The relevance to social interaction modulates bistable biological-motion perception

Social interaction, the process through which individuals act and react toward each other, is arguably the building block of society. As the very first step for successful social interaction, we need to derive the orientation and immediate social relevance of other people: a person facing toward us is much more likely to initiate communications than a person who is back to us. Reversely, however, it remains elusive whether the relevance to social interaction modulates how we perceive the other's orientation. Here, we adopted the bistable point-light walker (PLW) which is ambiguous in its in-depth orientation. Participants were asked to report the orientation (facing the viewer or facing away from the viewer) of the PLWs. Three factors that are task-irrelevant but critically pertinent to social interaction, the distance, the speed, and the size of the PLW, were systematically manipulated. The nearer a person is, the more likely it initiates interactions with us. The larger a person is, the larger influence it may exert. The faster a person is, the shorter time is left for us to respond. Results revealed that participants tended to perceive the PLW as facing them more frequently than facing away when the PLW was nearer, faster, or larger. These same factors produced different patterns of effects on a non-biological rotating cylinder. These findings demonstrate that the relevance to social interaction modulates the visual perception of biological motion and highlight that bistable biological motion perception not only reflects competitions of low-level features but is also strongly linked to high-level social cognition.