Short-term dynamic changes in interbrain synchrony during first social interaction between strangers

• Movement synchronization increases during an acquaintance among strangers • Movement synchronization predicts the dyadic feeling of connectedness • Interbrain synchrony gradually increases during the acquaintance • Dynamic changes in Interbrain synchrony were found in the left dlPFC-left IFG and the right PMC-right IFG • Interbrain synchrony in the right PMC-right IFG predicted movement synchronization Emerging evidence highlights the role of interbrain synchrony in fostering the formation of relationships. Yet a key question remains: how does interbrain synchrony dynamically evolve throughout a first meeting between strangers, and to what extent do interbrain networks exhibit dynamic changes during the interaction? In this study, we investigate short-term changes in behavioral and interbrain synchrony during initial face-to-face encounters between strangers. To assess experience-dependent changes in interbrain synchrony, we tracked these dynamics with functional near-infrared spectroscopy (fNIRS) over a five-minute interaction period, and examined whether these changes predict changes in movement synchronization. A total of 106 previously unacquainted participants were randomly paired into dyads and instructed to engage in a five-minute acquaintance conversation. Results indicate that movement synchronization increased throughout the interaction and predicted dyadic connectedness. The neuroimaging findings revealed heightened interbrain synchrony in the dorsolateral prefrontal cortex (dlPFC), inferior frontal gyrus (IFG), and premotor cortex (PMC). Critically, we observed a gradual increase in interbrain synchrony in the l.dlPFC- l.IFG and the r.PMC-r.IFG. Finally, we showed that interbrain synchrony in the r.PMC-r.IFG predicted movement synchronization only at the end of the meeting. These findings position interbrain synchrony as a rapidly plastic, experience-dependent coupling mechanism that emerges during first-time natural conversation, rather than a static correlate of shared state. It shows that minute-by-minute increases in behavioral and neural alignment play a foundational role in the formation of social bonds, potentially through motor pathways.