Kinematics of a coordinated goal-directed bimanual task

The experiments address the problem of bimanual coordination in a familiar task of everyday life. A goal-directed drawer-pulling task, with asymmetrical assignments among hands, was analyzed with the objective to detect discrete kinematic events ('anchors') that potentially could serve in proper goal synchronization. The left hand reached out for the drawer and opened it while the right hand performed a prehension movement to pick up a peg from the drawer. The task was smoothly performed, independently of vision. Typically, trajectories and velocity profiles of the leading pull-hand were more stereotypical than the more variable ones of the pick-hand. The pull-hand had a large velocity peak during reaching, followed by a small peak during pulling. Velocity profiles of the pick-hand were not bell-shaped and exhibited one or two broad waves, often with an irregular and probing evolution. Velocity profiles of both hands were aligned with the first or the second velocity peak of the leading pull-hand. In the majority of cases, temporal associations of events in the kinematics of the two limbs could thus be identified, which could serve to synchronize the hands at the goal. The nearly straight biphasic reach-and-pull trajectory of the leading hand contrasted with the more curved trajectory of the right pick-hand whereas, in the same unimanual action, the latter trajectories were quasi-rectilinear. Changing constraints (no vision, cutaneous anesthesia of pulling fingers) could change the coordination pattern. We argue that bimanual coordination relies on two interacting mechanisms: (1) feedforward control on the basis of sensorimotor memory; (2) temporal adjustments during the evolving bimanual synergy. Multiple strategies, imposed by the leading pull-hand, appeared to be responsible for feedback-induced corrections in the pick-hand and were found to contribute to the goal-invariance and to the principle of motor equivalence.