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Johansson ( 1973) was among the first to show that human observers are able to recognize a variety of human actions based on displays consisting only of moving dots that trace the positions of a small set of locations on the body (i.e., point-light displays). The ability to perceive the particular kinds of motion exhibited by active biological agents (i.e., biological-motion perception) is something shared by a wide variety of animals, including human beings (Bertenthal & Pinto, 1994 Blake & Shiffrar, 2007 Troje, 2013 Troje & Westhoff, 2006). We relate our findings to previous proposals of a distinction between form-based and motion-based mechanisms in biological-motion perception.

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A series of follow-up experiments suggested that observers were relying on a form-based cue with the isolated hands (specifically, the orientation of their path through space) and a motion-based cue with the isolated feet to achieve such high efficiencies. We found that efficiency was highest when the hands and the feet were shown in isolation. We then compared human performance to that of a statistically optimal observer (Gold, Tadin, Cook, & Blake, 2008), giving us a measure of humans' discriminative ability independent of the information available (a quantity known as efficiency). To dissociate these two possibilities, we measured people's ability to discriminate the walking direction of stimuli in which individual body parts (feet, hands) were removed or shown in isolation. Alternatively, the motion of the feet may simply be more discriminable than that of other parts of the body. One account of this effect is that the human visual system may have evolved specialized processes that are efficient for extracting information carried by the feet (Troje & Westhoff, 2006). Previous research on such biological-motion perception has suggested that the movements of the feet may play a particularly important role in making certain judgments about locomotion (Chang & Troje, 2009 Troje & Westhoff, 2006). People can reliably infer the actions, intentions, and mental states of fellow humans from body movements (Blake & Shiffrar, 2007).

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