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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.

Successful social behavior requires the accurate perception and interpretation of other peoples' actions. In the last decade, significant progress has been made in understanding how the human visual system analyzes bodily motion. Neurophysiological studies have identified two neural areas, the superior temporal sulcus (STS) and the premotor cortex, which play key roles in the visual perception of human movement. Patterns of neural activity in these areas are reflective of psychophysical measures of visual sensitivity to human movement. Both vary as a function of stimulus orientation and global stimulus structure. Human observers and STS responsiveness share some developmental similarities as both exhibit sensitivities that become increasingly tuned for upright, human movement. Furthermore, the observer's own visual and motor experience with an action as well as the social and emotional content of that action influence behavioral measures of visual sensitivity and patterns of neural activity in the STS and premotor cortex. Finally, dysfunction of motor processes, such as hemiplegia, and dysfunction of social processes, such as Autism, systematically impact visual sensitivity to human movement. In sum, a convergence of visual, motor, and social processes underlies our ability to perceive and interpret the actions of other people. WIREs Cogn Sci 2011 2 68-78 DOI: 10.1002/wcs.88 For further resources related to this article, please visit the WIREs website.

Mounting neurophysiological evidence indicates that the visual analysis of human movement differs from the visual analysis of other categories of complex movement. If different patterns of neural activity underlie visual percepts of human and nonhuman movement, then psychophysical measures should elucidate different patterns of visual sensitivity to human movement and similarly complex, but nonhuman movement. To test this prediction, two psychophysical studies compared visual sensitivity to human and animal motions. Using a simultaneous masking paradigm, observers performed a coherent motion detection task with point-light displays of human and horse gait, presented upright and inverted. While task performance indicated the use of configural processing during the detection of both human and horse motion, observers demonstrated greater visual sensitivity to coherent human motion than coherent horse motion. Recent experience influenced orientation dependence for both types of motion. Together with previous neurophysiological findings, these psychophysical results suggest that the visual perception of human movement is both distinct from and shares commonalities with the visual perception of similarly complex, nonhuman movement.

The texture of a rolling circle depicts the translational and rotational components of its motion. In the case of a homogeneous circle, however, visual cues to the rotational component of motion are absent. To examine how the visual system resolves undetermined motion cues, optically neutral circles were displaced so that changes in their orientation were invisible. Contextual cues systematically triggered the perception of illusory rotation, suggesting that the visual system uses contextual cues along with intrinsic surface cues to compute percepts of rolling objects. This might also explain why people rarely experience the perception of ambiguous motion.

We characterized magnetoencephalographic responses during observation of point-light displays of human and object motion. Time courses of grand-mean source estimates were computed and time frequency maps were calculated. For both conditions, activity began in the posterior occipital and mid-parietal areas. Further, late peaks were observed in the parietal, sensory-motor and left temporal regions. Only observation of human motion resulted in activation of the right temporal area. Both viewing conditions resulted in alpha and beta event-related desynchronization over the parietal, sensory-motor and temporal areas. A significant increase in beta activity was seen in the posterior temporal region in the human motion condition. The visual analyses of human and object motion appear to involve both overlapping and divergent patterns of neural activity.

Humans, being highly social creatures, rely heavily on the ability to perceive what others are doing and to infer from gestures and expressions what others may be intending to do. These perceptual skills are easily mastered by most, but not all, people, in large part because human action readily communicates intentions and feelings. In recent years, remarkable advances have been made in our understanding of the visual, motoric, and affective influences on perception of human action, as well as in the elucidation of the neural concomitants of perception of human action. This article reviews those advances and, where possible, draws links among those findings.

Johansson's point-light walker figures remain one of the most powerful and convincing examples of the role that motion can play in the perception of form (Johansson, 1973 Perception & Psychophysics14 201–211; 1975 Scientific American232(6) 76–88). In the current work, we use a dual-task paradigm to explore the role of attention in the processing of such stimuli. In two experiments we find striking differences in the degree to which direction-discrimination performance in point-light walker displays appears to rely on attention. Specifically, we find that performance in displays thought to involve top – down processing, either in time (experiment 1) or space (experiment 2) is adversely affected by dividing attention. In contrast, dividing attention has little effect on performance in displays that allow low-level, bottom – up computations to be carried out. We interpret these results using the active/passive motion distinction introduced by Cavanagh (1991 Spatial Vision5 303–309).

