Point-light Walkers Research Articles

Inbound friend or foe: how motion bistability is resolved under threat

Anxiety prepares us to deal with unpredictable threats, such as the approaching of an unknown person. Studies have shown our innate tendency to see approaching motion in ambiguous walkers in what was termed facing-the-viewer (FTV) bias. Here we investigated if anxiety states further contributed to this bias, hypothesizing that such states would increase overall FTV biases. Throughout three Experiments, we asked participants to judge the motion direction of ambiguous point-light walkers and measured their respective FTV biases under safe and anxiety-related conditions induced via imagery (Experiment 1), screaming sounds (Experiment 2), and threat of shock (Experiment 3). Across all experiments, we showed that anxiety does not affect our tendency to perceive an approaching behavior in ambiguous walkers. Based on our findings, and the discrepancies found in the literature, we emphasize the need for future studies to paint a clearer picture on the nature and aspects capable of affecting this bias.

Effect of spatial context on perceived walking direction.

Contextual modulation occurs for many aspects of high-level vision but is relatively unexplored for the perception of walking direction. In a recent study, we observed an effect of the temporal context on perceived walking direction. Here, we examined the spatial contextual modulation by measuring the perceived direction of a target point-light walker in the presence of two flanker walkers, one on each side. Experiment 1 followed a within-subjects design. Participants (n = 30) completed a spatial context task by judging the walking direction of the target in 13 different conditions: a walker alone in the center or with two flanking walkers either intact or scrambled at a flanker deviation of ±15°, ±30°, or ±45°. For comparison, participants completed an adaptation task where they reported the walking direction of a target after adaptation to ±30° walking direction. We found the expected repulsive effects in the adaptation task but attractive effects in the spatial context task. In Experiment 2 (n = 40), we measured the tuning of spatial contextual modulation across a wide range of flanker deviation magnitudes ranging from 15° to 165° in 15° intervals. Our results showed significant attractive effects across a wide range of flanker walking directions with the peak effect at around 30°. The assimilative versus repulsive effects of spatial contextual modulation and temporal adaptation suggest dissociable neural mechanisms, but they may operate on the same population of sensory channels coding for walking direction, as evidenced by similarity in the peak tuning across the walking direction of the inducers.

Atypical Time to Contact Estimation in Young Adults with Autism Spectrum Disorder.

Individuals with Autism Spectrum Disorder (ASD) present atypical sensory processing in the perception of moving stimuli and biological motion. The present study aims to explore the performance of young adults with ASD in a time to contact (TTC) estimation task involving social and non-social stimuli. TTC estimation involves extrapolating the trajectory of a moving target concealed by an occluder, based on the visible portion of its path, to predict the target's arrival time at a specific position. Sixteen participants with a diagnosis of level-1 ASD (M = 19.2 years, SE = 0.54 years; 3F, 13M) and sixteen participants with TD (M = 22.3 years, SE = 0.44 years; 3F, 13M) took part in the study and underwent a TTC estimation task. The task presented two object types (a car and a point-light walker), different object speeds, occluder lengths, motion directions and motion congruency. For the car object, a larger overestimation of TTC emerged for ASDs than for TDs, whereas no difference between ASDs and TDs emerged for the point-light walker. ASDs exhibited a larger TTC overestimation for the car object than for the point-light walker, whereas no difference between object types emerged for TDs. Our results indicated an atypical TTC estimation process in young adults with ASD. Given its importance in daily life, future studies should further explore this skill. Significant effects that emerged from the analysis are discussed.

Walk This Way: How Weight Distorts Gender Identification of Point-Light Walkers

Extensive research in social perception and biological motion has converged on the finding that humans are particularly accurate in identifying gender from the gait of minimal visual conspecific stimuli (e.g., point-light walkers). Despite the preponderance of evidence in favor of this ability, we return to the original paradigm and vary a single parameter—weight. Across nine pre-registered studies, participants (N = 3,196) were assigned to view the gait of point-light walkers based on actual human motion patterns. We find a decline in the accuracy of identifying the gender of female point-light walkers as their weight increases. However, as the weight of female walkers decreases, gender identification accuracy is recovered. These findings carry implications for the gendered nature of weight bias and the role of weight in human perception.

