Seeing life in motion: Animacy perception across species revealed by adaptation.

Biological motion displays depict a moving human figure by means of just a few isolated points of light attached to the major joints of the body. Naive observers readily interpret the moving pattern of dots as representing a human figure, despite the complete absence of form cues. This paper reports a series of experiments which investigated the visual processes underlying the phenomenon. Results suggest that (i) the effect relies upon responses in low-level motion-detecting processes, which operate over short temporal and spatial intervals and respond to local modulations in image intensity; and (ii) the effect does not involve hierarchical visual analysis of motion components, nor does it require the presence of dots which move in rigid relation to each other. Instead, movements of the extremities are crucial. Data are inconsistent with current theoretical treatments.

Studies of human locomotion have found that male and female walkers differ in terms of lateral body sway, with males tending to swing their shoulders from side to side more than their hips, and females tending to swing their hips more than their shoulders. Experiments reported here demonstrate that naive viewers can identify the gender of the figure in a biological motion display very reliably when the display contains gender-specific lateral body sway. Sensitivity to gender is high even for displays containing only a fraction of a step cycle. This dynamic cue dominates structural cues based on torso shape ('centre-of-moment') when the cues are set in opposition. It is mediated by gender-specific differences in the velocity of shoulder and hip dots, not by positional differences in shoulder and hip dots during the step cycle.

Biological motion (BM), the movement generated by living entities, transmits signals of life and conveys vital cues for animacy perception. In this review, we synthesize empirical findings from human and non-human animal studies to reveal how BM enjoys a unique position in visual perception as an animate motion and how it elicits animacy perception. Compared to non-biological and inanimate motions, BM engages specialized perceptual processing mechanisms and a dedicated cortical–subcortical network. Local motion cues, especially the foot movements of terrestrial animals, are pivotal in driving such specificity, and emerging evidence supports the existence of an innate, evolutionarily conserved “Life Detector” or “Step Detector” tuned to such information in the human and other vertebrate brains. The direct perception of animacy from BM relies on the processing of low-level kinematic features and mid-level motion features embedded in both intrinsic joint movements and extrinsic body motion. While ecological constraints and implied internal energy sources may serve as generic factors affecting animacy perception from visual motion, how precise BM features (both in intrinsic and extrinsic movements) combine to influence animacy percepts and the neural implementation remain largely unexplored. Addressing these gaps will help establish a framework for understanding BM through the lens of animate motion processing. This approach will offer deeper insights into how the life detection system hardwired in the vertebrate brain distinguishes animate from inanimate motion, further uncovering its broader cognitive and evolutionary implications.

Patterns of fMRI Activity Dissociate Overlapping Functional Brain Areas that Respond to Biological Motion

Auditory motion affects visual biological motion processing

Most vertebrates, including humans, are highly adept at detecting and encoding biological motion, even when it is portrayed by just a few point lights attached to the head and major joints. However, the function of subcortical regions in biological motion perception has been scarcely explored. Here, we investigate the role of the superior colliculus in local biological motion processing. Using high-field (3 T) and ultra-high-field (7 T) functional magnetic resonance imaging, we record the neural responses of the superior colliculus to scrambled point-light walkers (with local kinematics retained) in both humans and male macaque monkeys. Results show that the superior colliculus, especially the superficial layers, selectively responds to local biological motion. Furthermore, dynamic causal modeling analysis reveals a subcortical-cortical functional pathway that transmits local biological motion signals from the superior colliculus via the middle temporal visual complex to the posterior superior temporal sulcus in the human brain. These findings suggest the existence of a cross-species mechanism in the superior colliculus that facilitates the detection of local biological motion at the early stage of the visual processing stream.

