Adaptation of grasping responses to distorted object size and orientation.

The present study investigated the adaptive plasticity of the grasp component of prehensile movements. Subjects saw visual objects (V) of various sizes and orientations and were instructed to seize them with the thumb and index finger without manipulating them. The subjects' hand disappeared from view during the response and made contact with a haptic object (H), which could differ with respect to size or orientation from V. In Experiment A, we found that the grasp aperture adaptively increased when H > V and decreased when H < V. This confirms earlier observations and expands them by documenting that grasp size adaptation occurs even when grasping is not integrated into an intentional behavioral context (i.e., object manipulation). However, the magnitude of the observed adaptive size change was not monotonously related to the visual-haptic size difference, as one would expect for an adequate adaptive response. In Experiment B, we observed for the first time that the orientation of the grasp aperture adaptively changed when H was differently oriented than V. This change generalized to an unpracticed size and orientation of V, even in the absence of confirmative haptic feedback about dowel orientation. This outcome indicates that grasp orientation adaptation was at least partly based on the recalibration of sensory-to-motor transformation rules, rather than being completely due to strategic adjustments.