Modulation of biological motion perception in humans by gravity

Sensorimotor simulation and distributed processing of biological motion: Insights from healthy and paraplegic adults

The processing of biological motion (BM), particularly the local motion cues tracing the movements of crucial joints, is vital for social interaction and human survival. While numerous studies have focused on the brain mechanisms underlying BM processing, the contribution of sensorimotor simulation at peripheral effectors remains unclear. In this study, we examined healthy adults and paraplegic spinal cord injury participants to investigate this issue. For healthy adults, both intact BM stimuli and local BM cues without global configuration induced a temporal dilation effect when sitting (sensorimotor simulation accessible), but not when standing (sensorimotor simulation temporarily hindered). In contrast, for participants with permanently hindered sensorimotor simulation, the temporal dilation effect was observed only with intact BM stimuli but not with local BM cues, indicating a robust reliance on sensorimotor simulation during the processing of local BM cues and a selective compensation based on global configuration cues for the permanent loss of sensorimotor simulation. These findings highlight the role of embodied cognition in the distributed processing of biological motion and suggest the importance of selective compensation under damaged sensorimotor circuits.

Shared and Unique Neural Codes for Biological Motion Perception in Humans and Macaque Monkeys

Throughout evolution, living organisms have honed the ability to swiftly recognize biological motion (BM) across species. However, how the brain processes within- and cross-species BM, and the evolutionary progression of these processes, remain unclear. To investigate these questions, the current study examined brain activity in the lateral temporal areas of humans and monkeys as they passively observed upright and inverted human and macaque BM stimuli. In humans, the middle temporal area (hMT+) responded to both human and macaque BM stimuli, while the right posterior superior temporal sulcus (hpSTS) exhibited selective responses to human BM stimuli. This selectivity is evidenced by an increased feedforward connection from hMT+ to hpSTS during the processing of human BM stimuli. In monkeys, the MT region processed BM stimuli from both species, but no subregion in the STS anterior to MT is specific to conspecific BM stimuli. A comparison of these findings suggests that upstream brain regions (i.e., MT) may retain homologous functions across species, while downstream brain regions (i.e., STS) may have undergone differentiation and specialization throughout evolution. These results provide insights into the commonalities and differences in the specialized visual pathway engaged in processing within- and cross-species BMs, as well as their functional divergence during evolution.

Dynamics of visual reversals from ambiguous spinning biological-motion and rigid structure-from-motion

Three-dimensional rigid structure-from-motion (SFM) and structure from nonrigid biological point-light motion stimuli are perceptually ambiguous. This study investigated the dynamics of perceived reversals in two cases: a spinning point-light walker (PLW) and a spinning rigid human figure in a walking pose (SFM). It specifically focused on two key questions: Could the facing-the-viewer bias (FTV) account for the reversals for spinning PLW? To what extent do motion cues from limb motions or configural cues from the human shape contribute to the perceived reversals? In Experiment 1, participants reported reversals with more than twice the frequency while viewing the upright and inverted PLW than for the rigid structures, but an FTV bias was observed only for the upright walker. The perception of an ambulating living human shape of typically encountered walkers in an upright position thus plays a crucial role in obtaining an FTV bias for these spinning stimuli. In Experiment 2, the human figures walked or rigidly moved along a circular path while facing the motion direction, spinning at the same rate as in Experiment 1. A strong initial FTV bias was then observed, but the reversal rate was substantially reduced compared to reversals when spinning on the same spot. These findings highlight theoretically interesting distinct temporal dynamics of reversals and biases between biological motion and rigid SFM. It is argued that the differences in reversals between conditions have a common cause in the form of past experiences that differ between conditions.

Decoding Prosodic Information from Motion Capture Data: The Gravity of Co-Speech Gestures

In part due to correspondence in time, seeing how a speaking body moves can impact how speech is apprehended. Despite this, little is known about whether and which specific kinematic features of co-speech movements are relevant for their integration with speech. The current study uses machine learning techniques to investigate how co-speech gestures can be quantified to model vocal acoustics within an individual speaker. Specifically, we address whether kinetic descriptions of human movement are relevant for modeling their relationship with speech in time. To test this, we apply experimental manipulations that either highlight or obscure the relationship between co-speech movement kinematics and downward gravitational acceleration. Across two experiments, we provide evidence that quantifying co-speech movement as a function of its anisotropic relation to downward gravitational forces improves how well those co-speech movements can be used to predict prosodic dimensions of speech, as represented by the low-pass envelope. This study supports theoretical perspectives that invoke biomechanics to help explain speech-gesture synchrony and offers motivation for further behavioral or neuroimaging work investigating audiovisual integration and/or biological motion perception in the context of multimodal discourse.

