One-hertz subthreshold rTMS increases the threshold for evoking inhibition in the human motor cortex

Despite indisputable evidence that repetitive transcranial magnetic stimulation (rTMS) modulates motor cortical excitability, the effects of subthreshold low-frequency rTMS on intracortical inhibition (ICI) are controversial. In this paper we investigated whether increasing the level of baseline ICI increases the sensitivity of ICI for disclosing the after-effects of rTMS on cortical excitability. In experiment 1, we studied changes in ICI, tested at two different baseline levels, after a train of 900 subthreshold rTMS pulses delivered at 1 Hz. In experiment 2, we studied whether the same conditioning rTMS train changed the ICI threshold, and in experiment 3 whether it changed the facilitatory I-wave interaction. Conditioning rTMS reduced ICI tested at a baseline level of 75% but left ICI tested at a baseline level of 50% unchanged. It also increased the ICI threshold but left the facilitatory I-wave interaction unchanged. These findings suggest that conditioning rTMS selectively reduced ICI tested at a baseline level of 75% by increasing the threshold for evoking inhibition in the motor cortex. The inhibitory system mediating ICI may therefore be more efficient than other motor cortical systems in reducing high cortical excitability after external intervention. Hence studies investigating the after-effects of rTMS should standardize ICI levels at baseline.