Steven M. Thurman

Physical and Biological Constraints Govern Perceived Animacy of Scrambled Human Forms

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.

Complex interactions between spatial, orientation, and motion cues for biological motion perception across visual space

Human observers are adept at perceiving complex actions in point-light biological motion displays that represent the human form with a sparse array of moving points. However, the neural computations supporting action perception remain unclear, particularly with regards to central versus peripheral vision. We created novel action stimuli comprised of Gabor patches to examine the contributions of various competing visual cues to action perception across the visual field. The Gabor action stimulus made it possible to pin down form processing at two levels: (a) local information about limb angle represented by Gabor orientations and (b) global body structure signaled by the spatial arrangement of Gabor patches. This stimulus also introduced two types of motion signals: (a) local velocity represented by Gabor drifting motion and (b) joint motion trajectories signaled by position changes of Gabor disks over time. In central vision, the computational analysis of global cues based on the spatial arrangement of joints and joint trajectories dominated processing, with minimal influence of local drifting motion and orientation cues. In the periphery we found that local drifting motion and orientation cues interacted with spatial cues in sophisticated ways depending on the particular discrimination task and location within the visual field to influence action perception. This dissociation was evident in several experiments showing phantom action percepts in the periphery that contradicted central vision. Our findings suggest a highly flexible and adaptive system for processing visual cues at multiple levels for biological motion and action perception.

Perception of Social Interactions for Spatially Scrambled Biological Motion

It is vitally important for humans to detect living creatures in the environment and to analyze their behavior to facilitate action understanding and high-level social inference. The current study employed naturalistic point-light animations to examine the ability of human observers to spontaneously identify and discriminate socially interactive behaviors between two human agents. Specifically, we investigated the importance of global body form, intrinsic joint movements, extrinsic whole-body movements, and critically, the congruency between intrinsic and extrinsic motions. Motion congruency is hypothesized to be particularly important because of the constraint it imposes on naturalistic action due to the inherent causal relationship between limb movements and whole body motion. Using a free response paradigm in Experiment 1, we discovered that many naïve observers (55%) spontaneously attributed animate and/or social traits to spatially-scrambled displays of interpersonal interaction. Total stimulus motion energy was strongly correlated with the likelihood that an observer would attribute animate/social traits, as opposed to physical/mechanical traits, to the scrambled dot stimuli. In Experiment 2, we found that participants could identify interactions between spatially-scrambled displays of human dance as long as congruency was maintained between intrinsic/extrinsic movements. Violating the motion congruency constraint resulted in chance discrimination performance for the spatially-scrambled displays. Finally, Experiment 3 showed that scrambled point-light dancing animations violating this constraint were also rated as significantly less interactive than animations with congruent intrinsic/extrinsic motion. These results demonstrate the importance of intrinsic/extrinsic motion congruency for biological motion analysis, and support a theoretical framework in which early visual filters help to detect animate agents in the environment based on several fundamental constraints. Only after satisfying these basic constraints could stimuli be evaluated for high-level social content. In this way, we posit that perceptual animacy may serve as a gateway to higher-level processes that support action understanding and social inference.

Neural adaptation in pSTS correlates with perceptual aftereffects to biological motion and with autistic traits

The adaptive nature of biological motion perception has been documented in behavioral studies, with research showing that prolonged viewing of an action can bias judgments of subsequent actions towards the opposite of its attributes. However, the neural mechanisms underlying action adaptation aftereffects remain unknown. We examined adaptation-induced changes in brain responses to an ambiguous action after adapting to walking or running actions within two bilateral regions of interest: 1) human middle temporal area (hMT+), a lower-level motion-sensitive region of cortex, and 2) posterior superior temporal sulcus (pSTS), a higher-level action-selective area. We found a significant correlation between neural adaptation strength in right pSTS and perceptual aftereffects to biological motion measured behaviorally, but not in hMT+. The magnitude of neural adaptation in right pSTS was also strongly correlated with individual differences in the degree of autistic traits. Participants with more autistic traits exhibited less adaptation-induced modulations of brain responses in right pSTS and correspondingly weaker perceptual aftereffects. These results suggest a direct link between perceptual aftereffects and adaptation of neural populations in right pSTS after prolonged viewing of a biological motion stimulus, and highlight the potential importance of this brain region for understanding differences in social-cognitive processing along the autistic spectrum.

A comparison of form processing involved in the perception of biological and nonbiological movements.

Although there is evidence for specialization in the human brain for processing biological motion per se, few studies have directly examined the specialization of form processing in biological motion perception. The current study was designed to systematically compare form processing in perception of biological (human walkers) to nonbiological (rotating squares) stimuli. Dynamic form-based stimuli were constructed with conflicting form cues (position and orientation), such that the objects were perceived to be moving ambiguously in two directions at once. In Experiment 1, we used the classification image technique to examine how local form cues are integrated across space and time in a bottom-up manner. By comparing with a Bayesian observer model that embodies generic principles of form analysis (e.g., template matching) and integrates form information according to cue reliability, we found that human observers employ domain-general processes to recognize both human actions and nonbiological object movements. Experiments 2 and 3 found differential top-down effects of spatial context on perception of biological and nonbiological forms. When a background does not involve social information, observers are biased to perceive foreground object movements in the direction opposite to surrounding motion. However, when a background involves social cues, such as a crowd of similar objects, perception is biased toward the same direction as the crowd for biological walking stimuli, but not for rotating nonbiological stimuli. The model provided an accurate account of top-down modulations by adjusting the prior probabilities associated with the internal templates, demonstrating the power and flexibility of the Bayesian approach for visual form perception.

