Abstract
The present study investigated motor adaptation using point-to-point reaching movements that are perturbed by mechanically transduced and velocity-dependent force fields. Complete motor adaptation involves regaining the level of accuracy of reaching movements as it was prior to the exposure to the novel force field. Post-adaptation reaching movements made without the presence of perturbing forces may be accompanied with after-effects, mirror-symmetric deviations from an optimal trajectory. In a prior study it was found that after-effects occurred only when reaching movements were made while holding the handle of the mechanical device. Other studies found that they also occurred when making reaching movements in free space. The general research question of this study was to investigate if differences in the paradigm between these studies could account for the disparate results. More specifically, we studied whether training duration would influence the generalization of after-effects across different grip conditions. The first research hypothesis predicted that after a brief training period after- effects would only be present when reaching while holding the handle of the manipulandum, while after a longer training period they would also be present when reaching in free space. The second hypothesis predicted that after a longer training period the magnitude of after-effects would be smaller in the free-space reaching condition than III\\t when reaching while holding the manipulandum's handle. At the start of the training after-effects were only present while holding the handle, while the magnitude of after- effects was the same for both grip conditions. Following a short training after-effects were present in both grip conditions, and larger when holding the handle. Retention of performance took place in both grip conditions. These results suggest that motor adaptation can involve adaptation of both object and limb dynamics, but also that the outcome of the process is subject to the subtleties of the task and the environment.