Selective perturbation of visual input during prehension movements
Subjects were instructed to reach and grasp cylindrical objects, using a precision grip. The objects were two concentric dowels made of translucent material placed at 35 cm from the subject. The inner (“small”) dowel was 10 cm high and 1.5 cm in diameter. The outer (“large”) dowel was 6 cm high and 6 cm in diameter. Prehension movements were monitored using a Selspot system. The displacement of a marker placed at the wrist level was used as an index for the transport of the hand at the location of the object. Markers placed at the tips of the thumb and the index finger were used for measuring the size of aperture of the finger grip.
Kinematics of transport and grasp components were computed from the filtered displacement signals. Movement time (MT), time to peak velocity (TPV) and time to peak deceleration (TPD) of the wrist, time to peak velocity of grip aperture (TGV), time to maximum grip aperture (TGA) were the main parameters used for comparing the movements in different conditions. Spatial paths of the wrist, thumb and index markers were reconstructed in two dimensions. Variability of the spatial paths over repeated trials was computed as the surface of the ellipses defined by X and Y standard deviations from the mean path.
Computer controlled illumination of one of the dowels was the signal for reaching toward that dowel. Blocks of trials were made to the small dowel and to the large dowel. Mean MT during blocked trials was 550 ms. The acceleration phase of the movements (measured by parameter TPV) represented 33% of MT. About half of MT (52%) was spent after TPD in a low velocity phase while the hand was approaching the object. This kinematic pattern was not influenced by whether movements were directed at small or large dowels.
Grip aperture progressively increased during transport of the hand. TGA corresponded to about 60% of MT, that is, maximum grip aperture was reached during the low velocity phase of transport. Following TGA, fingers closed around the object until contact was made. This pattern of grip formation differed whether the movement was directed at the large or the small dowel: TGA occurred often earlier for the small dowel, and the size of the maximum grip aperture was larger for the large dowel. Variability of both the wrist and finger spatial paths was larger during the first half of MT, and tended to become very low as the hand approached the dowels.
Selective perturbations of dowel size were randomly produced at the onset of prehension movements. Perturbations involved increase in object size (the illumination was suddenly shifted from the small to the large dowel, S-L perturbation), or decrease in object size (from the large to the small dowel, L-S perturbations). During S-L perturbations MT was increased by 175 ms on average. As TPV and TPD did not differ from control unperturbed movements, increase in MT was entirely due to lengthening of the low velocity phase following TPD.
Grip formation was affected by the perturbation. Grip aperture first peaked to the size corresponding to the small dowel, then reincreased for accomodating the size of the large dowel. The time where grip aperture reincreased (as measured on the curve of grip velocity) occurred 329.8 ms on average after movement onset. Variability of wrist and finger spatial paths was increased with respect to controls, but it remained low during the final phase of the movement. L-S perturbations had similar effects, though attenuated with respect to S-L perturbations.
This relatively long time taken to initiate corrections in response to object size perturbations contrasts with the short time (about 100 ms) for initiating corrections during perturbations of object position. This difference suggests some degree of independence of the mechanisms generating finger movements during grip formation from those generating transport of the hand. In addition, the kinematic coupling of the two components (demonstrated here by lengthening of the low velocity phase of the transport during correction of finger grip size) suggests the existence of a different mechanism subserving temporal coordination of the two components.
Key wordsMotor control grasping Human
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