition, only limited progress has so far been made in understanding biological motion. After reviewing the relevant literature, this essay argues that future research needs to identify the contributions of three basic processes involved in understanding biological motion: perception of animacy, causality, and intention. The involvement of these basic processes needs to be investigated both in the typical healthy population as well as in populations with mental disorders, such as autism spectrum disorders and schizophrenia. We also suggest that a productive research approach should focus on more interactive actions of the sort often observed in the natural social environment, rather than solely using the single-actor displays that have been typical in previous work. It is further emphasized that there is a need for a theoretical and computational framework within which these different types of processing can be united. We propose that the predictive coding framework provides a good candidate. Keywords: biological motion; intentions; animacy; causality; predictive coding; interaction; prediction

Although the relationship between biological motion perception as depicted by point-light displays and social cognition has been investigated in recent decades, the developmental course of the integration of social cognition and the perception of biological motion is not well understood. To better understand this development, we investigated the ability of 9- and 12-month-old infants to shift their gaze toward a point-light upright human figure using a paradigm similar to that used by Yoon and Johnson (2009). We found that 12-month-old, but not 9-month-old, infants were able to follow the direction of attention of the upright point-light figure (Experiments 1 and 2). However, both the younger and older infants were able to follow the attentional shift of others under the full-view condition (Experimental 3). These results suggest that the ability to process the higher-level information provided by biological motion patterns, such as the attentional direction of others, develops by 12 months, but not by 9 months, of age. The relationship between the development of social cognition and that of biological motion perception is discussed.

The ability to detect biological motion (BM) and decipher the meaning therein is essential to human survival and social interaction. However, at the individual level, we are not equally equipped with this ability. In particular, impaired BM perception and abnormal neural responses to BM have been observed in autism spectrum disorder (ASD), a highly heritable neurodevelopmental disorder characterized by devastating social deficits. Here, we examined the underlying sources of individual differences in two abilities fundamental to BM perception (i.e., the abilities to process local kinematic and global configurational information of BM) and explored whether BM perception shares a common genetic origin with autistic traits. Using the classical twin method, we found reliable genetic influences on BM perception and revealed a clear dissociation between its two components-whereas genes account for about 50% of the individual variation in local BM processing, global BM processing is largely shaped by environment. Critically, participants' sensitivity to local BM cues was negatively correlated with their autistic traits through the dimension of social communication, with the covariation largely mediated by shared genetic effects. These findings demonstrate that the ability to process BM, especially with regard to its inherent kinetics, is heritable. They also advance our understanding of the sources of the linkage between autistic symptoms and BM perception deficits, opening up the possibility of treating the ability to process local BM information as a distinct hallmark of social cognition.

Schizophrenia patients have impairments of biological motion (BM) perception, which provides critical information about social cognition. Because social cognition is underpinned by attention, the impairments of BM perception in schizophrenia could be partially attributable to altered attention. To elucidate the impairments in attention and social perception in schizophrenia, we investigated the neural basis of impaired BM processing using MRI in respect to attention deficits by eye tracker. Voxel-based morphometry was performed to evaluate the relationship between BM perception and gray matter (GM) volume. The temporo-parietal junction (TPJ) and anterior superior temporal sulcus (aSTS) were related to task accuracy. However, when the effect of attention (i.e., eye movement) was controlled, the relationship in TPJ became non-significant, while aSTS showed a significant relationship with BM perception. Alteration in TPJ might be associated with inefficient attentional strategy, whereas dysfunctional aSTS might be correlated with deficit in higher order BM processing per se. Several cognitive levels as well as corresponding brain areas are possibly involved in the manifestation of social cognitive deficits in schizophrenia.

Perception of biological motion is an important social cognitive ability that has been mapped to specialized brain regions. Perceptual deficits and neural differences during biological motion perception have previously been associated with autism, a disorder classified by social and communication difficulties and repetitive and restricted interests and behaviors. However, the traits associated with autism are not limited to diagnostic categories, but are normally distributed within the general population and show the same patterns of heritability across the continuum. In the current study, we investigate whether self-reported autistic-like traits in healthy adults are associated with variable neural response during passive viewing of biological motion displays. Results show that more autistic-like traits, particularly those associated with the communication domain, are associated with increased neural response in key regions involved in social cognitive processes, including prefrontal and left temporal cortices. This distinct pattern of activation might reflect differential neurodevelopmental processes for individuals with varying autistic-like traits, and highlights the importance of considering the full trait continuum in future work.