Previous research has shown that motion processing is significantly impaired in both autistic and schizophrenic patients. One specific area of motion perception that is affected by both disorders is the processing of biological motion. Stimuli that are often used to investigate biological motion perception are point-light walkers (PLW), for which point-lights are attached to the joints of a moving person, which alone are enough to perceive the human figure. PLWs are useful stimuli to investigate biological motion perception, because they contain the local motion information of each dot, and in addition, by grouping the local elements of the walkers into a global form, they have a high-level interpretation of a moving person (normal walker). By scrambling the positions of the points on the screen, the local motion of each dot is preserved while obscuring the underlying form (scrambled walker). Similarly, by randomizing the position of the dots along the underlying skeleton each frame of the moving sequence, the local image motion is lost, whereas the global form of the walker is preserved (random-position walker). Also inverting the walker in a way that it appears to be walking on the ceiling can obscure the global form of the walker (inverted walker). Using these four kinds of walkers, it is possible to investigate the relative contribution of form and motion information to biological motion perception using one single paradigm. We have successfully used these walkers before to investigate biological motion perception in ageing. Here, we tested differences in biological motion processing in healthy young people with different schizotypic (N=38) and autistic traits (N=35). We asked participants to perform a biological motion direction discrimination task for normal, inverted, scrambled and random-position walkers. In addition, they had to complete questionnaires on autistic and schizotypic traits. Using both reaction time and accuracy measures we found differences for processing local motion and global form information between participants with high or low scores on both questionnaires.

Point-light biological motion stimuli have particular spatiotemporal properties that enable them to be processed with remarkable efficiency. Human observers can readily recognize action, gender, emotion, and identity information conveyed by dynamic point-light walkers. All these processes require temporal integration, yet little is known about the encoding of biological motion temporal information. Here we report a novel temporal illusion: biological motion signals significantly lengthen perceived temporal duration in a manner that is independent of conscious awareness of biological nature. In a parametric time discrimination paradigm, we showed that the same presentation duration of an upright biological motion sequence was perceived significantly longer compared with that of an inverted motion sequence. This overestimation effect was not due to the familiarity of the global configuration, since an upright static biological motion frame showed no difference when compared with an inverted one. Moreover, such temporal illusion persisted with spatially scrambled biological motion signals, whose global configurations were completely disrupted, independent of the observers' explicit knowledge of the nature of the stimuli. These findings provide strong evidence that biological motion has particular temporal properties that distinguish itself from other forms of motion and highlight the intrinsic sensitivity of the human visual system to local biological motion signals.

BackgroundPerception of biological motion is linked to the action perception system in the human brain, abnormalities within which have been suggested to underlie impairments in social domains observed in autism spectrum conditions (ASC). However, the literature on biological motion perception in ASC is heterogeneous and it is unclear whether deficits are specific to biological motion, or might generalize to form-from-motion perception.Methodology and Principal FindingsWe compared psychophysical thresholds for both biological and non-biological form-from-motion perception in adults with ASC and controls. Participants viewed point-light displays depicting a walking person (Biological Motion), a translating rectangle (Structured Object) or a translating unfamiliar shape (Unstructured Object). The figures were embedded in noise dots that moved similarly and the task was to determine direction of movement. The number of noise dots varied on each trial and perceptual thresholds were estimated adaptively. We found no evidence for an impairment in biological or non-biological object motion perception in individuals with ASC. Perceptual thresholds in the three conditions were almost identical between the ASC and control groups.Discussion and ConclusionsImpairments in biological motion and non-biological form-from-motion perception are not across the board in ASC, and are only found for some stimuli and tasks. We discuss our results in relation to other findings in the literature, the heterogeneity of which likely relates to the different tasks performed. It appears that individuals with ASC are unaffected in perceptual processing of form-from-motion, but may exhibit impairments in higher order judgments such as emotion processing. It is important to identify more specifically which processes of motion perception are impacted in ASC before a link can be made between perceptual deficits and the higher-level features of the disorder.

