"Interpersonal coordination and observation are crucial to social interactions, whether we are engaging in conversation with other people, jointly assembling furniture, or performing in a musical duet. Previous research in social cognition has so far been mostly restricted to measuring behaviour and brain signatures of isolated individuals, in the absence of a real interaction. There has been a recent movement in the field, away from individuals in isolation and towards a joint action approach, employing studies of two or more individuals engaged in an interaction. Interpersonal mechanisms of joint action have largely been addressed by studying behavioural synchronization between people’s movements, gazes, and postures as they engage in real-time, joint, rhythmic actions. Recent studies have also begun to investigate physiological synchronization between romantic partners, choir singers, and performers and observers during high arousal rituals. However, the physiological mechanisms underlying joint action remain largely unknown, and their relationship to action coordination and observation has not yet been established. Here, we propose to investigate the relationship between interpersonal behavioural and physiological coupling at the autonomic nervous system level, during observation and participation in joint coordination. The first objective of the proposed project is to investigate the relationship between behavioural coupling and physiological coupling, during coordinated action in a musical setting. The second objective is to investigate physiological coupling between performers and observers of behavioural synchronization in online and offline scenarios. Finally, the third objective is to investigate the effect of physiological coupling, enabled using biofeedback training, on behavioural synchronization. This research takes a novel direction towards understanding coordination and human bonding beyond behaviour and brain activity."