Seeing relationships: The specialization for a two-body shape in human visual perception

Abstract The human social nature manifests in, among other ways, a particular visual sensitivity to social entities such as faces and bodies. But, the core of a social representation is not as much the social entity as the relation that binds multiple entities together. We asked whether human vision exhibits a special sensitivity to socially relevant spatial relations, beyond the visual sensitivity to likely members of those relations. Some social relations reliably correlate with spatial relations: people are likely to physically interact face-to-face more than back-to-back. Using functional MRI and behavioral measures, we show that visual sensitivity to social stimuli extends to images encompassing two bodies facing toward (vs. away from) each other. In particular, the visual-perception lateral occipital cortex showed an organization with the inferior part encoding of the number of bodies (one vs. two) and the superior part, encoding the spatial relation between bodies (facing vs. nonfacing). Moreover, the functionally localized body-selective cortex responded to facing bodies more strongly than to identical but not facing bodies. Finally, a multivariate pattern analysis showed that representations of single bodies in facing (vs. nonfacing) dyads were sharpened, suggesting that the spatial positioning cuing interaction put pressure on the discrimination of body postures. Finally, the cost of body inversion (upside-down rotation) on body recognition, a behavioral signature of the specialized mechanism for body perception, was larger for facing than for nonfacing dyads. Thus, the human visual perception system registers spatial relations between multiple bodies, with tuning for those that cue social interaction. Public Significance Statement The humans’ social nature shapes visual perception. Here, we show that human vision is not only attuned to socially relevant entities, such as bodies, but also to socially relevant spatial relations between those entities. The body-selective visual cortex responds more strongly to multiple bodies that appear to interact (i.e., face-to-face), relative to unrelated bodies, and shows more accurate representation of single body postures in interacting scenarios. Moreover, recognition of facing bodies is particularly susceptible to perturbation by upside-down rotation, indicative of a particular visual sensitivity to the canonical appearance of facing bodies. Encoding of relations between multiple bodies, in the same areas for body-shape recognition, suggests that the context in which a body is seen, deeply affects its processing.