Repetitive TMS of the motor cortex improves ipsilateral sequential simple finger movements

TLDR

Disrupting cortical activity can improve behavior, and because motor cortex transcallosal inhibition normally suppresses the contralateral side, transiently reducing M1 excitability with rTMS may alter interhemispheric balance. The authors aimed to determine whether lowering M1 excitability by rTMS would enhance ipsilateral hand motor performance by relieving transcallosal inhibition of the contralateral M1. Sixteen healthy volunteers performed a sequential index‑finger key‑press task before and after a 10‑minute 1‑Hz rTMS train delivered below motor threshold to ipsilateral or contralateral M1, ipsilateral premotor cortex, or vertex. rTMS of M1 shortened ipsilateral hand task execution time for at least 10 minutes, had no effect on contralateral performance, and was associated with increased excitability in the unstimulated M1.

Abstract

<b><i>Background:</i></b> Disruption of cortical function can improve behavior. Motor cortex (M1) transcallosal interactions are mainly inhibitory; after unilateral damage to M1, there is increased excitability of the unaffected M1. Repetitive transcranial magnetic stimulation (rTMS) of M1 produces a temporary reduction in cortical excitability in the same M1 that outlasts the duration of the rTMS train. The authors hypothesize that reducing cortical excitability of M1 by rTMS may improve motor performance in the ipsilateral hand by releasing the contralateral M1 from transcallosal inhibition. <b><i>Methods:</i></b> Sixteen healthy volunteers participated. Using a sequential key-pressing task with the index finger, motor performance was monitored before and after rTMS (1 Hz for 10 minutes with the intensity below motor threshold) applied to the ipsilateral M1, contralateral M1, ipsilateral premotor area, or vertex (Cz). <b><i>Results:</i></b> rTMS of M1 shortened execution time of the motor task with the ipsilateral hand without affecting performance with the contralateral hand. This effect outlasted rTMS by at least 10 minutes, was specific for M1 stimulation, and was associated with increased intracortical excitability in the unstimulated M1. <b><i>Conclusions:</i></b> The authors' results support the concept of an interhemispheric "rivalry." They demonstrate the utility of repetitive transcranial magnetic stimulation to explore the functional facilitation of the unstimulated counterpart motor cortex, presumably via suppression of activity in the stimulated motor cortex and transcallosal inhibition.