Single limb performance following contralateral bimanual limb training.

Recent studies on intermanual transfer of reaching movements suggest that this transfer is conducted over an "extrinsic" coordinate system. We hypothesize that training reaching movements in a force field with both hands at the same time, in the same position (bimanual grip) will be more beneficial in promoting transfer of the learned skill to the dominant hand than training the unimpaired limb on the same movements in the same force field since the representation of the movement should be invariant of the limb. However, unlike intermanual transfer, bimanual transfer has the potential to involve infinite number of actuator combinations, or joint configurations, interfering with consistent transfer. The efficacy of this method of transfer has implications for people with hemiparesis since the less-affected arm could potentially "instruct" the more-affected arm how to move. Here, we report on an experiment that evaluates and compares the skill transfer between limbs in a reaching task: 1) intermanual transfer (from the nondominant to the dominant hand) and 2) bimanual transfer (from a bimanual grip to the dominant hand) with healthy subjects. We used two methods from which to judge the transfer: performance in the presence of the force field or by errors made during "catch trials" when the forces were unexpectedly removed as subjects changed hands (known as after effects of adaptation). We found only a small amount of transfer (20% of that seen in the practiced limb) with both types of training, and surprisingly there was no significant difference in the movement accuracy between these two training methods. Moreover, the direction of the after effects supports the assertion that the nervous system generalizes these movements in an extrinsic coordinate system. Accordingly, the limb must experience the dynamics singularly in order to develop an internal model.