Most studies and theories of object recognition have addressed the perception of rigid objects. Yet, physical objects may also move in a nonrigid manner. A series of priming studies examined the conditions under which observers can recognize novel views of objects moving nonrigidly. Observers were primed with 2 views of a rotating object that were linked by apparent motion or presented statically. The apparent malleability of the rotating prime object varied such that the object appeared to be either malleable or rigid. Novel deformed views of malleable objects were primed when falling within the object's motion path. Priming patterns were significantly more restricted for deformed views of rigid objects. These results suggest that moving malleable objects may be represented as continuous events, whereas rigid objects may not. That is, object representations may be "dynamically remapped" during the analysis of the object's motion.

Depth rotations can reveal new object parts and result in poor recognition of “static” objects (Biederman & Gerhardstein, 1993) . Recent studies have suggested that multiple object views can be associated through temporal contiguity and similarity Edelman & Weinshall, 1991 , Lawson et al., 1994 , Wallis, 1996 . Motion may also play an important role in object recognition since observers recognize novel views of objects rotating in the picture plane more readily than novel views of statically re-oriented objects (Kourtzi & Shiffrar, 1997) . The series of experiments presented here investigated how different views of a depth-rotated object might be linked together even when these views do not share the same parts. The results suggest that depth rotated object views can be linked more readily with motion than with temporal sequence alone to yield priming of novel views of 3D objects that fall in between “known” views. Motion can also enhance path specific view linkage when visible object parts differ across views. Such results suggest that object representations depend on motion processes.

Recovering a reliable 3D percept from the retinal sampling of dynamic images requires the linkage of motion signals across space and time. In this paper, we review recent experimental results that enhance our understanding of the perceptual processes of motion integration, segmentation, and selection that are necessary to solve this inverse optics problem. Simple paradigms involving the presentation of moving contours are used to assess our ability to link sparse motion information. Our results indicate that human motion perception strongly depends on both primitive stimulus characteristics, such as contrast, eccentricity, and duration, as well as higher level characteristics such as feature classification and spatial configurations. Further, the perceived direction of a moving object depends little upon its familiarity. Finally, pursuit eye movements of compositional stimuli are highly correlated with perceived motion coherence. This ensemble of results is analysed within the context of current theories of motion perception.

How do people recognize an object in a novel orientation? Psychophysical and neurophysiological studies have suggested that extensive practice is required before observers can recognize an object that has been rotated to a new orientation Because object orientation frequently varies with object movement, we examined whether observers might more readily recognize a moving object in a new orientation Results from a priming study indicate that motion significantly and readily enhances the recognition of new object orientations when those orientations fall within the path of the motion That is motion promotes view-invariant object recognition without practice

To understand the visual analysis of biological motion, subjects viewed dynamic, stick figure renditions of a walker, car, or scissors through apertures. As a result of the aperture problem, the motion of each visible edge was ambiguous. Subjects readily identified the human figure but were unable to identify the car or scissors through invisible apertures. Recognition was orientation specific and robust across a range of stimulus durations, and it benefited from limb orientation cues. The results support the theory that the visual system performs spatially global analyses to interpret biological logical motion displays.

Previous research demonstrates that meaningfully related sounds enhance visual sensitivity to point-light displays of human movement. Here we report two psychophysical studies that investigated whether, and if so when, this facilitation is modulated by the temporal relationship between auditory and visual stimuli. In Experiment 1, participants detected point-light walkers in masks while listening to footsteps that were either synchronous or out-of-phase with point-light footfalls. The relative timing of auditory and visual walking did not impact performance. Experiment 2 further tested the importance of multisensory timing by disrupting the rhythm of the auditory and visual streams. Participants detected point-light walkers while listening to footstep or tone sounds that were either synchronous or temporally random with regards to point-light footfalls. Heard footsteps improved visual sensitivity over heard tones regardless of timing. Taken together, these results suggest that during the detection of others' actions, the perceptual system makes use of meaningfully related sounds whether or not they are synchronous. These results are discussed in relation to the unity assumption theory as well as recent empirical data that suggest that temporal correspondence is not always a critical factor in multisensory perception and integration.

