Processing third-party social interactions in the human infant brain.

The understanding of developing social brain functions during infancy relies on research that has focused on studying how infants engage in first-person social interactions or view individual agents and their actions. Behavioral research suggests that observing and learning from third-party social interactions plays a foundational role in early social and moral development. However, the brain systems involved in observing third-party social interactions during infancy are unknown. The current study tested the hypothesis that brain systems in prefrontal and temporal cortex, previously identified in adults and children, begin to specialize in third-party social interaction processing during infancy. Infants (N = 62), ranging from 6 to 13 months in age, had their brain responses measured using functional near-infrared spectroscopy (fNIRS) while viewing third-party social interactions and two control conditions, infants viewing two individual actions and infants viewing inverted social interactions. The results show that infants preferentially engage brain regions localized within the dorsomedial prefrontal cortex when viewing third-party social interactions. These findings suggest that brain systems processing third-party social interaction begin to develop early in human ontogeny and may thus play a foundational role in supporting the interpretation of and learning from social interactions.