Biological motion perception in the theoretical framework of perceptual decision-making: An event-related potential study

Biological motion perception plays a critical role in various decisions in daily life. Failure to decide accordingly in such a perceptual task could have life-threatening consequences. Neurophysiology and computational modeling studies suggest two processes mediating perceptual decision-making. One of these signals is associated with the accumulation of sensory evidence and the other with response selection. Recent EEG studies with humans have introduced an event-related potential called Centroparietal Positive Potential (CPP) as a neural marker aligned with the sensory evidence accumulation while effectively distinguishing it from motor-related lateralized readiness potential (LRP). The present study aims to investigate the neural mechanisms of biological motion perception in the framework of perceptual decision-making, which has been overlooked before. More specifically, we examine whether CPP would track the coherence of the biological motion stimuli and could be distinguished from the LRP signal. We recorded EEG from human participants while they performed a direction discrimination task of a point-light walker stimulus embedded in various levels of noise. Our behavioral findings revealed shorter reaction times and reduced miss rates as the coherence of the stimuli increased. In addition, CPP tracked the coherence of the biological motion stimuli with a tendency to reach a common level during the response, albeit with a later onset than the previously reported results in random-dot motion paradigms. Furthermore, CPP was distinguished from the LRP signal based on its temporal profile. Overall, our results suggest that the mechanisms underlying perceptual decision-making generalize to more complex and socially significant stimuli like biological motion.

Limb articulation of biological motion can induce illusory motion perception during self-motion.

When one walks toward a crowd of pedestrians, dealing with their biological motion while controlling one's own self-motion is a difficult perceptual task. Limb articulation of a walker is naturally coupled to the walker's translation through the scene and allows the separation of optic flow generated by self-motion from the biological motion of other pedestrians. Recent research has shown that if limb articulation and translation mismatch, such as for walking in place, self-motion perception becomes biased. This bias may reflect an illusory motion attributed to the pedestrian crowd from the articulation of their limbs. To investigate this hypothesis, we presented observers with a simulation of forward self-motion toward a laterally moving crowd of point-light walkers and asked them to report the perceived lateral speed of the crowd. To investigate the dependence of the crowd speed percept on biological motion, we also included conditions in which the points of the walker were spatially scrambled to destroy body form and limb articulation. We observed illusory crowd speed percepts that were related to the articulation rate of the biological motion. Scrambled walkers also produced illusory motion but it was not related to articulation rate. We conclude that limb articulation induces percepts of crowd motion that can be used for interpreting self-motion toward crowds.

Think positive! Resolving human motion ambiguity in the presence of disease threat

ABSTRACT Recently, approach-avoidance tendencies and visual perception biases have been increasingly studied using bistable point-light walkers (PLWs). Prior studies have found a facing-the-viewer bias when one is primed with general threat stimuli (e.g. angry faces), explained by the “error management theory”, as failing to detect a threat as approaching is riskier than the opposite. Importantly, no study has explored how disease threat – linked to the behavioural immune system – might affect this bias. This study aimed to explore whether disease-signalling cues can alter how we perceive the motion direction of ambiguous PLWs. Throughout 3 experiments, participants indicated the motion direction of a bistable PLW previously primed with a control or disease-signalling stimuli – that is, face with a surgical mask (Experiment 1), sickness sound (Experiment 2), or face with a disease cue (Experiment 3). Results showed that sickness cues do not significantly modulate the perception of approach-avoidance behaviours. However, a pattern emerged in Experiments 2 and 3, suggesting that sickness stimuli led to more facing away percepts. Unlike other types of threat, this implies that disease-related threat stimuli might trigger a distinct perceptual bias, indicating a preference to avoid a possible infection source. Nonetheless, this finding warrants future investigations.

Aesthetic preferences for prototypical movements in human actions

A commonplace sight is seeing other people walk. Our visual system specializes in processing such actions. Notably, we are not only quick to recognize actions, but also quick to judge how elegantly (or not) people walk. What movements appear appealing, and why do we have such aesthetic experiences? Do aesthetic preferences for body movements arise simply from perceiving others’ positive emotions? To answer these questions, we showed observers different point-light walkers who expressed neutral, happy, angry, or sad emotions through their movements and measured the observers’ impressions of aesthetic appeal, emotion positivity, and naturalness of these movements. Three experiments were conducted. People showed consensus in aesthetic impressions even after controlling for emotion positivity, finding prototypical walks more aesthetically pleasing than atypical walks. This aesthetic prototype effect could be accounted for by a computational model in which walking actions are treated as a single category (as opposed to multiple emotion categories). The aesthetic impressions were affected both directly by the objective prototypicality of the movements, and indirectly through the mediation of perceived naturalness. These findings extend the boundary of category learning, and hint at possible functions for action aesthetics.

Post-migration living difficulties and poor mental health associated with increased interpretation bias for threat.

