Temporal constraints in the control of prehensile movement.

Three experiments were conducted to investigate the control of the manipulation (i.e., finger-thumb aperture) and transportation (i.e., wrist velocity) components in prehensile movement (Jeannerod, 1981, 1984). In all experiments, subjects were seated and instructed to grasp a dowel mounted on a joystick following a discrete movement over a set distance. Thus, the amount of dowel movement following the grasp could be determined. In Experiment 1, the tolerance (i.e., the amount of allowable dowel movement) was manipulated using a computer-generated boundary around the dowel. The results indicated that the transportation component changed dependent on the tolerance condition, and there were trends that maximum aperture was also affected. Experiment 2 manipulated both tolerance and dowel size (i.e., diameter) factorially in a within-subject design. Dowel size affected only the manipulation component, supporting Jeannerod's (1981) earlier work, but tolerance clearly influenced both components. Experiment 3 investigated Wing, Turton, and Fraser's (1986) proposition that speed of movement influences aperture size. Distance and movement time were combined factorially to produce conditions with different average velocities. Maximum aperture was dependent on the movement time rather than the speed of movement. The relation between the control of the components was examined by using a new method of calculating within-trial correlations between aperture size and wrist velocity in Experiments 2 and 3. The correlations were related to the temporal aspects of the movement with higher correlations in the rapid movement time conditions. Also, the temporal occurrence of maximum aperture remained invariant across the different movement conditions. In general, the results suggest a strong functional linkage between the two components, which may be dependent on the temporal characteristics of the movement.