Plasticity of Interhemispheric Interference in an Asymmetric Bimanual Task

Abstract

While asymmetric bimanual actions abound in normal life, we frequently experience that dissociating the two hands is difficult, as seen anecdotally in rubbing the stomach and patting the head. To examine to what extent interhemispheric communication can be modified through practice, a discrete/rhythmic bimanual task in healthy subjects was tested. æWhile seated, subjects were asked to rotate their forearms in the horizontal plane with the elbow positions fixed. They were instructed to move their left arm as fast as possible to a target without disturbing the continuous right-arm that performed smooth oscillations. The task goal was to simultaneously show fast velocity of the discrete movement and smooth oscillation of the rhythmic movement, similar to unimanual performance. 18 right-handed subjects performed 20 practice sessions, and three 3-month follow-up sessions. The left-arm velocity increased across practice, without statistical difference with unimanual performance. For the right arm, subjects were unable to improve the smoothness. For the 3-month retention sessions, subjects showed the same level of performance for both arms as they performed at the last practice session. These results suggest an asymmetry in the adaptability of the two armsÍ tasks, and long-term retention of the acquired asymmetry. æThe results have important implications for rehabilitation, as hemiplegic stroke patients are known to also show deficits on their ñunaffectedî side due to the overflow of signals from the lesioned hemisphere. Better understanding the nature of this interhemispheric communication and how to decrease it could lead to improvements in stroke rehabilitation techniques.