Do obstacles affect the selection of grasping points?

The selection of grasping points, the positions at which the digits make contact with an object’s surface in order to pick it up, depends on several factors. In this study, we examined the influence of obstacles on the selection of grasping points. Subjects reached to grasp a sphere placed on a table. Obstacles were placed either near the anticipated grasping points or near the anticipated elbow position at the time of contact with the object. In all cases, subjects adjusted the way they moved when there was an obstacle nearby, but only obstacles near the thumb had a consistent influence across subjects. In general, the influence of the obstacle increased as it was placed closer to the digit or elbow, rather than the subject grasping in a manner that would be appropriate for all conditions. This suggests that under these circumstances the configuration of the arm and hand at the moment of contact was a critical factor when selecting at which points to grasp the objects.