The influence of target object shape on maximum grip aperture in human grasping movements

The shape of a target object could influence maximum grip aperture in human grasping movements in several different ways. Maximum grip aperture could be influenced by the required precision of digit placement, by the aim to avoid colliding with the wrong parts of the target objects, by the mass of the target objects, or by (mis)judging the width or the volume of the target objects. To examine the influence of these five factors, we asked subjects to grasp five differently shaped target objects with the same maximal width, height and depth and compared their maximum grip aperture with what one would expect for each of the five factors. The five target objects, a cube, a three-dimensional plus sign, a rectangular block, a cylinder and a sphere, were all grasped with the same final grip aperture. The experimentally observed maximum grip apertures correlated poorly with the maximum grip apertures that were expected on the basis of the required precision, the actual mass, the perceived width and the perceived volume. They correlated much better with the maximum grip apertures that were expected on the basis of avoiding unintended collisions with the target object. We propose that the influence of target object shape on maximum grip aperture might primarily be the result of the need to avoid colliding with the wrong parts of the target object.