Accurate interpretations of image motion require the segmentation of motion signals produced by different objects with the simultaneous integration of motion signals produced by the same object. We investigated a motion integration paradigm in which the direction of an object's motion could only be determined from an integration of motion signals across the disconnected object edges. In a series of experiments we show that observers' ability to determine object motion depends significantly upon stimulus duration, luminance contrast and edge width. These effects suggest that the visual system, after some delay, relies upon relatively thick, luminance defined contour discontinuities to segment moving images. Copyright © 1996 Elsevier Science Ltd.

A translating homogeneous edge viewed through an aperture is an ambiguous stimulus, while a translating edge discontinuity is unambiguous. Under what conditions does the visual system use unambiguous velocity estimates to interpret ambiguous velocity estimates? We considered a translating rectangle visible through a set of stationary apertures. One aperture displayed a rectangle edge while the other apertures displayed corners. Observers reported the direction in which the edge appeared to translate. The results suggest that collinearity and terminator proximity determine whether the unambiguous corner velocity was used to interpret the ambiguous edge velocity. These results suggest some of the ways in which the visual system controls the integration of velocity estimates across image space.

To interpret the projected image of a moving object, the visual system must integrate motion signals across different image regions. Traditionally, researchers have examined this process by focusing on the integration of equally ambiguous motion signals. However, when the motions of complex, multi-featured images are measured through spatially limited receptive fields, the resulting motion measurements have varying degrees of ambiguity. In a series of experiments, we examine how human observers interpret images containing motion signals of differing degrees of ambiguity. Subjects judged the perceived coherence of images consisting of an ambiguously translating grating and an unambiguously translating random dot pattern. Perceived coherence of the dotted grating depended upon the degree of concurrence between the velocities of the grating terminators and dots. Depth relationships also played a critical role in the motion integration process. When terminators were suppressed with occlusion cues, coherence increased. When dots and gratings were presented at different depth planes, coherence decreased. We use these results to outline the conditions under which the visual system uses unambiguous motion signals to interpret object motion.

I. Introduction Chapter 1: Making Great Strides: Advances in Research on the Perception of the Human Body Chapter 2: Gunnar Johansson, Events, and Biological Motion II. Psychophysics Chapter 3: Top-Down versus Bottom-up Processing of Biological Motion Chapter 4: Seeing You through Me: Creating Self-Other Correspondences for Body Perception Chapter 5: What Does " Really Mean? Differentiating Visual Percepts of Human, Animal, and Non-biological Motions Chapter 6: Shape-Independent Processing of Biological Motion Chapter 7: Action Perception from a Common Coding Perspective III. Development and Individual Differences Chapter 8: Developmental Origins of Biological Motion Perception Chapter 9: Experience and the Perception of Biological Motion Chapter 10: Variability in the Visual Perception of Human Motion as a Function of the Observer's Autistic Traits Chapter 11: Development of Body Motion Processing in Normalcy and Pathology IV. Social Perspectives Chapter 12: Person (Mis)Perception? On the Biased Representation of the Human Body. Chapter 13: It's the Way You Walk: Kinematic Specification of Vulnerability to Attack Chapter 14: Coordinating Social Beings in Motion Chapter 15: Functionalism Redux: How Adaptive Action Constrains Perception, Simulation, and Evolved Intuitions V. Neurophysiology Chapter 16: Neural mechanisms for action observation Chapter 17: Neural Mechanisms for Biological Motion and Animacy Chapter 18: The How, When, and Why of Configural Processing in the Perception of Human Movement Chapter 19: Brain Mechanisms for Social Perception: Moving towards an Understanding of Autism Chapter 20: From Body Perception to Action Preparation: A Distributed Neural System for Viewing Bodily Expressions of Emotion Chapter 21: Sensory and Motor Brain Areas Subserving Biological Motion Perception: Neuropsychological and Neuroimaging Studies Chapter 22: Computational Mechanisms of the Visual Processing of Action Stimuli Index

In demonstrations of apparent motion, observers typically report seeing motion along the shortest possible path between two sequentially presented objects. Recent work has demonstrated that violations of this shortest path rule occur with realistic photographs of a human body displayed for sufficiently long temporal intervals when a longer path is more anatomically plausible than the shortest path. The current set of experiments investigated the mechanisms by which information about biomechanical motion constrains apparent motion perception. In Experiment I, we demonstrated, first, that the availability of extra processing time does not simply—in and of itself—result in the perception of longer paths of apparent motion. Second, we rejected the hypothesis that the perception of biomechanically correct paths of apparent motion depends on biologically appropriate velocities. In Experiment 2, we discovered that the longer the motion path required to satisfy the biomechanical movement limitations of the stimulus, the longer the time needed to construct and therefore perceive that path. These findings together suggest that additional processing time is necessary, but not sufficient, for interpolations of longer paths.