Previous research has found associations between mental health difficulties and interpretation biases, including heightened interpretation of threat from neutral or ambiguous stimuli. Building on this research, we explored associations between interpretation biases (positive and negative) and three constructs that have been linked to migrant experience: mental health symptoms (Global Severity Index [GSI]), Post-Migration Living Difficulties (PMLD), and Perceived Ethnic Discrimination Questionnaire (PEDQ). Two hundred thirty students who identified as first- (n = 94) or second-generation ethnic minority migrants (n = 68), and first-generation White migrants (n = 68) completed measures of GSI, PEDQ, and PMLD. They also performed an interpretation bias task using Point Light Walkers (PLW), dynamic stimuli with reduced visual input that are easily perceived as humans performing an action. Five categories of PLW were used: four that clearly depicted human forms undertaking positive, neutral, negative, or ambiguous actions, and a fifth that involved scrambled animations with no clear action or form. Participants were asked to imagine their interaction with the stimuli and rate their friendliness (positive interpretation bias) and aggressiveness (interpretation bias for threat). We found that the three groups differed on PEDQ and PMLD, with no significant differences in GSI, and the three measured were positively correlated. Poorer mental health and increased PMLD were associated with a heightened interpretation for threat of scrambled animations only. These findings have implications for understanding of the role of threat biases in mental health and the migrant experience.

Untangling the confounded relationship between perceiver sex and walker sex on the bistable perception of emotion displaying point-light walkers

Point-light displays of walking gait carries an assortment of information about the individual and this information is often perceivable to others. Some of these bits of information are entangled, with some facilitating and others inhibiting each other. We sought to untangle the perception of basic threat emotions from sex of the walker and the perceiver, as expressed through the bistable perception of anticipated approaching or withdrawing point-light walkers. Stationary point-light walkers displaying anger or fear were shown to 164 psychology student perceivers, who were told that the walkers would be either walking towards them or walking away from them. Perceivers were asked to identify the displayed emotion for each walker stimulus. Expected walker direction showed no influence on the perception of either emotion, across either sex of the perceivers or walkers. Anger was identified better on male walkers and fear was identified better on female walkers. Female perceivers were able to identify both emotions better than male perceivers, but only when displayed by female walkers. The sex of both the perceiver and the walker interact to influence the perception of basic threat emotions displayed through point-light walking gait, with implications for the development of inter-sexual and intra-sexual group cohesion programs.

Tuning in to a hip-hop beat: Pursuit eye movements reveal processing of biological motion

Smooth pursuit eye movements are mainly driven by motion signals to achieve their goal of reducing retinal motion blur. However, they can also show anticipation of predictable movement patterns. Oculomotor predictions may rely on an internal model of the target kinematics. Most investigations on the nature of those predictions have concentrated on simple stimuli, such as a decontextualized dot. However, biological motion is one of the most important visual stimuli in regulating human interaction and its perception involves integration of form and motion across time and space. Therefore, we asked whether there is a specific contribution of an internal model of biological motion in driving pursuit eye movements. Unlike previous contributions, we exploited the cyclical nature of walking to measure eye movement's ability to track the velocity oscillations of the hip of point-light walkers. We quantified the quality of tracking by cross-correlating pursuit and hip velocity oscillations. We found a robust correlation between signals, even along the horizontal dimension, where changes in velocity during the stepping cycle are very subtle. The inversion of the walker and the presentation of the hip-dot without context incurred the same additional phase lag along the horizontal dimension. These findings support the view that information beyond the hip-dot contributes to the prediction of hip kinematics that controls pursuit. We also found a smaller phase lag in inverted walkers for pursuit along the vertical dimension compared to upright walkers, indicating that inversion does not simply reduce prediction. We suggest that pursuit eye movements reflect the visual processing of biological motion and as such could provide an implicit measure of higher-level visual function.

Flying enhances viewing from above bias on ambiguous visual stimuli.

The human spatial orientation system is well designed on the ground but is imperfect in the aeronautical three-dimensional (3D) environment. However, human perception systems perform Bayesian statistics based on encountered environments and form shortcuts to improve perceptual efficiency. It is unknown whether our perception of spatial orientation is modified by flying experience and forms perceptual biases. The current study tested pilot perceptual biases on ambiguous visual stimuli, the bistable point-light walkers, and found that flying experiences increased the pilot's tendency to perceive himself as higher than the target and the target as farther away from them. Such perceptual effects due to flight are likely to be attributed to experience of variable vestibular state in a higher position in 3D space, rather than the experience of a higher viewpoint. Our findings suggest that flying experience will modifies our visual perceptual biases, and that more attention should be paid to the enhanced viewing from above bias when flying to avoid overestimating altitude or viewing angle when the visual conditions are ambiguous.

