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When viewing ambiguous stimuli, people tend to perceive some interpretations more frequently than others. Such perceptual biases impose various types of constraints on visual perception, and accordingly, have been assumed to serve distinct adaptive functions. Here we demonstrated the interaction of two functionally distinct biases in bistable biological motion perception, one regulating perception based on the statistics of the environment – the viewing-from-above (VFA) bias, and the other with the potential to reduce costly errors resulting from perceptual inference – the facing-the-viewer (FTV) bias. When compatible, the two biases reinforced each other to enhance the bias strength and induced less perceptual reversals relative to when they were in conflict. Whereas in the conflicting condition, the biases competed with each other, with the dominant percept varying with visual cues that modulate the two biases separately in opposite directions. Crucially, the way the two biases interact does not depend on the dominant bias at the individual level, and cannot be accounted for by a single bias alone. These findings provide compelling evidence that humans robustly integrate biases with different adaptive functions in visual perception. It may be evolutionarily advantageous to dynamically reweight diverse biases in the sensory context to resolve perceptual ambiguity.

Point-light walkers have been useful to study the contribution of form and motion to biological motion perception by manipulating the lifetime, number, or spatial distribution of the light points. Recent studies have also manipulated the light points themselves, replacing them with small images of objects. This manipulation degraded the recognizability of biological motion, particularly for local images of human bodies. This result suggests an interference of body form information in the local images with the body form analysis necessary for global biological motion recognition at the global level. We further explored this interference with respect to its selectivity for body orientation and motion. Participants had to either discriminate the facing direction (left/right) or the walking direction (forward/backward) of a global walker composed of local stick figures that could face left or right and either stand still or walk forward or backward. Local stick figures interfered stronger with the facing direction task if they were facing in the same direction as the global walker. Walking (forward/backward/static) of the stick figures influenced neither the facing direction task nor the walking direction task. We conclude that the interference is highly specific since it concerns not only the category (human form), but even the facing direction.

Recent models have proposed a two-stage process of biological motion recognition. First, template or snapshot neurons estimate the body form. Then, motion is estimated from body form change. This predicts separate aftereffects for body form and body motion. We tested this prediction. Observers viewing leftward- or rightward-facing point-light walkers that walked forward or backward subsequently experienced oppositely directed aftereffects in stimuli ambiguous in the facing or the walking direction. These aftereffects did not originate from adaptation to the motion of the individual light points, because they occurred for limited-lifetime stimuli that restrict local motion. They also occurred when the adaptor displayed a random sequence of body postures that did not induce the walking motion percept. We thus conclude that biological motion gives rise to separate form and motion aftereffects and that body form representations are involved in biological motion perception.

Biological motion is easily perceived by neurotypical observers when encoded in point-light displays. Some but not all relevant research shows significant deficits in biological motion perception among those with ASD, especially with respect to emotional displays. We tested adults with and without ASD on the perception of masked biological motion and the perception of direction from coherent and scrambled biological motion. Within the autism spectrum group, there was a large and statistically significant relationship between IQ and the ability to perceive directionality in masked biological motion. There were no group differences in sensitivity to biological motion or the ability to identify the direction of motion. Possible explanations are discussed, including the possible use of compensatory strategies in high IQ ASD.

Typically developing human infants preferentially attend to biological motion within the first days of life 1 . This ability is highly conserved across species 2,3 and is believed to be critical for filial attachment and for detection of predators 4 . The neural underpinnings of biological motion perception are overlapping with brain regions involved in perception of basic social signals such as facial expression and gaze direction 5 , and preferential attention to biological motion is seen as a precursor to the capacity for attributing intentions to others 6 . However, in a serendipitous observation 7 , we recently found that an infant with autism failed to recognize point-light displays of biological motion, but was instead highly sensitive to the presence of a non-social, physical contingency that occurred within the stimuli by chance. This observation raised the possibility that perception of biological motion may be altered in children with autism from a very early age,withcascading consequencesforbothsocialdevelopment and the lifelong impairments in social interaction that are a hallmark of autism spectrum disorders 8 . Here we show that two-year-olds withautismfailtoorienttowardspoint-lightdisplaysofbiological motion, and their viewing behaviour when watching these pointlight displays can be explained instead as a response to non-social, physicalcontingencies—physicalcontingenciesthataredisregarded by control children. This observation has far-reaching implications for understanding the altered neurodevelopmental trajectory of brain specialization in autism 9 . Preferentialattentiontobiologicalmotionisafundamentalmechanism facilitating adaptive interaction with other living beings. It is present throughout a wide range of species, from humans 10,11 to monkeys 12 to birds 13 . Developmentally, it can be found in newly hatched chicks 14 and in human infants as young as 2days old 1 . Recognitionofbiologicalmotionremainsintactinavarietyofforms, from degraded presentations, through varying states of occlusion, and in cases when information-bearing components are reduced to

We addressed the issue of how display orientation affects the perception of biological motion. In Experiment 1, spontaneous recognition of a point-light walker improved abruptly with image-plane display rotation from inverted to upright orientation. Within a range of orientations from 180 degrees to 90 degrees, it was dramatically impeded. Using ROC analysis, we showed (Experiments 2 and 3) that despite prior familiarization with a point-light figure at all orientations, its detectability within a mask decreased with a change in orientation from upright to a range of 90 degrees-180 degrees. In Experiment 4, a priming effect in biological motion was observed only if a prime corresponded to a range of deviations from upright orientation within which the display was spontaneously recognizable. The findings indicate that display orientation nonmonotonically affects the perception of biological motion. Moreover, top-down influence on the perception of biological motion is limited by display orientation.

