Interpersonal Neural Synchrony Across Levels of Interpersonal Closeness and Social Interactivity

Interpersonal neural synchrony is a key marker of social interactions, offering insights into the neural mechanisms underlying human connection and developmental outcomes. So far, hyperscanning studies have examined synchrony across diverse dyads and tasks, leading to inconsistencies and limiting cross-study comparability. This variability challenges the establishment of a unified theoretical framework for neural synchrony. This study investigated the effects of interpersonal closeness and social interactivity on neural synchrony using functional near-infrared spectroscopy hyperscanning. We recorded brain activity from 142 dyads (70 close-friend, 39 romantic-partner, and 33 mother-child dyads) across three interaction conditions: video co-watching (passive), a cooperative game (structured active), and free interaction (unstructured active). Neural synchrony was computed between participants' bilateral inferior frontal gyrus (IFG) and temporoparietal junction (TPJ) using wavelet transform coherence. Results showed that true dyads exhibited significantly higher synchrony than non-interacting surrogate dyads (q < .001), particularly in combinations involving the right IFG, left IFG, and left TPJ. Mother-child dyads displayed lower synchrony than adult-adult dyads at the global (p < .001) and local level of analysis. At the global level, synchrony was highest during video co-watching, followed by the cooperative game and free interaction (p < .001). However, left IFG-left IFG and left IFG-right TPJ synchrony peaked during the cooperative game. These findings highlight the role of brain maturation and social task structure in shaping neural synchrony. By providing an experimental framework, this study lays the groundwork for future hypothesis-driven hyperscanning research, advancing our understanding of the neural mechanisms underlying human social interactions.