Interpersonal visual interaction induces local and global stabilisation of rhythmic coordination.

Perceptual coupling between people can lead to the spontaneous synchronisation of their rhythmic movements. In the current study, we hypothesised that the sight of a co-actor generates anchoring (local stabilisation around specific spatiotemporal points within movement cycles), and that such anchoring supports the occurrence and stability of spontaneous interpersonal synchronisation (global stabilisation across cycles). To test these hypotheses, we re-examined previously published data from a study where participants were required to perform auditory-motor coordination of whole-body movements with versus without visual contact. Paired participants performed two kinds of coordination task - either knee flexion or extension repeatedly with auditory metronome beats (Flexion-on-the-beat and Extension-on-the-beat conditions) while standing face-to-face or back-to-back to manipulate visual interaction. The analysis of individual movement trajectories showed that visual interaction led to decreased variability along the entire trajectory, except the maximum extension position. The results also indicated that the strength of this anchoring was correlated with the degree to which the variability of interpersonal phase relations decreased with visual coupling, suggesting that local stabilisation supported global interpersonal stabilisation. Therefore, the sight of a co-actor generates anchoring effects that may play a crucial role in the stabilisation of spontaneous interpersonal synchronisation.