| Abstract No.: | C-D3146 |
| Country: | Canada |
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| Title: | REACHING TO GRASP AS A SINGLE PROCESS NOT RELYING ON PLANNING OF MOTION COMPONENTS |
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| Authors/Affiliations: | 1 Fang Yang*; 1 Anatol Feldman;
1 Department of Physiology, University of Montreal, QC, Canada
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| Content: | Objectives. We tested the hypothesis that although reaching and grasping an object can be seen as consisting of several components, including the hand transport to and grasping the object, they are not programmed by the nervous system. Instead, they emerge following a single process that tends to minimize the distance and difference in shapes between the referent surface of the hand modified by the brain and the actual hand surface. Different movement components and the actual hand surface emerge following this minimization process constrained by central and reflex interactions of neuromuscular elements between themselves and with external forces acting before and during grasping the object. These elements thus behave together, as a coherent ensemble guided by the minimization process in achieving the motor goal. Mechanical perturbations that delay the performance of one movement component may postpone the appearance of other components. We tested this implication of the minimization hypothesis. Methods. In response to a sound signal, sited subjects moved the right (dominant) hand to reach and grasp an object (cube) placed within (experiment 1) or beyond (experiment 2) the reach of the arm. In experiment 2, subject not only moved the arm but also bended the trunk forward to reach and grasp the object. The vision was blocked simultaneously with the signal to move. In 30 % of randomly selected trials, the wrist or trunk motion in the respective experiments was prevented by an electromagnetic device at different phases of motion, for different time.
Results. When wrist motion was mechanically blocked (experiment 1), changes in the hand aperture ceased and resumed only after the wrist was unblocked. By varying the onset time and duration of perturbation, it was possible to cease these changes at any phase of motion, even then fingers began to close on the object. Similar influences on the hand aperture were observed when trunk motion was blocked in movements beyond the arm’s reach (experiment 2). Preventing the trunk motion, however, did not influence the hand transport to the object, unless the trunk block was initiated after the arm reached its extension limits.
Conclusions. The finding that changes in the hand aperture can be interrupted at any phase by arresting the wrist or trunk is consistent with the minimization hypothesis. The same hypothesis helps explain why the trunk arrest in experiment 2 influenced the hand aperture. It has been previously shown that the contribution of trunk motion to the hand movement extent in motion beyond the reach of the arm is initially neutralized by compensatory changes in the arm joint angles until the arm reaches its extension limits. Therefore, trunk arrest prior to this moment could not influence the hand transport. In contrast, changes in the hand aperture critically depended on whether or not the trunk motion could eventually bring the hand to the object, such that preventing the trunk motion discontinued the aperture changes. |
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