Bayesian integration of position and orientation cues in perception of biological and non-biological forms

Visual form analysis is fundamental to shape perception and likely plays a central role in perception of more complex dynamic shapes, such as moving objects or biological motion. Two primary form-based cues serve to represent the overall shape of an object: the spatial position and the orientation of locations along the boundary of the object. However, it is unclear how the visual system integrates these two sources of information in dynamic form analysis, and in particular how the brain resolves ambiguities due to sensory uncertainty and/or cue conflict. In the current study, we created animations of sparsely-sampled dynamic objects (human walkers or rotating squares) comprised of oriented Gabor patches in which orientation could either coincide or conflict with information provided by position cues. When the cues were incongruent, we found a characteristic trade-off between position and orientation information whereby position cues increasingly dominated perception as the relative uncertainty of orientation increased and vice versa. Furthermore, we found no evidence for differences in the visual processing of biological and non-biological objects, casting doubt on the claim that biological motion may be specialized in the human brain, at least in specific terms of form analysis. To explain these behavioral results quantitatively, we adopt a probabilistic template-matching model that uses Bayesian inference within local modules to estimate object shape separately from either spatial position or orientation signals. The outputs of the two modules are integrated with weights that reflect individual estimates of subjective cue reliability, and integrated over time to produce a decision about the perceived dynamics of the input data. Results of this model provided a close fit to the behavioral data, suggesting a mechanism in the human visual system that approximates rational Bayesian inference to integrate position and orientation signals in dynamic form analysis.

Causal Action: A Fundamental Constraint on Perception and Inference About Body Movements

The human body navigates the environment via locomotory movements that leverage gravity and limb biomechanics to propel the body in a particular direction. This process creates a causal link between limb movements and whole-body translation. However, it is unknown whether humans use this causal relation as a constraint in perception and inference with body movements. In the present study, participants rated actions of other individuals as more natural when limb movements (as a cause) occurred before body displacements (as an effect) than when limb movements temporally lagged behind body displacements. This causal expectation for human body movements not only affected perceptual impressions regarding the naturalness of observed actions but also guided the interpretation of motion cues within a more generalized causal context. We interpret these results within a framework of causality as evidence that the constraint of causal action plays an important role in perception and inference with body movements.

Revisiting the importance of common body motion in human action perception

Human actions are complex dynamic stimuli comprised of two principle motion components: 1) common body motion, which represents the translation of the body when a person moves through space, and 2) relative limb movements, resulting from articulation of limbs after factoring out common body motion. Historically, most research in biological motion has focused primarily on relative limb movements while discounting the role of common body motion in human action perception. The current study examined the relative contribution of posture change resulting from relative limb movements and translation of body position resulting from common body motion in discriminating human walking versus running actions. We found that faster translation speeds of common body motion evoked significantly more responses consistent with running when discriminating ambiguous actions morphed between walking and running. Furthermore, this influence was systematically modulated by the uncertainty associated with intrinsic cues as determined by the degree of limited-lifetime spatial sampling. The contribution of common body motion increased monotonically as the reliability of inferring posture changes on the basis of intrinsic cues decreased. These results highlight the importance of translational body movements and their interaction with posture change as a result of relative limb movements in discriminating human actions when visual input information is sparse and noisy.

Neural correlates of action aftereffects triggered by adaptation to biological motion

A hallmark of human vision is the capacity to recognize diverse actions from point-light displays of biological motion (BM). The adaptive nature of BM perception is documented in behavioral studies where prolonged viewing of a stimulus can bias judgments of subsequent stimuli towards the opposite of its attributes. However, the neural mechanisms underlying action adaptation aftereffects remain unknown. We used functional magnetic resonance imaging (fMRI) to measure neural adaptation after prolonged viewing of a BM stimulus (n=12). Using an event-related design with topping-up adaptation, we measured neural aftereffects from brain responses to morphed actions after adapting to walking or running actions within two bilateral regions of interest: 1) human medial temporal area (hMT+), a lower-level motion-sensitive region of cortex, and 2) superior temporal sulcus (pSTS), a higher-level action-selective area. Neural adaptation in hMT+ was observed only when the adapting and testing stimuli were in the same location. In contrast, neural adaptation in the action-sensitive area pSTS was found to be location-invariant. Importantly, we found a significant correlation across subjects between the strength of neural aftereffects in the right pSTS and perceptual aftereffects measured behaviorally. We also measured fMR-adaptation to repeated actions (e.g. repetition suppression) and found significant effects in the right pSTS, although the strength of fMR-adaptation did not correlate with perceptual aftereffects, suggesting that distinct mechanisms are involved in action aftereffects and repetition suppression. Interestingly, the magnitudes of behavioral and neural aftereffects were significantly correlated with individual differences in autistic traits (Baron-Cohen et al., 2001). Participants with more autistic traits exhibited less modulation of brain responses in right pSTS and correspondingly weaker perceptual aftereffects. These results suggest a direct link between perceptual adaptation and neural adaptation in right pSTS, and suggest this as a core brain region for understanding social and perceptual deficits in Autism Spectrum Condition. Meeting abstract presented at VSS 2015.