Suppression of ipsilateral motor cortex facilitates motor skill learning

Abstract

The primary motor cortex (M1) plays a critical role in early aspects of motor skill learning. Given the notion of inter-hemispheric competition, unilateral disruption of M1 may increase excitability of the unaffected motor cortex and thus improve motor learning with the ipsilateral hand. We applied slow-frequency repetitive transcranial magnetic stimulation (rTMS) before the initiation of practice of a simple motor skill. Participants were randomly divided into three stimulation groups: (i) ipsilateral M1; (ii) contralateral M1; and (iii) Cz (control site). The mean execution time and error rate were recorded in four sessions distributed over 2 days. Disruption of M1 with rTMS slowed down skill acquisition with the contralateral hand, albeit non significantly, but paradoxically enhanced learning with the ipsilateral hand. This was evidenced by a significant decrease of execution time at the end of day 1 in the group that received rTMS over the ipsilateral M1 compared with both control groups (Cz and contralateral M1 stimulation). This supports the notion of inter-hemispheric competition and provides novel insights that may be applicable to neurorehabilitation.

Figure 1

Finger movement sequence. Subjects were instructed to press the keys in the numbered order with their left index finger, as accurately and quickly as possible.

Figure 2

A) Changes in execution time. Each plot represents mean execution time in one practice block. Data from the initial block on Day-1 (baseline), the first block on Day-2, and the last one in each practice session are plotted. Execution time improved with practice sessions. In the group with rTMS to the ipsilateral M1, the execution time at the end of Day-1 (after S2) was significantly shorter than in the other two groups (*, p < 0.05). The group with rTMS of the contralateral M1 improved their execution times slower than the other two groups, but not significantly. Filled symbols indicate significantly shorter values than baseline (p < 0.05). Error bars represent standard error. B) Changes of execution time during the ten blocks of the first session (S1). Subjects with rTMS to the ipsilateral M1 improved their execution times faster than the other two groups. Conversely, subjects with rTMS to the contralateral M1 improved their execution times slower than the other two groups. Asterisks show the values that are significantly different from those of controls, indicating shorter or longer values. Filled symbols represent significantly shorter values than baseline, indicating significantly improved performance.