Visual perception of travel distance for self-motion through crowds.

Humans can use visual motion to estimate the distance they have traveled. In static environments, optic flow generated by self-motion provides a pattern of expanding motion that is used for the estimation of travel distance. When the environment is populated by other people, their biological motion destroys the one-to-on correspondence between optic flow and travel distance. We investigated how observers estimate travel distance in a crowded environment. In three conditions, we simulated self-motion through a crowd of standing, approaching, or leading point-light walkers. For a standing crowd, optic flow is a veridical signal for distance perception. For an approaching crowd, the visual motion is the sum of the self-motion-induced optic flow and the optic flow produced by the approaching walkers. If only optic flow were to be used, travel distance estimates would be too high because of the approaching direction of the crowd toward the observer. If, on the other hand, cues from biological motion could be used to estimate the speed of the crowd, then the excessive optic from the approaching crowd flow might be compensated. In the leading crowd condition, in which walkers of the crowd keep their distance from the observer as they walk along with the observer, no optic flow is produced. In this condition, travel distance estimation would have to rely solely on biological motion information. We found that distance estimation was quite similar across these three conditions. This suggests that biological motion information can be used (a) to compensate for excessive optic flow in the approaching crowd condition and (b) to generate distance information in the leading crowd condition.

Recognition of humans from biological motion in infants.

Infant studies have suggested that the detection of biological motion (BM) might be an innate capacity, based on newborns' spontaneous preference for BM. However, it is unclear if, like adults, infants recognize humans from BM and are able to build the representation of bodies and faces. To address this issue, we tested whether exposure to BM influences subsequent face recognition in 3- to 8-month-old infants. After familiarization with a point-light walker (PLW) of either a female or a male, the infant's preference for female and male faces was measured. If infants can build the representation of not only the body but also the face from PLWs, the familiarization effect of gender induced by the PLW might be generalized to faces. We found that infants at 7 to 8 months looked for longer at the face whose gender was opposite to that of the PLW, whereas 3- to 4- and 5- to 6-month-old infants did not. These results suggest that infants can access the representation of humans from BM and extract gender, which is shared across bodies and faces, from at least 7 to 8 months of age.

Cortical encoding of rhythmic kinematic structures in biological motion

Biological motion (BM) perception is of great survival value to human beings. The critical characteristics of BM information lie in kinematic cues containing rhythmic structures. However, how rhythmic kinematic structures of BM are dynamically represented in the brain and contribute to visual BM processing remains largely unknown. Here, we probed this issue in three experiments using electroencephalogram (EEG). We found that neural oscillations of observers entrained to the hierarchical kinematic structures of the BM sequences (i.e., step-cycle and gait-cycle for point-light walkers). Notably, only the cortical tracking of the higher-level rhythmic structure (i.e., gait-cycle) exhibited a BM processing specificity, manifested by enhanced neural responses to upright over inverted BM stimuli. This effect could be extended to different motion types and tasks, with its strength positively correlated with the perceptual sensitivity to BM stimuli at the right temporal brain region dedicated to visual BM processing. Modeling results further suggest that the neural encoding of spatiotemporally integrative kinematic cues, in particular the opponent motions of bilateral limbs, drives the selective cortical tracking of BM information. These findings underscore the existence of a cortical mechanism that encodes periodic kinematic features of body movements, which underlies the dynamic construction of visual BM perception.

The gender-based facing bias in 3-D biological motion perception.

In biological motion perception, movements of several point lights can evoke a vivid impression of living animals, including humans. Recent studies have reported that male point-light walkers tend to be perceived as facing toward the viewer more than female walkers, and have hypothesized that the gender-based facing bias arises from motion signals. The purpose of this study was to test this hypothesis under experimental conditions where binocular disparity was added to biological motion stimuli. In the two experiments reported here, participants were presented with disparity-defined female and male point-light figures facing toward or away from the viewer. In Experiment 1, we measured "facing-the-viewer" responses in upright and inverted walker configurations. It was found that the facing bias was greater for the male walker than for the female walker in most disparity magnitudes, regardless of walker inversion. In Experiment 2, the walker stimuli were replaced by static snapshots of the walkers. The results showed that the facing bias did not differ between the female and male static figures. These results suggest that motion signals play an important role in producing the gender-based facing bias, even when binocular disparity is added to biological motion stimuli.