Hierarchical and distinct biological motion processing in macaque visual cortical areas MT and MST

It is widely accepted that biological motion (BM) perception involves the posterior superior temporal sulcus (pSTS). Yet, how individual neurons and neural circuits in pSTS encode BM remains unclear. Here we combined electrophysiological recordings with neural network modeling to elucidate BM computations in two subregions of pSTS. We recorded single-cell activity from the middle temporal area (MT) and the medial superior temporal area (MST) of three macaque monkeys when they viewed point-light displays portraying BM walking in different directions (left vs. right), orientations (upright vs. inverted), and forms (intact vs. scrambled). We found that, while individual neurons in both MT and MST showed selectivity for these features, neural populations in MST but not MT exhibit BM-specific encoding, i.e., preferential representation of intact upright BM-the defining characteristic of BM recognition. A neural network model trained to replicate these neurophysiological findings implicated that, BM-specific encoding in MST may arise from feedforward connectivity patterns, i.e., MT subpopulations selective for linear translational motion and nonlinear optic flow projected preferentially to distinct MST cells. Taken together, our findings highlight hierarchical and distinct BM processing in MT and MST, advancing our understanding of BM computations in pSTS at the single-cell and neural circuit levels in the primate brain.

Cortical tracking of hierarchical rhythms orchestrates the multisensory processing of biological motion

When observing others' behaviors, we continuously integrate their movements with the corresponding sounds to enhance perception and develop adaptive responses. However, how the human brain integrates these complex audiovisual cues based on their natural temporal correspondence remains unclear. Using electroencephalogram (EEG), we demonstrated that rhythmic cortical activity tracked the hierarchical rhythmic structures in audiovisually congruent human walking movements and footstep sounds. Remarkably, the cortical tracking effects exhibit distinct multisensory integration modes at two temporal scales: an additive mode in a lower-order, narrower temporal integration window (step cycle) and a super-additive enhancement in a higher-order, broader temporal window (gait cycle). Furthermore, while neural responses at the lower-order timescale reflect a domain-general audiovisual integration process, cortical tracking at the higher-order timescale is exclusively engaged in the integration of biological motion cues. In addition, only this higher-order, domain-specific cortical tracking effect correlates with individuals' autistic traits, highlighting its potential as a neural marker for autism spectrum disorder. These findings unveil the multifaceted mechanism whereby rhythmic cortical activity supports the multisensory integration of human motion, shedding light on how neural coding of hierarchical temporal structures orchestrates the processing of complex, natural stimuli across multiple timescales.

No enhancement of vestibular stimulation on visual working memory for actions

Holding and processing actions in visual working memory (VWM) is crucial for daily functioning, but it is capacity-limited. The vestibular system, which is activated by both vestibular signals and visual gravitational motion, may be involved in this cognitive process. Past research indicates that galvanic vestibular stimulation (GVS) is a non-invasive technique that can enhance motor functions and various cognitive functions. However, the impact of GVS on VWM for actions has not been explored. This study addresses this gap by investigating whether GVS can improve VWM capacity for both upright and inverted actions. Using a virtual reality platform, we assessed VWM capacity in a change detection task, applying either STIM-current (0.8 mA) or SHAM-current (0 mA) GVS. The results showed no significant effect of GVS on VWM for either upright or inverted actions, at least for non-noisy GVS with stimulation intensity below cutaneous threshold. Despite the absence of significant effects, this study provides valuable insights into the interactions between the vestibular system and cognitive processes, laying the groundwork for future research on GVS applications in cognitive enhancement.

Theta oscillations in observers' temporal cortex index postural instability of point-light displays

This study investigates whether postural equilibration follows the same principles of motor resonance as goal-oriented actions, namely, whether an individual activates the same neuronal substrates when experiencing postural perturbation as when observing another individual in this condition. To address this question, we examined electroencephalographic dynamics while subjects observed point-light displays featuring an unstable human display, a stable human display, and their respective scrambled counterparts lacking shape information and biological motion. We focused on theta band (4-7 Hz), which is a fundamental frequency for modulating brain activity during challenging balance tasks and reflects postural stability monitoring. Rather than mirroring activity, our findings suggest an inhibitory response to postural instability. Theta event-related synchronization in the left temporal cortex was dampened for the unstable display as compared to its scramble counterpart and to the stable display. This low theta response coincided with an increase in left temporal-prefrontal connectivity, compatible with top-down inhibitory mechanisms. By contrast, the stronger theta response to the stable display as compared to the unstable one could be due to the difficulty of recognizing low-motion biological stimuli, or alternatively, to a facilitation of stimulus processing and strengthening of the mirroring response. The response facilitation for stable posture, coupled with a diminished response to the unstable display, could contribute to a broader mechanism mitigating postural threats and ensuring stable balance. Future investigations should leverage these findings to explore how posture-related responses correlate with perceptual and motor expertise, and to more clearly define these mechanisms during dynamic social interactions.

