The effect of obstacle position on reach-to-grasp movements

Numerous everyday tasks require the nervous system to program a prehensile movement towards a target object positioned in a cluttered environment. Adu lt humans are extremely proficient in avoiding contact with any non-target objects (obstacles) whilst carrying out such movements. A number of recent studies have highlighted the importance of considering the control of reach -to-grasp (prehension) movements in the presence of such obstacles. The cur rent study was constructed with the aim of beginning the task of studying t he relative impact on prehension as the position of obstacles is varied wit hin the workspace. The experimental design ensured that the obstacles were positioned within the workspace in locations where they did not interfere p hysically with the path taken by the hand when no obstacle was present. In all positions, the presence of an obstacle caused the hand to slow down and the maximum grip aperture to decrease. Nonetheless, the effect of the obst acle varied according to its position within the workspace. In the situatio n where an obstacle was located a small distance to the right of a target o bject, the obstacle showed a large effect on maximum grip aperture but a re latively small effect on movement time. In contrast, an object positioned i n front and to the right of a target object had a large effect on movement speed but a relatively small effect on maximum grip aperture. It was found that the presence of two obstacles caused the system to decrease further th e movement speed and maximum grip aperture. The position of the two obstacl es dictated the extent to which their presence affected the movement parame ters. These results show that the antic ipated likelihood of a collision wi th potential obstacles affects the planning of movement duration and maximu m grip aperture in prehension.