Observers are remarkably sensitive to point-light displays of human movement. The Superior Temporal Sulcus (STS) and premotor cortex are implicated in the visual perception of point-light human actions and the integration of perceptual signals across modalities. These neurophysiological findings suggest that auditory information might impact visual sensitivity to point-light displays of human actions. Previous research has demonstrated that coincident, action-consistent sounds enhance visual sensitivity to the presence of coherent point-light displays of human movement. Here we ask whether visual detection sensitivity is modulated specifically by the meaningfulness of sounds that are coincident with observed point-light actions. To test this hypothesis, two psychophysical studies were conducted wherein participants detected the presence of coherent point-light walkers in a mask under unimodal or audiovisual conditions. Participants in audiovisual conditions heard either tones or actual footfalls coincident with the seen walkers' footsteps. Detection sensitivity increased when visual displays were paired with veridical auditory cues (footfalls), but not when paired with simple tones. The footfall advantage disappeared when the visual stimuli were inverted. These results suggest that the visual system makes use of auditory cues during the visual analysis of human action when there is a meaningful match between the auditory and visual cues.

Individual motion measurements are inherently ambiguous since the component of motion parallel to a homogeneous translating edge cannot be measured. Numerous models have proposed that the visual system solves this ambiguity through the integration of motion measurements across disparate contours. To examine this proposal, subjects observed a translating diamond through four stationary apertures. Since the diamond's motion could not be determined from any single contour, motion integration across contours was required to determine the diamond's direction of motion. We demonstrate that observers have difficulty accurately integrating motion information across space. Performance improved when the diamond stimulus was presented at 7 degrees eccentricity, through jagged apertures, or at low contrast. Taken together, these results imply that integration across space is more likely when the motion of contour terminators is less salient or reliable.

Observers viewed pairs of alternating photographs of a human body in different positions. Shortest-path motion solutions were pitted against anatomically possible movements. With short stimulus onset asynchronies (SOAs), observers tended to report the shortest path despite violations of anatomical constraints. However, with longer SOAs observers became increasingly likely to report the anatomically possible, but longer, paths. This finding, in conjunction with those from a second study, challenges the accepted wisdom that apparent motion paths are independent of the object. Instead, our findings suggest that when given enough time and appropriate stimuli, the visual system prefers at least some object-appropriate apparent motion paths.

Human observers demonstrate impressive visual sensitivity to human movement. What defines this sensitivity? If motor experience influences the visual analysis of action, then observers should be most sensitive to their own movements. If view-dependent visual experience determines visual sensitivity to human movement, then observers should be most sensitive to the movements of their friends. To test these predictions, participants viewed sagittal displays of point-light depictions of themselves, their friends, and strangers performing various actions. In actor identification and discrimination tasks, sensitivity to one's own motion was highest. Visual sensitivity to friends', but not strangers', actions was above chance. Performance was action dependent. Control studies yielded chance performance with inverted and static displays, suggesting that form and low-motion cues did not define performance. These results suggest that both motor and visual experience define visual sensitivity to human action.

Why are human observers particularly sensitive to human movement?' Seven experiments examined the roles of visual experience and motor processes In human movement perception by comparing visual sensitivities to point-light displays of familiar. unusual, and impossible gaits across gait-speed and identity discrimination tasks. In both tasks, visual sensitivity to physically possible gaits was superior to visual sensitivity to physically impossible gaits, supporting perception-action coupling theories of human movement perception. Visual experience influenced walker-identity perception but not gait-speed discrimination. Thus. both motor experience and visual experience defile visual sensitivity to human movement. An ecological perspective can he used to define the conditions necessary for experience-dependent sensitivity to human movement.