Detecting biological motion signals in human and monkey superior colliculus: a subcortical-cortical pathway for biological motion perception

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.

Computational account for the naturalness perception of others' jumping motion based on a vertical projectile motion model

The visual naturalness of a rendered character's motion is an important factor in computer graphics work, and the rendering of jumping motions is no exception to this. However, the computational mechanism that underlies the observer's judgement of the naturalness of a jumping motion has not yet been fully elucidated. We hypothesized that observers would perceive a jumping motion as more natural when the jump trajectory was consistent with the trajectory of a vertical projectile motion based on Earth's gravity. We asked human participants to evaluate the naturalness of point-light jumping motions whose height and duration were modulated. The results showed that the observers' naturalness rating varied with the modulation ratios of the jump height and duration. Interestingly, the ratings were high even when the height and duration differed from the actual jump. To explain this tendency, we constructed computational models that predicted the theoretical trajectory of a jump based on the projectile motion formula and calculated the errors between the theoretical and observed trajectories. The pattern of the errors correlated closely with the participants' ratings. Our results suggest that observers judge the naturalness of observed jumping motion based on the error between observed and predicted jump trajectories.

A stochastic world model on gravity for stability inference

The fact that objects without proper support will fall to the ground is not only a natural phenomenon, but also common sense in mind. Previous studies suggest that humans may infer objects' stability through a world model that performs mental simulations with a priori knowledge of gravity acting upon the objects. Here we measured participants' sensitivity to gravity to investigate how the world model works. We found that the world model on gravity was not a faithful replica of the physical laws, but instead encoded gravity's vertical direction as a Gaussian distribution. The world model with this stochastic feature fit nicely with participants' subjective sense of objects' stability and explained the illusion that taller objects are perceived as more likely to fall. Furthermore, a computational model with reinforcement learning revealed that the stochastic characteristic likely originated from experience-dependent comparisons between predictions formed by internal simulations and the realities observed in the external world, which illustrated the ecological advantage of stochastic representation in balancing accuracy and speed for efficient stability inference. The stochastic world model on gravity provides an example of how a priori knowledge of the physical world is implemented in mind that helps humans operate flexibly in open-ended environments.

Life motion signals modulate visual working memory

Previous research has demonstrated that biological motion (BM) cues can induce a reflexive attentional orienting effect, a phenomenon referred to as social attention. However, it remains undetermined whether BM cues can further affect higher-order cognitive processes, such as visual working memory (WM). By combining a modified central pre-cueing paradigm with a traditional WM change detection task, the current study investigated whether the walking direction of BM, as a non-predictive central cue, could modulate the encoding process of WM. Results revealed a significant improvement in WM performance for the items appearing at the location cued by the walking direction of BM. The observed effect disappeared when the BM cues were shown inverted, or when the critical biological characteristics of the cues were removed. Crucially, this effect could be extended to upright feet motion cues without global configuration, reflecting the key role of local BM signals in modulating WM. More importantly, such a BM-induced modulation effect was not observed with inanimate motion cues, although these cues can also elicit attentional effects. Our findings suggest that the attentional effect induced by life motion signals can penetrate to higher-order cognitive processes, and provide compelling evidence for the existence of "life motion detector" in the human brain from a high-level cognitive function perspective.

Eye pupil signals life motion perception

The ability to readily detect and recognize biological motion (BM) is fundamental to survival and interpersonal communication. However, perception of BM is strongly disrupted when it is shown upside down. This well-known inversion effect is proposed to be caused by a life motion detection mechanism highly tuned to gravity-compatible motion cues. In the current study, we assessed the inversion effect in BM perception using a no-report pupillometry. We found that the pupil size was significantly enlarged when observers viewed upright BMs (gravity-compatible) compared with the inverted counterparts (gravity-incompatible). Importantly, such an effect critically depended on the dynamic biological characteristics, and could be extended to local feet motion signals. These findings demonstrate that the eye pupil can signal gravity-dependent life motion perception. More importantly, with the convenience, objectivity, and noninvasiveness of pupillometry, the current study paves the way for the potential application of pupillary responses in detecting the deficiency of life motion perception in individuals with socio-cognitive disorders.

