GRADUAL MOLDING OF THE HAND TO OBJECT CONTOURS

Subjects were asked to reach to and to grasp 15 similarly sized object s with the foul fingers opposed to the thumb. The objects' contours di ffered: some presented a concave surface to the fingers, others a flat one, and yet others a convex surface. Flexion/extension at the metaca rpal-phalangeal and proximal interphalangeal joints of the fingers was recorded during the reaching movement. We used discriminant analysis, cluster analysis, and information theory to determine the extent to w hich the shape of the hand was affected by the objects' shapes along a convexity/concavity gradient. Maximum aperture of the hand was reache d about midway in the reaching movement. At that time, the hand's post ure was influenced by the shape of the object to be grasped but imperf ectly. The information transmitted by hand posture about object shape increased gradually and monotonically as the hand approached the objec t, reaching a maximum at the time the object was in the grasp of the h and. We also asked subjects to shape the hand so as to grasp the objec t without moving the arm. Their performance was poorer on this task in the sense that hand shape discriminated among fewer objects and that trial-to-trial variability was greater than when the distal and proxim al components of the motion were linked. The results indicate that the hand is molded only gradually to the contours of an object to be gras ped. Because other parameters of the motion, such as movement directio n, for example, already are specified fully early on in a movement, th e results also suggest that the specification of diverse aspects of a movement does not evolve at a uniform rate.