Gradual Molding of the Hand to Object Contours. Marco Santello and John F. Soechting. Department of Physiology, University of Minnesota, Minneapolis, MN 55455.
APStracts 4:325N, 1997.
ABSTRACT
Subjects were asked to reach to and to grasp 15 similarly sized objects with the four fingers opposed to the thumb. The objects' contours differed: some presented a concave surface to the fingers, others a flat one and yet others a convex surface. Flexion/extension at the metacarpal-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 which the shape of the hand was affected by the objects' shapes along a convexity/concavity gradient. Maximum aperture of the hand was reached about midway in the reaching movement. At that time, the hand's posture was influenced by the shape of the object to be grasped, but imperfectly. The information transmitted by hand posture about object shape increased gradually and monotonically as the hand approached the object, reaching a maximum at the time the object was in the grasp of the hand. We also asked subjects to shape the hand so as to grasp the object, 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 proximal components of the motion were linked. The results indicate that the hand is molded only gradually to the contours of an object to be grasped. Since other parameters of the motion, such as movement direction, for example, are already fully specified early on in a movement, the results also suggest that the specification of diverse aspects of a movement does not evolve at a uniform rate. 

Received 9 September 1997; accepted in final form 14 November 1997.
APS Manuscript Number J744-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 12 December 1997