Does interpersonal movement synchronization differ from synchronization with a moving object?

We examined whether movement synchronization is different during coordination with another person than during coordination with a moving object. In addition, the influence of belief in the other person's agency was assessed. Participants synchronized their lower-arm movements with a computer-controlled rhythmic reference movement. The reference movements were pre-recorded, biological movements and were identical in all conditions. They were presented either by means of a confederate's arm in a motor-driven manipulandum or by means of movements of the manipulandum alone. To assess the influence of the belief in the confederate's agency, participants either were or were not informed that the confederate's movements were motor driven. The strength of coupling between the participant's movements and the reference movements was assessed in terms of the standard deviation of relative phase and the time needed to re-establish the coordination pattern after an unexpected perturbation of the reference signal. Mean relative phase indicated whether the participant was leading or lagging the reference movements. Coupling strength was not affected by the presence of another person in the coordination task, nor by the belief in this person's agency. However, participants had a stronger tendency to lead while synchronizing with the manipulandum, indicating that they responded differently to the observed kinematics of this moving object than to the kinematics of the confederate's arm movements, at least when the confederate's agency was assumed. Hence, although neither the involvement of another person nor the participant's belief in this person's agency affected coupling strength, the form of the coupling seemed to be influenced by the former factor, suggesting a different attunement to the reference movements during a joint-action situation. Future research is required to determine whether these interpretations extend to unintentional and bidirectional coordination, in which agency is not only assumed but actually effectuated.