Coordination and control of bimanual prehension: effects of perturbing object location.

The purpose of the present study was to investigate the coordination of the two effectors when one or both targets were displaced in a bimanual prehension task. Sixteen right-handed volunteers were asked to reach 20 cm to grasp and lift two cubic objects with the right and left hands. Upon initiation of the reach: (1) both objects could remain at the initial position (NN); (2) the right object could be displaced toward the subject (NJ); (3) the left object could be displaced (JN); or (4) both objects could be displaced (JJ). Generally, the results indicated that the hand moving to the perturbed object was reorganized to reach the target efficiently, but hovered to somewhat couple object lift for the two hands. In contrast, adjustments were seen in the velocity profiles of the hand moving to the non-perturbed target, including a premature deceleration phase and corrective movements to reach the target location. Together, these results indicate that when the perturbation of one object occurs during the performance of a bimanual prehension task, visual information is used to independently update the control process for the limb moving to the perturbed object. Additionally, interference causes the limb moving to the non-perturbed target to be inappropriately adjusted in response to the perturbation. Our results also indicated that perceptual and motor factors such as time allotted for the use of feedback and the direction of movement may play a role in the independence/dependence relationship between the hands during bimanual tasks. Furthermore, subjects' expectations about the performance and goal of the task could have a further influence on the level of interference seen during bimanual movements. Finally, despite interference effects which caused multiple accelerations and decelerations, the hand moving to the non-perturbed target still achieved the target location in the same movement time as during control conditions. This final result indicates the efficiency with which subjects can reorganize both limbs in the face of altered task requirements.