Does the type of prehension influence the kinematics of reaching?

Kinematic studies have indicated that when a subject reaches to grasp an object, the movement consists of two primary components: (a) a transport phase whereby the hand is brought towards the object and (b) a grip phase whereby the hand changes shape in anticipation of the grasp. Using a visual perturbation paradigm, we investigated the effect of different grip component strategies upon the transport phase. The distal strategy was determined by the size of the object to be grasped: for the small object (1.5 cm o.d.) subjects naturally adopted a precision grip between the index finger and thumb; for the large object (6 cm o.d.) subjects used a whole hand prehensile grip. During 20% of the reaching trials the perturbation was introduced by unexpectedly changing the object size. The results showed that corrections to the distal program in response to the perturbation were preceded by changes in the deceleration phase of the proximal component. The data supported previous findings of two visuo-motor channels for this prehensile movement but indicated that when unanticipated shifts of only the distal program are required, both channels show modifications.