In a cartoon, we often receive an animacy impression from a dynamic nonanimate object, such as a sponge or a flour sack, which does not have an animal-like shape. We hypothesize that the animacy impression of a nonanimal object could stem from dynamic patterns that are possibly fundamental for biological motion perception. Here we show that observers recognize the animacy of human jump actions from the combination of deformation and translation. We extracted vertical motion vectors from the uppermost and lowermost points in point-light jumper stimuli and assigned the vectors to a uniform rectangle. The participants’ task was to rate the animacy and jump impressions for the rectangle. Results showed that both animacy and jump impressions for the rectangle movements were comparable to those for the original point-light movements. The impressions decreased for stimuli having a deformation or translation component alone, which was extracted from the original motion vectors. By mathematically simulating deformation and translation in a human jump, we also found that the temporal relation between deformation and translation plays a critical role in the determination of jump impressions but only has a moderate effect for animacy impressions. On the basis of the results, we discuss how cartoon techniques take advantage of the properties of biological motion perception.

When viewing ambiguous stimuli, people tend to perceive some interpretations more frequently than others. Such perceptual biases impose various types of constraints on visual perception, and accordingly, have been assumed to serve distinct adaptive functions. Here we demonstrated the interaction of two functionally distinct biases in bistable biological motion perception, one regulating perception based on the statistics of the environment – the viewing-from-above (VFA) bias, and the other with the potential to reduce costly errors resulting from perceptual inference – the facing-the-viewer (FTV) bias. When compatible, the two biases reinforced each other to enhance the bias strength and induced less perceptual reversals relative to when they were in conflict. Whereas in the conflicting condition, the biases competed with each other, with the dominant percept varying with visual cues that modulate the two biases separately in opposite directions. Crucially, the way the two biases interact does not depend on the dominant bias at the individual level, and cannot be accounted for by a single bias alone. These findings provide compelling evidence that humans robustly integrate biases with different adaptive functions in visual perception. It may be evolutionarily advantageous to dynamically reweight diverse biases in the sensory context to resolve perceptual ambiguity.

People are very adept at perceiving biological motion (e.g., Johansson, 1973). This ability has been an essential life skill to members of this social species. The human niche during the ice age was socially coordinated hunting for big game. Being able to judge the location targeted by the throw of a conspecific would be a valuable perceptual ability that we now study to investigate 2 competing theories of biological motion perception: Common Coding (CC; Prinz, 1997) and Kinematic Specification of Dynamics (KSD; Runeson & Frykholm, 1983). The 2 theories diverge in attributing perceptual ability to either motor or visual experience, respectively. To test predictions of the CC theory, we performed 3 experiments to manipulate observers' specific motor experience while they judged the targeted location of throwing by watching point-light displays. In Experiment 1, we tested whether the identity of the thrower in the display mattered. In Experiment 2, we tested whether the motor expertise of the observer matte...

The ability to recognize the movements of other biological entities, such as whether a person is walking toward you, is essential for survival and social interaction. Previous studies have shown that the visual system is particularly sensitive to approaching biological motion. In this study, we examined whether the gender of walkers and observers influenced the walking direction discrimination of approaching point-light walkers in fine granularity. The observers were presented a walker who walked in different directions and were asked to quickly judge the walking direction (left or right). The results showed that the observers demonstrated worse direction discrimination when the walker was depicted as male than when the walker was depicted as female, probably because the observers tended to perceive the male walkers as walking straight ahead. Intriguingly, male observers performed better than female observers at judging the walking directions of female walkers but not those of male walkers, a result indicating perceptual advantage with evolutionary significance. These findings provide strong evidence that the gender of walkers and observers modulates biological motion perception and that an adaptive perceptual mechanism exists in the visual system to facilitate the survival of social organisms.

Current models suggest that human form and motion information are initially processed through separate pathways, then integrated in action perception. Testing such a sequential model requires techniques with high temporal resolution. Prior work demonstrated sensitivity of a posterior temporal event-related potential (ERP) effect — the N2 — to biological motion, but did not test whether the N2 indexes biological motion perception specifically, or human form/action perception more generally. We recorded ERPs while participants viewed stimuli across 3 blocks: (1) static (non-moving) point-light displays of humans performing actions; (2) static stick figures with clear forms; and (3) point-light biological motion. A similar sequence of ERP components was elicited by human forms in all blocks (stationary and moving), and reliably discriminated between human and scrambled forms. The N2 showed similar scalp distribution and sensitivity to stimulus manipulations for both stick figures and biological motion, suggesting that it indexes integration of form and motion information, rather than biological motion perception exclusively — and that form and motion information are therefore integrated by approximately 200 ms. We identified a component subsequent to the N2, which we label the medial parietal positivity/ventral–anterior negativity (MPP/VAN), that was also sensitive to both human form and motion information. We propose that the MPP/VAN reflects higher-order human action recognition that occurs subsequent to the integration of form and motion information reflected by the N2.

