Hand Orientation and Perturbation Effects during Reaching and Grasp

Hand and arm coordination and orientation during reach to grasp tasks were studied. Subjects were instructed to perform reaching and making a grip-type grasp on targets with various orientation angles and locations. We tested three conditions: movements to targets of fixed location and orientation, movements to targets with a predictably perturbed orientation, and movements to targets with a randomly perturbed orientation. For fixed target orientations the hand rotated quickly together with the initiation of reaching movement. The amount of rotation was proportional to the final required orientation of the hand for the grip. In the predictable orientation task, the hand would first make a rotation required for the original target orientation and then correct for the final target orientation. Such correction occurred approximately 170 ms after the onset of the target orientation change. This latency was gradually reduced as the subjects accustomed to the perturbation and the hand orientation trajectory started to shift to match closer to that for the final target orientation. This adaptation was different under the random perturbation condition. No clear orientation trajectory adaptation but a generally faster correction was observed.