The visual interpretation of object motion requires the integration of motion information within the same object and segmentation of motion signals across different objects. How does the visual system balance these two requirements simultaneously? A series of experiments using a variety of methodologies suggests two answers to this question. Firstly, simple form cues direct the integration and segmentation of motion signals during the analysis of object motion. This suggests that motion perception is not a modular process. Secondly, motion analyses may differ significantly across different object categories. Behavioral, neurophysiological, and imaging studies suggest that the visual perception of human movement may benefit from a convergence of motor and visual processes in a manner that differs from other types of motion perception. Thus, interpretation of an object’s motion depends upon local form cues and object category.

Perception of apparent motion operates somewhat differently for objects and human figures. Depending on the interstimulus interval, the latter d may give rise to either perception of a direct path (i.e. biologically impossible) or indirect path (i.e. biologically possible). Here, PET was used to investigate whether a change in brain activity accompanies this perceptual shift. We found neural encoding of apparent motion to be a function of the intrinsic properties of the stimulus presented (object vs human) as well as the kind of human movement path perceived (biomechanically possible vs impossible). Motor and parietal cortex were only involved for possible motion which suggests that these regions are selectively activated to process actions which conform to the capabilities of the observer.

Psychophysical and neurophysiological studies suggest that human body motions can be readily recognized. Human bodies are highly articulated and can move in a nonrigid manner. As a result, we perceive highly dissimilar views of the human form in motion. How does the visual system integrate multiple views of a human body in motion so that we can perceive human movement as a continuous event? The results of a set of priming experiments suggest that motion can readily facilitate the linkage of different views of a moving human. Positive priming was found for novel views of a human body that fell within the path of human movement. However, no priming was observed for novel views outside the path of motion. Furthermore, priming was restricted to those views that satisfied the biomechanical constraints of human movement. These results suggest that visual representation of human movement may be based upon the movement limitations of the human body and may reflect a dynamic interaction of motion and object-recognition processes.

We report four experiments examining processes that contribute to the perception of point-light displays of human locomotion. In three experiments, we employed a simultaneous masking paradigm to examine the visual system’s use of configural information in global analyses of biological motion displays. In the fourth experiment, we obtained descriptions of our stimulus displays from naive observers. Performance in both the detection and identification studies suggests that the visual system responded equivalently to figures exhibiting any organization of limbs that is consistent with the human form. Moreover, the subconfigurations best detected were also most likely to be described independently as depicting a human figure. Thus our findings provide evidence that the visual system can exploit characteristic subconfigurations of the human form in the perception of human locomotion.

To function adeptly within our environment, we must perceive and interpret the movements of others. What mechanisms underlie our exquisite visual sensitivity to human movement? To address this question, a set of psychophysical studies was conducted to ascertain the temporal characteristics of the visual perception of human locomotion. Subjects viewed a computer-generated point-light walker presented within a mask under conditions of apparent motion. The temporal delay between the display frames as well as the motion characteristics of the mask were varied. With sufficiently long trial durations, performance in a direction discrimination task remained fairly constant across inter-stimulus interval (ISI) when the walker was presented within a random motion mask but increased with ISI when the mask motion duplicated the motion of the walker. This pattern of results suggests that both low-level and high-level visual analyses are involved in the visual perception of human locomotion. These findings are discussed in relation to recent neurophysiological data suggesting that the visual perception of human movement may involve a functional linkage between the visual and motor systems., peer-reviewed

In this study, the perceived speed of a tilted line translating horizontally (for a duration of 167 msec) is evaluated with respect to a vertical line undergoing the same translation. Perceived speed of the oblique line is shown to be underestimated when compared to the vertical line. This bias increases: (1) when the line is further tilted, (2) with greater line lengths, (3) with lower contrasts, and finally (4) with a speed of 2.1 deg/sec as compared to a higher speed of 4.2 deg/sec. These results may be accounted for by considering that two velocity signals are used by the visual system to estimate the speed of the line: the translation of this line (this signal does not depend on the line's orientation) and the motion component normal to the line (this signal depends on orientation). We suggest that these two signals are encoded by different types of units and that the translation signal is specifically extracted at the line endings. We further suggest that these signals are integrated by a weighted average process according to their perceptual salience. Other interpretations are considered at the Ught of current models dealing with the two-dimensional integration of different velocity signals.