Important feature identification for perceptual sex of point-light walkers using supervised machine learning.

The present study aimed to elucidate the dynamic features that are highly predictive in the biological and perceptual sex classification of point-light walkers (PLWs) and how these features behave in sex classification using supervised machine learning. Fifteen observers judged the sex of 21 PLWs from a side view. A fast Fourier transform was applied to retrieve the spectral components from the multiphasic hip and shoulder movements. An exhaustive search identified the most important features for biological and perceptual sex classifications. An individual conditional expectation (ICE) with a support vector machine (SVM) model was used to interpret the behavior of each important feature. The observers judged the biological sex from side-view PLWs with an accuracy of 62.9% for 10 male PLWs and of 57.0% for 11 female PLWs. The SVM model for biological sex prediction demonstrated that the third harmonic of hip motion played a dominant role in achieving a high predictive accuracy of 90.5% with few feature interactions. In the model of perceptual sex prediction, however, an accurate prediction of 85.7% was achieved using five spectral components of hip and shoulder motions, where the ICE plots of the features followed heterogeneous courses, suggesting feature interactions. The machine learning model suggests that biological sex classification depends mainly on local cues of the PLW. However, the high-performance model of perceptual sex classification involves interactions of various frequency components of hip and shoulder motions, suggesting more complex processes in sex perception.

Looking more masculine among females: Spatial context modulates gender perception of face and biological motion.

Perception of visual information highly depends on spatial context. For instance, perception of a low-level visual feature, such as orientation, can be shifted away from its surrounding context, exhibiting a simultaneous contrast effect. Although previous studies have demonstrated the adaptation aftereffect of gender, a high-level visual feature, it remains largely unknown whether gender perception can also be shaped by a simultaneously presented context. In the present study, we found that the gender perception of a central face or a point-light walker was repelled away from the gender of its surrounding faces or walkers. A norm-based opponent model of lateral inhibition, which accounts for the adaptation aftereffect of high-level features, can also excellently fit the simultaneous contrast effect. But different from the reported contextual effect of low-level features, the simultaneous contrast effect of gender cannot be observed when the centre and the surrounding stimuli are from different categories, or when the surrounding stimuli are suppressed from awareness. These findings on one hand reveal a resemblance between the simultaneous contrast effect and the adaptation aftereffect of high-level features, on the other hand highlight different biological mechanisms underlying the contextual effects of low- and high-level visual features.

Using EEG movement tagging to isolate brain responses coupled to biological movements

Detecting biological motion is essential for adaptive social behavior. Previous research has revealed the brain processes underlying this ability. However, brain activity during biological motion perception captures a multitude of processes. As a result, it is often unclear which processes reflect movement processing and which processes reflect secondary processes that build on movement processing. To address this issue, we developed a new approach to measure brain responses directly coupled to observed movements. Specifically, we showed 30 male and female adults a point-light walker moving at a pace of 2.4 Hz and used EEG frequency tagging to measure the brain response coupled to that pace (‘movement tagging’). The results revealed a reliable response at the walking frequency that was reduced by two manipulations known to disrupt biological motion perception: phase scrambling and inversion. Interestingly, we also identified a brain response at half the walking frequency (i.e., 1.2 Hz), corresponding to the rate at which the individual dots completed a cycle. In contrast to the 2.4 Hz response, the response at 1.2 Hz was increased for scrambled (vs. unscrambled) walkers. These results show that frequency tagging can be used to capture the visual processing of biological movements and can dissociate between global (2.4 Hz) and local (1.2 Hz) processes involved in biological motion perception, at different frequencies of the brain signal.

Following Other People’s Footsteps: A Contextual-Attraction Effect Induced by Biological Motion

Our visual system is bombarded with numerous social interactions that form intangible social bonds among people, as exemplified by synchronized walking in crowds. Here, we investigated whether these perceived social bonds implicitly intrude on visual perception and induce a contextual effect. Using multiple point-light walkers and a classical contextual paradigm, we tested 72 college-age adults across six experiments and found that the perceived direction of the central walker was attracted toward the direction of the surrounding walkers. The observed contextual-attraction effect occurred even when the surrounding walkers differed from the central walker in gender and walking speed but disappeared when they were asynchronously presented or replaced by inanimate motion. Strikingly, this contextual-attraction effect partially persisted in the context of local motion rather than static figures. These findings, in contrast to the typical contextual-repulsion effect, lend support for the distinctiveness of perceived social bonds on contextual modulation and suggest a specialized contextual mechanism tuned to social factors.