Dissecting social interaction: Dual-fMRI reveals patterns of interpersonal brain-behaviour relationships that dissociate among dimensions of social exchange.

During social interactions, each individual's actions are simultaneously a consequence of and an antecedent to their interaction partner's behaviour. Capturing online the brain processes underlying such mutual dependency requires simultaneous measurements of all interactants' brains during real-world exchange ("hyperscanning"). This demands a precise characterisation of the type of interaction under investigation, however, and analytical techniques capable of capturing interpersonal dependencies. We adapted an interactive task capable of dissociating between two dimensions of inter-dependent social exchange: goal structure (co-operation vs. competition) and interaction structure (concurrent vs. turn-based). Performing dual-fMRI hyperscanning on pairs of individuals interacting on this task, and modelling brain responses in both interactants as systematic reactions to their partner's behaviour, we investigated interpersonal brain-behaviour dependencies (iBBD) during each dimension. This revealed patterns of iBBD that differentiated among exchanges: In players supporting the actions of another, greater brain responses to the co-player's actions were expressed in regions implicated in social cognition, such as the medial prefrontal cortex, precuneus, and temporal cortices. Stronger IBBD during concurrent competitive exchanges was observed in brain systems involved in movement planning and updating, however, such the supplementary motor area. This demonstrates the potential for hyperscanning to elucidate neural processes underlying different forms of social exchange.