Time structure of a goal-directed bimanual skill and its dependence on task constraints.

The aim of the study was to elucidate the underlying principles of bimanual coordination and to establish quantitative coordination criteria. Healthy human subjects were instructed to open a loaded drawer with the left hand and to grasp, lift and reinsert with the right hand a small peg in the drawer recess. This bimanual goal-oriented task was executed promptly and consistently after a few trials. The temporal structure of the individual limb actions was assessed for computing interlimb synchronization and temporal correlation. In all subjects, both hands were well synchronized at the goal with high intermanual correlation in reaching the goal (event times of drawer opening and grasping the peg). This temporal goal-invariance was independent of movement speed and of the highly variable timing of the individual hands and persisted when subjects were blindfolded. Unilateral loading of the pulling hand and cutaneous anesthesia of the left index finger and thumb used for grasping the drawer handle significantly increased the pull-phase. This slowing of the left hand was matched by an adaptive delay of the right non-disturbed hand, thus preserving goal invariance. As a working hypothesis, we propose that multimodal sensory signals generated in the leading arm be transmitted centrally to re-parameterize the non-disturbed arm.