Coordination dynamics of reaching and grasping: pattern switching, hysteresis and trajectory formation

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 clock face. Object orientation was systematically varied as a control parameter in either a clockwise or counterclockwise direction. Two reaching patterns were defined: 1) forward projection with counterclockwise (pronation) wrist rotation; and 2) forward projection with clockwise (supination) wrist rotation. The relative timing between projection and rotation of wrist remained quite consistent across conditions. Switching between patterns and hysteresis were 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 not or did induce pattern switching. In the latter case, previously quiescent biomechanical degrees of freedom were spontaneously activated.