MULTIFUNCTIONALITY AND SWITCHING IN THE COORDINATION DYNAMICS OF REACHING AND GRASPING

The coordination dynamics of reaching and grasping an object was studied kinematically. Subjects reached for, grasped and rotated a bar that was positioned at thirty-five different orientations corresponding to positions on a clockface. Object orientation was systematically varied as a control parameter in either a clockwise or counterclockwise direction. Two reaching patterns were defined: (1) forward projection of the hand with supination of the forearm (anti-phase reach); and (2) forward projection of the hand with pronation of the forearm (in-phase reach). The relative timing between wrist forward projection and wrist angular rotation remained quite consistent across conditions. Switching between patterns and hysteresis was observed for a range of object positions. Further, perturbations of the bar across the hysteresis region induced switching between reach patterns. The trajectory of the end effector, whether rectilinear or curvilinear, was dependent on whether the perturbation did or did not induce pattern switching. In the latter case, previously quiescent biomechanical degrees of freedom were spontaneously activated.