The Contribution of the Thumb to Reaching Movements

Abstract

Transport of the hand towards an object and the formation of grasp are logically separable components of reaching. It has been suggested that, although the two components must be temporally co-ordinated, their spatial parameters are under the control of independent visuo-motor channels. A case study of reaching by a proficient user of a manually-operated artificial hand is presented. A pattern of natural hand usage was observed in which the index finger rather than the thumb was responsible for reduction of grasp aperture as the hand approached an object. The same pattern of usage was also observed in the artificial hand even though the mechanics of that hand make it no easier to move the finger than the thumb. This suggests that the relative stability of the thumb in the natural hand is determined, not simply by anatomy, but by a role in guiding the transport component of reaching. At least part of the spatial aspect of grasp formation is closely related to the transport component of reaching and this is evidence against theories postulating two independent visuo-motor channels controlling the spatial parameters of grasp and transport.