Vascular smooth muscle cell-specific miRNA-214 deficiency alleviates simulated microgravity-induced vascular remodeling

The human cardiovascular system has evolved to accommodate the gravity of Earth. Microgravity during spaceflight has been shown to induce vascular remodeling, leading to a decline in vascular function. The underlying mechanisms are not yet fully understood. Our previous study demonstrated that miR-214 plays a critical role in angiotensin II-induced vascular remodeling by reducing the levels of Smad7 and increasing the phosphorylation of Smad3. However, its role in vascular remodeling evoked by microgravity is not yet known. This study aimed to determine the contribution of miR-214 to the regulation of microgravity-induced vascular remodeling. The results of our study revealed that miR-214 expression was increased in the forebody arteries of both mice and monkeys after simulated microgravity treatment. In vitro, rotation-simulated microgravity-induced VSMC migration, hypertrophy, fibrosis, and inflammation were repressed by miR-214 knockout (KO) in VSMCs. Additionally, miR-214 KO increased the level of Smad7 and decreased the phosphorylation of Smad3, leading to a decrease in downstream gene expression. Furthermore, miR-214 cKO protected against simulated microgravity induced the decline in aorta function and the increase in stiffness. Histological analysis showed that miR-214 cKO inhibited the increases in vascular medial thickness that occurred after simulated microgravity treatment. Altogether, these results demonstrate that miR-214 has potential as a therapeutic target for the treatment of vascular remodeling caused by simulated microgravity.

Audiovisual correspondence facilitates the visual search for biological motion

Hearing synchronous sounds may facilitate the visual search for the concurrently changed visual targets. Evidence for this audiovisual attentional facilitation effect mainly comes from studies using artificial stimuli with relatively simple temporal dynamics, indicating a stimulus-driven mechanism whereby synchronous audiovisual cues create a salient object to capture attention. Here, we investigated the crossmodal attentional facilitation effect on biological motion (BM), a natural, biologically significant stimulus with complex and unique dynamic profiles. We found that listening to temporally congruent sounds, compared with incongruent sounds, enhanced the visual search for BM targets. More intriguingly, such a facilitation effect requires the presence of distinctive local motion cues (especially the accelerations in feet movement) independent of the global BM configuration, suggesting a crossmodal mechanism triggered by specific biological features to enhance the salience of BM signals. These findings provide novel insights into how audiovisual integration boosts attention to biologically relevant motion stimuli and extend the function of a proposed life detection system driven by local kinematics of BM to multisensory life motion perception.

Action observation network: domain-specific or domain-general?

The action observation network (AON) has traditionally been thought to be dedicated to recognizing animate actions. A recent study by Karakose-Akbiyik et al. invites rethinking this assumption by demonstrating that the AON contains a shared neural code for general events, regardless of whether those events involve animate or inanimate entities.

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.

Up right, not right up: Primacy of verticality in both language and movement

When describing motion along both the horizontal and vertical axes, languages from different families express the elements encoding verticality before those coding for horizontality (e.g., going up right instead of right up). In light of the motor grounding of language, the present study investigated whether the prevalence of verticality in Path expression also governs the trajectory of arm biological movements. Using a 3D virtual-reality setting, we tracked the kinematics of hand pointing movements in five spatial directions, two of which implied the vertical and horizontal vectors equally (i.e., up right +45° and bottom right −45°). Movement onset could be prompted by visual or auditory verbal cues, the latter being canonical in French (“en haut à droite”/up right) or not (“à droite en haut”/right up). In two experiments, analyses of the index finger kinematics revealed a significant effect of gravity, with earlier acceleration, velocity, and deceleration peaks for upward (+45°) than downward (−45°) movements, irrespective of the instructions. Remarkably, confirming the linguistic observations, we found that vertical kinematic parameters occurred earlier than horizontal ones for upward movements, both for visual and congruent verbal cues. Non-canonical verbal instructions significantly affected this temporal dynamic: for upward movements, the horizontal and vertical components temporally aligned, while they reversed for downward movements where the kinematics of the vertical axis was delayed with respect to that of the horizontal one. This temporal dynamic is so deeply anchored that non-canonical verbal instructions allowed for horizontality to precede verticality only for movements that do not fight against gravity. Altogether, our findings provide new insights into the embodiment of language by revealing that linguistic path may reflect the organization of biological movements, giving priority to the vertical axis.

Gravity-Dependent Animacy Perception in Zebrafish

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.