Seeing Social: A Neural Signature for Conscious Perception of Social Interactions

Social information is some of the most ambiguous content we encounter in our daily lives, yet in experimental contexts, percepts of social interactions-that is, whether an interaction is present and if so, the nature of that interaction-are often dichotomized as correct or incorrect based on experimenter-assigned labels. Here, we investigated the behavioral and neural correlates of subjective (or conscious) social perception using data from the Human Connectome Project in which participants ( n = 1049; 486 men, 562 women) viewed animations of geometric shapes during fMRI and indicated whether they perceived a social interaction or random motion. Critically, rather than experimenter-assigned labels, we used observers' own reports of "Social" or "Non-social" to classify percepts and characterize brain activity, including leveraging a particularly ambiguous animation perceived as "Social" by some but "Non-social" by others to control for visual input. Behaviorally, observers were biased toward perceiving information as social (vs non-social); and neurally, observer reports (compared with experimenter labels) explained more variance in activity across much of the brain. Using "Unsure" reports, we identified several regions that responded parametrically to perceived socialness. Neural responses to social versus non-social content diverged early in time and in the cortical hierarchy. Finally, individuals with higher internalizing trait scores showed both a higher response bias toward "Social" and an inverse relationship with activity in default mode and visual association areas while scanning for social information. Findings underscore the subjective nature of social perception and the importance of using observer reports to study percepts of social interactions. SIGNIFICANCE STATEMENT Simple animations involving two or more geometric shapes have been used as a gold standard to understand social cognition and impairments therein. Yet, experimenter-assigned labels of what is social versus non-social are frequently used as a ground truth, despite the fact that percepts of such ambiguous social stimuli are highly subjective. Here, we used behavioral and fMRI data from a large sample of neurotypical individuals to show that participants' responses reveal subtle behavioral biases, help us study neural responses to social content more precisely, and covary with internalizing trait scores. Our findings underscore the subjective nature of social perception and the importance of considering observer reports in studying behavioral and neural dynamics of social perception.