Repeated interpersonal interaction leads to generalized long-term changes in interbrain synchrony: evidence from a psychotherapy study

Social interaction is increasingly conceptualized as a process of brain-to-brain coupling, as high-quality, close interactions involve synchrony in brain activity between the people involved. However, whether such coupling can undergo long-term plastic change remains unclear. The current study examines whether inter-brain synchrony is not only an ephemeral byproduct of a single interaction, but a stable mechanism underlying overall interpersonal functioning. We use recurring interactions during psychotherapy as a test case, in a longitudinal design informed by precision neuroscience.We used longitudinal functional near-infrared spectroscopy (fNIRS) hyperscanning to examine twenty-one participants with major depressive disorder across sixteen psychotherapy sessions, with eight hyperscanning sessions per participant (every other session, total N = 143 40-min imaging sessions). We also measured inter-brain synchrony between patients and clinical interviewers before, at the middle of and after the 16 weeks (N = 48 interviews with imaging). Inter-brain synchrony between patients’ right inferior frontal gyrus (IFG) and all therapist brain regions significantly increased over the course of therapy, while synchrony between patients’ left pre-motor region and all therapist regions significantly decreased. These changes were associated with better perceived overall interpersonal functioning. Finally, overall inter-brain synchrony between patients and interviewers significantly increased. These findings indicate that repeated interpersonal interaction induces long-term changes in inter-brain synchrony, consistent with experience-dependent inter-brain plasticity, aligned with changes in overall interpersonal functioning. The brain regions involved suggest a shift from motion attention to emotional understanding. Thus, our findings demonstrate that repeated interpersonal interactions can invoke durable inter-brain plasticity that generalizes to other relationships.