Changes of muscle excitability of the hand contralateral to a task performing one: A transcranial magnetic stimulation study

It used to be considered that unilateral movements of distal limb parts are associated only with contralateral motor cortical activity. Recent neuroimaging studies, however, suggest that the motor cortex ipsilateral to a task-performing hand is also activated, and that motor patterns in one hand affect the degree of the activity of the ipsilateral motor cortex. If so, muscles of the hand contralateral to a task-performing one may change those excitability depending on types of tasks. We studied eight subjects who performed three different finger tasks by one hand: (a) pinch, (b) sequential finger opposition, and (c) tactile discrimination. Transcranial magnetic stimulation was delivered by a figure eight coil over the hemisphere ipsilateral to a task-performing hand. Motor evoked potentials and background electromyographic activities were recorded from the opponens pollicis muscle contralateral to the stimulated hemisphere. On average, the motor evoked potentials were larger during tactile discrimination task than those at rest in either hand (p<0.01). Background electromyographic activities in the left hand increased significantly during right hand tactile discrimination task (p<0.01), whilst those in the right hand did not change during the left hand performance (p>0.05). These findings suggest the followings: (1) the hand muscle contralateral to a task performing one changes its excitability depending on types of tasks; and (2) increment of excitability of the left hand muscle associated with right hand tactile discrimination is greater than that of the right hand one in association with the same task by the left hand, thus supporting the idea that there is a functional asymmetry between the right and left motor cortex in respect of motor performance.