The visual perception of human movement from sparse point-light walkers is often believed to rely on local motion analysis. We investigated the role of local motion in the perception of human walking, viewed from the side, in different tasks. The motion signal was manipulated by varying point lifetime. We found the task of coherence discrimination, commonly used in biological motion studies, to be inappropriate for testing the role of motion. A task requiring temporal information showed a strong performance drop when fewer points were used or when the image sequence was sampled and displayed at a reduced frame rate. Irrespective of the frame rate, performance did not vary with point lifetime. We concluded that local motion is not required for the perception of tested biological movements, suggesting that the analysis of biological motion does not benefit from examining local motion. The reliance of perception on the number of displayed points and frames supports the idea that biological motion is perceived from a sequence of spatiotemporally sampled forms.

Deficient local biological motion perception in migraineurs: Results from a duration discrimination paradigm

A two-stage framework for neural processing of biological motion

Biological motion perception is referred to as the ability to recognize a moving human figure from no more than a few moving point lights. Such point-light stimuli contain limited form information about the shape of the body and local image motion signals from the moving points. The contributions of form and motion to the vivid perception of point-light displays are subject to controversy in the discussion. While some studies claim that local motion signals are critical, others emphasize the role of global form cues. Here, we present a template-matching approach to investigate the role of global form analysis. We used a template-matching method that derives biological motion exclusively from form information. The algorithm used static postures monitored from walking humans as stored templates. We compared the simulation results to psychophysical experiments with the commonly used point-light walker and a variant point-light walker with near-absent local motion signals. The common result in all experiments was a high correlation between simulation results and psychophysical data. The results show that the limited form information in point-light stimuli might be sufficient to perceive biological motion. We suggest that it is possible for humans to extract the sparse form information in point-light walkers and to use it to perceive biological motion by integrating dynamic form information over time.

Point-light animations of biological motion are perceived quickly and spontaneously, giving rise to an irresistible sensation of animacy. However, the mechanisms that support judgments of animacy based on biological motion remain unclear. The current study demonstrates that animacy ratings increase when a spatially scrambled animation of human walking maintains consistency with two fundamental constraints: the direction of gravity and congruency between the directions of intrinsic and extrinsic motion. Furthermore, using a reverse-correlation method, we show that observers employ structural templates, or form-based “priors,” reflecting the prototypical mammalian body plan when attributing animacy to scrambled human forms. These findings reveal that perception of animacy in scrambled biological motion involves not only analysis of local intrinsic motion, but also its congruency with global extrinsic motion and global spatial structure. Thus, they suggest a strong influence of prior knowledge about characteristic features of creatures in the natural environment.

Biological motion (BM), depicted by a handful of point lights attached to the major joints, conveys rich animacy information, which is significantly disrupted if BM is shown upside down. This well-known inversion effect in BM perception is conserved in terrestrial vertebrates and is presumably a manifestation of an evolutionarily endowed perceptual filter (i.e., life motion detector) tuned to gravity-compatible BM. However, it remains unknown whether aquatic animals, living in a completely different environment from terrestrial animals, perceive BM in a gravity-dependent manner. Here, taking advantage of their typical shoaling behaviors, we used zebrafish as a model animal to examine the ability of teleosts to discriminate between upright (gravity-compatible) and inverted (gravity-incompatible) BM signals. We recorded their swimming trajectories and quantified their preference based on dwelling time and head orientation. The results obtained from three experiments consistently showed that zebrafish spent significantly more time swimming in proximity to and orienting towards the upright BM relative to the inverted BM or other gravity-incompatible point-light stimuli (i.e., the non-BM). More intriguingly, when the recorded point-light video clips of fish were directly compared with those of human walkers and pigeons, we could identify a unique and consistent pattern of accelerating movements in the vertical (gravity) direction. These findings, to our knowledge, demonstrate for the first time the inversion effect in BM perception in simple aquatic vertebrates and suggest that the evolutionary origin of gravity-dependent BM processing may be traced back to ancient aquatic animals.