Subjective, not objective, socialness drives activity in the brain's third visual pathway

Recent work, including the third visual stream hypothesis, frames social perception as a largely automatic visual process. Simple animations of shapes engaged in "social" behaviors such as chasing have long been used to study social perception; however, both behavioral and neural responses are typically modeled against discrete, experimenter-assigned stimulus labels. This approach is insufficient, given that perceptions of such stimuli vary not only between people but also across trials. Here, we directly compared the neural responses to objective (stimulus-based) and subjective (response-based) socialness in an fMRI study in which the degree of chase between two agents was varied algorithmically. Participants (n=24) rated their perception of a chase on a continuous scale. Subjective ratings better predicted activity in visuo-social brain regions, predominantly in area MT, even after controlling for optic flow. Our findings underscore the visual nature of social cognition and highlight the importance of analyses sensitive to idiosyncrasies in perception.