Effects of object shape on hand configuration and visual feedback during grasping

Normal subjects gradually preshape their hands during a grasping movement in order to conform the hand to the shape of a target object. The evolution of hand preshaping may depend on visual feedback about arm and hand position as well as on target shape and location at specific times during the movement. The present study manipulated object shape in order to produce differentiable patterns of finger placement along two orthogonal dimensions (flexion/extension and abduction/adduction), and manipulated the amount of available visual information during a grasp. Normal subjects were asked to reach to and grasp a set of objects presented in a randomized fashion at a fixed spatial location in three visual feedback conditions: Full Vision (both hand and target visible), Object Vision (only the object was visible but not the hand) and No Vision (vision of neither the hand nor the object during the movement). Flexion/extension angles of the metacarpophalangeal and proximal interphalangeal joints of the index, ring, middle and pinkie fingers as well as the abduction/adduction angles between the index-middle and middle-ring fingers were recorded. Kinematic analysis revealed that as visual feedback was reduced, movement duration increased and time to peak aperture of the hand decreased, in accord with previously reported studies. Analysis of the patterns of joint flexion/extension and abduction/adduction per object shape revealed that preshaping based on the abduction/adduction dimension occurred early during the reach for all visual feedback conditions (similar to45% of normalized movement time). This early preshaping across visual feedback conditions suggests the existence of mechanisms involved in the selection of basic hand configurations. Furthermore, while configuration changes in the flexion/extension dimension resulting in well-defined hand configurations occurred earlier during the movement in the Object Vision and No Vision conditions (45%), those in the Full Vision condition were observed only after 75% of the movement, as the moving hand entered the central region of the visual field. The data indicate that there are at least two control mechanisms at work during hand preshaping, an early predictive phase during which grip selection is attained regardless of availability of visual feedback and a late responsive phase during which subjects may use visual feedback to optimize their grasp.