Handing over a cube: spatial features of physical joint-action.

Even though joint action is highly developed in humans, not much is known about motor control in physical joint-action tasks. Here we investigated a physical handover task: one subject sequentially passed wooden cubes to another without communicating verbally. Temporal parameters such as reaction time decreased on a trial-to-trial basis, showing that the efficiency of the task is optimized on-line by implicit negotiation between the partners. In contrast, the spatial position of the handover was found to be invariant and trial-independent. Thus, our results suggest that physical joint-action is guided by on-line adaptation and a priori assumptions.