The present study examines whether race-specific features affect biological motion perception. Activation of the neural action observation and imitation network was measured using functional MRI. During scanning, individuals were asked to imitate and observe basic hand movements of own-race and other-race actors. Results indicate that three key areas often associated with action observation and imitation, the inferior parietal lobule, superior parietal lobule, and superior temporal sulcus, were more active when participants imitated and observed hand movements of own-race relative to other-race actors. These findings indicate that several regions associated with the neural imitation/observation network are sensitive to race-related features.

The ability to detect potentially interacting agents in the surrounding environment is acknowledged to be one of the first perceptual tasks developed by humans, supported by the ability to recognise biological motion. The precocity of this ability suggests that it might be based on rather simple motion properties, and it can be interpreted as an atomic building block of more complex perception tasks typical of interacting scenarios, as the understanding of non-verbal communication cues based on motion or the anticipation of others’ action goals. In this article, we propose a novel perspective for video analysis, bridging cognitive science and machine vision, which leverages the use of computational models of the perceptual primitives that are at the basis of biological motion perception in humans. Our work offers different contributions. In a first part, we propose an empirical formulation for the Two-Thirds Power Law , a well-known invariant law of human movement, and thoroughly discuss its readability in experimental settings of increasing complexity. In particular, we consider unconstrained video analysis scenarios, where, to the best of our knowledge, the invariant law has not found application so far. The achievements of this analysis pave the way for the second part of the work, in which we propose and evaluate a general representation scheme for biological motion characterisation to discriminate biological movements with respect to non-biological dynamic events in video sequences. The method is proposed as the first layer of a more complex architecture for behaviour analysis and human-machine interaction, providing in particular a new way to approach the problem of human action understanding.

The ability to infer deceptive intents from nonverbal behavior is critical for social interactions. By combining single-pulse and repetitive transcranial magnetic stimulation (TMS) in healthy humans, we provide both correlational and causative evidence that action simulation is actively involved in the ability to recognize deceptive body movements. We recorded motor-evoked potentials during a faked-action discrimination (FAD) task: participants watched videos of actors lifting a cube and judged whether the actors were trying to deceive them concerning the real weight of the cube. Seeing faked actions facilitated the observers' motor system more than truthful actions in a body-part-specific manner, suggesting that motor resonance was sensitive to deceptive movements. Furthermore, we found that TMS virtual lesion to the anterior node of the action observation network, namely the left inferior frontal cortex (IFC), reduced perceptual sensitivity in the FAD task. In contrast, no change in FAD task performance was found after virtual lesions to the left temporoparietal junction (control site). Moreover, virtual lesion to the IFC failed to affect performance in a difficulty-matched spatial-control task that did not require processing of spatiotemporal (acceleration) and configurational (limb displacement) features of seen actions, which are critical to detecting deceptive intent in the actions of others. These findings indicate that the human IFC is critical for recognizing deceptive body movements and suggest that FAD relies on the simulation of subtle changes in action kinematics within the motor system.

Biological motion perception (BMP) is a vivid perception of the moving form of a human figure from a few light points on the joints of the body. BMP is commonplace and important, but there is great inter-individual variability in this ability. This study used multiple regression model analysis to explore the association between the BMP performance and intrinsic brain activity, in order to investigate the neural substrates underlying inter-individual variability of BMP performance. The resting-state functional magnetic resonance imaging (rs-fMRI) and BMP performance data were collected from 24 healthy participants. For each participant, the intrinsic brain network was constructed, and a graph-based network efficiency metric was measured. Then, a multiple linear regression model was used to explore the association between network regional efficiency and BMP performance. We found that the local and global network efficiency of many regions was significantly correlated with the BMP performance. Further analysis showed that the local efficiency rather than global efficiency could be used to explain most of the BMP inter-individual variability, and the regions involved were predominately located at the Default Mode Network (DMN). Additionally, the discrimination analysis showed that the local efficiency over regions including thalamus could be used to classify BMP performance across participants. Notably, the association pattern between the network nodal efficiency and the BMP was different from the association pattern that of the static directional/gender information perception. Overall, these findings showed that intrinsic brain network efficiency may be considered as a neural factor that explains BMP inter-individual variability. Keywords: Biological motion; Resting-state network; Network efficiency; Multiple linear regression model; Brain-behavior analysis

International audience; The human brain contains specialized circuits for observing and understanding actions. Previous studies have not distinguished whether this "mirror system" uses specialized motor representations or general processes of visual inference and knowledge to understand observed actions. We report the first neuroimaging study to distinguish between these alternatives. Purely motoric influences on perception have been shown behaviorally, but their neural bases are unknown. We used fMRI to reveal the neural bases of motor influences on action observation. We controlled for visual and knowledge effects by studying expert dancers. Some ballet moves are performed by only one gender. However, male and female dancers train together and have equal visual familiarity with all moves. Male and female dancers viewed videos of gender-specific male and female ballet moves. We found greater premotor, parietal, and cerebellar activity when dancers viewed moves from their own motor repertoire, compared to opposite-gender moves that they frequently saw but did not perform. Our results show that mirror circuits have a purely motor response over and above visual representations of action. We understand actions not only by visual recognition, but also motorically. In addition, we confirm that the cerebellum is part of the action observation network.

