Differential contribution of between and within‐brain coupling to movement synchronization

A fundamental characteristic of the human brain that supports behavior is its capacity to create connections between brain regions. A promising approach holds that during social behavior, brain regions not only create connections with other brain regions within a brain, but also coordinate their activity with other brain regions of an interaction partner. Here we ask whether between‐brain and within‐brain coupling contribute differentially to movement synchronization. We focused on coupling between the inferior frontal gyrus (IFG), a brain region associated with the observation‐execution system, and the dorsomedial prefrontal cortex (dmPFC), a region associated with error‐monitoring and prediction. Participants, randomly divided into dyads, were simultaneously scanned with functional near infra‐red spectroscopy (fNIRS) while performing an open‐ended 3D hand movement task consisting of three conditions: back‐to‐back movement, free movement, or intentional synchronization. Results show that behavioral synchrony was higher in the intentional synchrony compared with the back‐to‐back and free movement conditions. Between‐brain coupling in the IFG and dmPFC was evident in the free movement and intentional synchrony conditions but not in the back‐to‐back condition. Importantly, between‐brain coupling was found to positively predict intentional synchrony, while within‐brain coupling was found to predict synchronization during free movement. These results indicate that during intentional synchronization, brain organization changes such that between‐brain networks, but not within‐brain connections, contribute to successful communication, pointing to shift from a within‐brain feedback loop to a two‐brains feedback loop.