Neural and behavioral dynamics of dyadic rhythm coordination across limb pairings.

Interpersonal motor synchronization relies on precise neural coordination, yet its underlying brain mechanisms remain incompletely understood. Guided by mutual prediction theory, we investigated how temporal structure and effector-specific constraints shape dyadic coordination. Using functional near-infrared spectroscopy (fNIRS) hyperscanning, we recorded inter-brain activity from pairs of participants performing a synchronization-continuation tapping task. The task included three types of effector pairings (hand-hand, hand-foot, foot-foot) and two rhythmic contexts (with or without temporal subdivision). Behaviorally, external rhythms enhanced synchronization accuracy, while rhythmic subdivision impaired temporal alignment. Neuroimaging findings revealed increased inter-brain synchronization (IBS) in the left temporoparietal junction and motor cortex during foot-foot coordination, potentially reflecting adaptive and task-dependent engagement of neural networks under higher control demands. IBS in the motor cortex was also enhanced in the continuation phase, consistent with greater reliance on internal timing. Brain-behavior associations indicated that different effectors engage distinct neural systems in supporting temporal alignment. These findings demonstrate that interpersonal synchronization is shaped by both temporal structure and bodily effectors, with upper- and lower-limb coordination relying on partially dissociable neural dynamics. The results provide novel insights into the neural dynamics of joint motor behavior, contributing to our understanding of embodied cognition and the neural bases of human interaction.