One of the more stunning examples of the resourcefulness of human vision is the ability to see 'biological motion', which was first shown with an adaptation of earlier cinematic work: illumination of only the joints of a walking person is enough to convey a vivid, compelling impression of human animation, although the percept collapses to a jumble of meaningless lights when the walker stands still. The information is sufficient to discriminate the sex and other details of the walker, and can be interpreted by young infants. Here we measure the ability of the visual system to integrate this type of motion information over space and time, and compare this capacity with that for viewing simple translational motion. Sensitivity to biological motion increases rapidly with the number of illuminated joints, far more rapidly than for simple motion. Furthermore, this information is summed over extended temporal intervals of up to 3 seconds (eight times longer than for simple motion). The steepness of the summation curves indicates that the mechanisms that analyse biological motion do not integrate linearly over space and time with constant efficiency, as may occur for other forms of complex motion, but instead adapt to the nature of the stimulus.

The sex of human walkers can be recognized without familiarity cues from displays of pointlight sources mounted on major joints. Static versions of these abstract displays do not permit accurate recognition of sex. Variation in the degree of armswing or in walking speed generally interferes with recognition, except that faster speeds are associated somewhat with improved recognition of females. Lights on upper-body joints permit more accurate guesses than do Lights on lower-body joints, but identification is possible even from minimal displays, with lights placed only on the ankles. No feedback was given to observers. Confidence judgments of sex relate to the accuracy of responses in a manner that suggests that viewers know what they are doing.

Several temporal and spatial factors affect gender recognition of a walker when portrayed, without familiarity cues, as a dynamic point-light display. We demonstrate that, among temporal parameters, the duration of the dynamic stimulus must be longer than 1.6 sec, but that 2.7 sec is fully adequate. Given the speed of our walkers, the recognition threshold appears to be roughly two step cycles. In addition, presentation rate of the stimulus must be near to normal, perhaps because nonnormal rates alter apparent gravity and obscure the normal relationship between output and conservation of energy. We demonstrate that, among spatial factors, the discreteness of the joint information must be maintained for accurate recognition. We go on to argue that it is the information about the shoulder and the hip of a walker that is of primary importance. Finally, inversion of the stimulus display produces the unexpected effect of reversing the apparent sex of most walkers. That is, when presented upside down, male walkers appear female and female walkers appear male.

In three experiments we tried to mask the motions of human gait. We represented human walkers as a set of 11 computer-generated elements on a display monitor, moving as a nested hierarchy of motions that mimicked the motions of the head and major joints. The walker was seen in sagittal view, facing either right or left and walking as if on a treadmill. On the walker was superimposed a simultaneous mask composed of elements with the same brightness, shape, and subtense as those of the walker. We varied the mask parameters—particularly the number of elements and style of motion—to discern what masks best camouflaged the walker’s direction. In general, many kinds of masks impeded viewer performance at durations of 200 msec, but only relatively complex masks continued to impede performance to 400 msec and beyond. Four results stand out concerning the concurrent perceptual organization of target and mask. First, if the mask is easily divided into groups by its motion parameters, viewer performance with respect to the stimulus is generally impeded by increasing the number of groups in the mask. Second, the most successful masks are those composed of scrambled parts of walkers. Third, given a sufficient number of scrambled-walker elements, viewer performance does not improve above chance even at 800 msec. And fourth, this lack of improvement appears to be confined to scrambled-walker masks that share the particular gait parameters of the walker target.

Viewers can recognize themselves and others in an abstract display of their movements. Light sources mounted on joints prominent during the act of walking are sufficient cues for identification. No other information, no feedback, and little practice with such a display are needed. This procedure, developed by Johansson, holds promise for inquiry into the dimensions and features of event perception: It is both naturalistic and experimentally manageable.

This paper reports the first phase of a research program on visual perception of motion patterns characteristic of living organisms in locomotion. Such motion patterns in animals and men are termed here as biological motion. They are characterized by a far higher degree of complexity than the patterns of simple mechanical motions usually studied in our laboratories. In everyday perceptions, the visual information from biological motion and from the corresponding figurative contour patterns (the shape of the body) are intermingled. A method for studying information from the motion pattern per se without interference with the form aspect was devised. In short, the motion of the living body was represented by a few bright spots describing the motions of the main joints. It is found that 10–12 such elements in adequate motion combinations in proximal stimulus evoke a compelling impression of human walking, running, dancing, etc. The kinetic-geometric model for visual vector analysis originally developed in the study of perception of motion combinations of the mechanical type was applied to these biological motion patterns. The validity of this model in the present context was experimentally tested and the results turned out to be highly positive.