Most animal species group together and coordinate their behavior in quite sophisticated manners for mating, hunting or defense purposes. From pouring water at the dinner table to the skillful coordination expressed by orchestras or dance ensembles, humans also manifest remarkable abilities in interpersonal behavioral coordination. Given the deep implication that interpersonal coordination has on brain development and adult cognitive functioning (1), these phenomena have been deemed as the dark matter of neuroscience (2). Indeed a large body of research has flourished to understand the conditions that promote behavioral coordination (3), the impact that better coordination has on social life (4) and the effect that psychiatric disorders have on coordination indexes that could eventually turn into objective biomarkers of pathology, disease progression and/or remission after pharmacological/behavioral treatment (5). Yet, a mechanistic understanding of how the neurobehavioral machinery required to organize our own behavior is flexibly tuned to adapt to others’ behavior is still missing. Current research deals with the ‘macroscopic’ structure of interpersonal rhythmic coordination (6, 7). That is, the spontaneous or intentional production of coordinated movements along a natural, task-related or negotiated common pace. Little has however been done to unearth the lower-level mechanisms by which our own action is finely adjusted online based on other’s action. Important advance has been made in the understanding of how cortical and subcortical neuronal networks issue commands to the spinal cord (and then muscles) and how the sensory reafference is integrated for feedback-based control within tight sensorimotor loops. While it is evident that mutual exchange of sensorimotor information is essential for behavioral co-adaptation, it is not clear how sensorimotor loops effectively come into play for integrating one’s own actions and the other’s (re)actions. The present project seeks to fill this fundamental gap by zooming into the ‘microscopic’ structure of interpersonal coordination. It is known that corrective force pulses are naturally engraved in our feedback-based movement control system (8). These subtle but highly consistent kinematic discontinuities (i.e., sub-movements) reflect the intermittent nature of motor control and represent a clear and objective proxy for the closing of visuo-motor loops (9, 10). Yet, no study has investigated whether sub-movements constitute the visible part of the low-level sensorimotor machinery by which coordination is achieved between individuals. Preliminary data collected by our group (3 behavioral experiments with more than 60 participants in total) show that sub-movements are reciprocally adjusted (‘inter-locked’) across individuals during interpersonal coordination. This novel and intriguing finding suggests that visuo-motor loop dynamics might couple across participants to optimize synchronization of the sense-and-correct process required to coordinate. This prior work sets an important theoretical as well as technical ground for the present project. In fact, we will exploit the same task (slow finger flexion-extension movements during solo performance, or during in-phase / anti-phase coordination with a partner), the same analytical tools (spectral decomposition and phase-locking analyses) and the same state-of-the-art research equipment (motion capture lab with 10 high resolution infra-red cameras) to uncover the individual-level and dyad-level neural mechanisms subtending coordination at the microscopic – sub-movements – scale. The research plan includes two electroencephalography (EEG, 64Chs) data acquisition sessions. The first one will explore individual-level neural dynamics of visuo-motor loop control during solo performance. Specifically, we will use directed connectivity metrics (e.g. Granger causality) and lag-dependent phase-locking estimates (Tomassini et al., In Press) to outline descending (motor) and ascending (visual) contributions to submovements-locked (within-) brain oscillatory activity. The second session will involve dual-EEG recording and will aim at tracking the (inter-) brain dynamics underlying sub-movements synchronization during dyadic interaction. To do so, we will exploit advanced time-resolved analytic approaches that will enable us to relate behavioral (between-partners) synchronization to corresponding inter-brain synchronization on a point-by-point fashion (i.e., by means of an index of “instantaneous” coupling strength/goodness). This will grant us a unique opportunity to make a substantial step forward towards understanding the low-level visuo-motor neural machinery at the basis of social coordination.