Perturbation of object location during bimanual prehension: the role of visual feedback.

In this series of studies on the coordination of the two hands during a bimanual perturbation task, 10 right-handed volunteers were asked to reach to grasp and lift two illuminated cubic objects. Upon initiation of the reach a perturbation could occur by extinguishing one or both objects and illuminating new objects located directly away from the start position in the para-sagittal plane (Experiment 1) or toward the start position in the para-sagittal plane (Experiment 2). In Experiment 2 we also manipulated position of the targets within the visual span by having the targets move toward the midline or away from the midline. Dependent measures included kinematic data for the reach movement as well as the timing of eye movements. Results of both experiments indicated little interference between the hands when one object was perturbed while the other remained stationary. We hypothesize that when visual feedback about limb movement is available, participants can independently reorganize the trajectory of the perturbed limb with minimal interference on the non-perturbed limb. Furthermore, results of Experiment 2 indicated that the position of the targets within the visual span at the final target location dictates the number of eye movements made to acquire both targets and can lead to asynchronies at movement termination in a task-dependent manner. Finally, we found that when targets were perturbed away from the body movement time results indicated a right-hand advantage for dealing with a single perturbation. In contrast, perturbations toward the body abolished the movement time advantage. We suggest that differences in the use of visual feedback when working in the upper versus lower visual fields may influence hand advantages.