Body movement perception is shaped by both generic and actor-specific models of human bodies

Body movement perception is shaped by knowledge of the human body biomechanics. Apparent motion elicited by rapidly alternating pictures follows the shortest path between two body postures only if it is biomechanically plausible. And although we tend to perceive the location of a moving body part as slightly shifted forward along its trajectory, this perceptual extrapolation is absent (or reduced) if the movement would have been unlikely to continue along the same trajectory because of the body biomechanical constraints. The received view is that perception is shaped by a model of the observers own body. Here, we report three lines of evidence suggesting otherwise. First, we report the typical influence of knowledge of the upper limb biomechanics on apparent movement perception and perceptual extrapolation in two individuals born completely without (upper and lower) limbs (Experiments 1 and 2). Second, we report that we failed to find evidence that observers own biomechanics influence perceptual extrapolation of body movements (Experiment 3). Third, we show that participants perception is influenced by their knowledge of actor-specific biomechanics (Experiments 4- 8). We conclude that body movement perception is shaped by visually acquired models of both generic and actor-specific body biomechanics. Public significance statementAlthough information about others body movements reaches our visual system with a considerable delay, we can interact with them in real-time. Although we often blink, we perceive uninterrupted body movements. To solve the problems posed by informational lag and intermittence, the visual system appears to rely on knowledge of the human body biomechanics to extrapolate the most likely future (or unseen) position of observed limb movements. For the last 20 years, the received view has been that perceptual "filling-in" and extrapolation of others body movements are underpinned by a model of the observers own body, acquired through motor experience and proprioceptive feedback. Here, we report three lines of evidence indicating that body movement perception is rather shaped by visually acquired models of both generic and actor-specific body biomechanics. These findings provide new insight into how the human visual system extrapolates future body positions.