Social interaction increases brain synchrony during co-watching of novel movies

How does co-presence change our neural experience of the world? Can a conversation change how we synchronise with our partner during later events? Using fNIRS hyperscanning, we measured brain activity from 27 pairs of familiar adults simultaneously over frontal, temporal and parietal regions bilaterally, as they co-watched two different episodes of the BBC DipDap series. In-between the two episodes, each pair engaged in a face-to-face conversation on topics unrelated to the cartoon episodes. Brain synchrony was calculated using wavelet transform coherence and computed separately for real pairs and shuffled (pseudo) pairs. Findings reveal that real pairs showed increased brain synchrony over left Temporo-Parietal Junction (TPJ) and reduced brain synchrony over left Superior Parietal Lobe (SPL), compared to pseudo pairs (who had never seen each other and watched the same movie at different times). In addition, after a conversation, real pairs showed increased synchrony over right TPJ during subsequent novel movie co-watching compared to pseudo pairs. Crucially, conversation was unrelated to the content of the movie. This data suggests that social factors (co-presence and interaction) modulate brain synchrony during non-interactive shared experiences, even for later unrelated events. This happens via both coupling and de-coupling of brain activity, which may reflect mechanisms of attunement and shared common-ground on one hand, and internal cognitive processes such as body self-consciousness and body-ownership on the other.