The recognition of the movements and ac­tions of others is of great importance for social interaction. The visual motion pat­tern projected on the retina when watching somebody else act is called biological mo­tion. Because of the many degrees of free­dom of the body, biological motion is a rel­atively complicated motion pattern, much more variable, for example, than optic flow or object motion. The regularities of biolog­ical motion are contained in its relationship to the body, i.e. in the constraints imposed by the articulation of the limbs on the move­ment of the body parts. The neural mecha­nisms of biological motion perception, there­fore, take body form information into ac­count. I describe a model of biological mo­tion perception that starts from a representa­tion of body form and posture and retrieves biological motion as the transformation of the body posture over time. Essentially, this proposes a ventral pathway to motion per­ception that is distinct from the other motion pathways in the dorsal stream, and special­ized for body motion.

In our everyday life, processing complex dynamic scenes such as crowds and traffic is of critical importance. Further, it is well documented that there is an age-related decline in complex perceptual-cognitive processing, which can be reversed with training. It has been suggested that a specific dynamic scene perceptual-cognitive training procedure [the three-dimensional multiple object tracking speed task (3D-MOT)] helps observers manage socially relevant stimuli such as human body movements as seen in crowds or during sports activities. Here, we test this assertion by assessing whether training older observers on 3D-MOT can improve biological motion (BM) perception. Research has shown that healthy older adults require more distance in virtual space between themselves and a point-light walker to integrate BM information than younger adults. Their performances decreased markedly at a distance as far away as 4 m (critical for collision avoidance), whereas performance in young adults remained constant up to 1 m. We trained observers between 64 and 73 years of age on the 3D-MOT speed task and looked at BM perception at 4 and 16 m distances in virtual space. We also had a control group trained on a visual task and a third group without training. The perceptual-cognitive training eliminated the difference in BM perception between 4 and 16 m after only a few weeks, whereas the two control groups showed no transfer. This demonstrates that 3D-MOT training could be a good generic process for helping certain observers deal with socially relevant dynamic scenes.

Typical adult observers demonstrate enhanced behavioral sensitivity to human movement compared to animal movement. Yet, the neural underpinnings of this effect are unknown. We examined the tuning of brain mechanisms for the perception of biological motion to the social relevance of this category of motion by comparing neural response to human and non-human biological motion. In particular, we tested the hypothesis that the response of the right posterior superior temporal sulcus (pSTS) varies according to the social relevance of the motion, responding most strongly to those biological motions with the greatest social relevance (human > dog). During a functional magnetic resonance imaging (fMRI) session, typical adults viewed veridical point-light displays of human, dog, and tractor motions created from motion capture data. A conjunction analysis identified regions of significant activation during biological motion perception relative to object motion. Within each of these regions, only one brain area, the right pSTS, revealed an enhanced response to human motion relative to dog motion. This finding demonstrates that the pSTS response is sensitive to the social relevance of a biological motion stimulus.

The presence of information in a visual display does not guarantee its use by the visual system. Studies of inversion effects in both face recognition and biological-motion perception have shown that the same information may be used by observers when it is presented in an upright display but not used when the display is inverted. In our study, we tested the inversion effect in scrambled biological-motion displays to investigate mechanisms that validate information contained in the local motion of a point-light walker. Using novel biological-motion stimuli that contained no configural cues to the direction in which a walker was facing, we found that manipulating the relative vertical location of the walker’s feet significantly affected observers’ performance on a direction-discrimination task. Our data demonstrate that, by themselves, local cues can almost unambiguously indicate the facing direction of the agent in biological-motion stimuli. Additionally, we document a noteworthy interaction between local and global information and offer a new explanation for the effect of local inversion in biological-motion perception.

We investigate the occurrence of perspective reversals for a depth-ambiguous point-light figure. In addition, we exploit the phenomenon of reversibility to search for stimulus features relevant in the process of depth assignment. Experiment 1 shows that perceptual switches indeed occur during prolonged viewing, although the switches occur infrequently. The reversibility is confirmed in Experiment 2, in which the perceptual ambiguity of the point-light action is manipulated as well as observers’ intention to perceive a particular alternative. In addition, the pattern of eye movements reveals local stimulus features specifically associated with the perception of the different alternatives. In Experiment 3, the importance of these features as determining factors of the initial interpretation is investigated by manipulating the location of the first fixation on the stimulus. Implications for a better understanding of biological motion perception are discussed.