Modulation of excitability of human motor cortex (M1) by 1 Hz transcranial magnetic stimulation of the contralateral M1

Objective: Previous studies demonstrated that single-pulse transcranial magnetic stimulation (TMS) of one motor cortex (M1) exerts a brief inhibitory effect on the contralateral M1. The purpose of this study was to test the hypothesis that 30 min of 1 Hz TMS of M1 will result in a lasting increase in excitability in the contralateral M1. Methods: Healthy volunteers were tested on 2 separate days, before (baseline) and after one of two interventions: (a) stimulation of M1 with 1 Hz TMS for 30 min at 115% of resting motor threshold, and (b) sham stimulation. Recruitment curves to TMS, pinch force, and simple reaction time were assessed in the hand contralateral to the unstimulated motor cortex. Results: The main finding of this study was that 30 min of 1 Hz significantly increased recruitment curves in the contralateral motor cortex in the real stimulation condition relative to sham (P<0.005, factorial analysis of variance (ANOVA)). This change outlasted the stimulation period for at least 15 min and occurred in the absence of changes in pinch force or reaction time. Conclusions: These results raise the potential for inducing lasting modulation of excitability in M1 by 1 Hz TMS of the other M1, a phenomenon possibly reflecting modulation of interhemispheric interactions. Significance: It is conceivable that 1 Hz TMS applied to M1 may be used to modulate excitability in the opposite motor cortex for therapeutic purposes.