Further evidence to support different mechanisms underlying intracortical inhibition of the motor cortex

Paired-pulse magnetic stimulation has been widely used to study intracortical inhibition of the motor cortex. Inhibition at interstimulus intervals (ISIs) of 1–5 ms is ascribed to a GABAergic inhibitory system in the motor cortex. However, Fisher et al. have proposed that different mechanisms are operating at an ISI of 1 ms and 2.5 ms. In order to confirm their concept and clarify whether inhibition at all these intervals is produced by a single mechanism, we compared effects of paired-pulse stimulation at ISIs of 1 ms, 2 ms, and 3–5 ms. We evaluated how intracortical inhibition affected the I3-wave, I1-wave, magnetic D-wave, and anodal D-wave components of electromyographic (EMG) responses using previously reported methods. The data suggest that three separate effects occur within these ISIs. At ISIs of 3–5 ms, inhibition was evoked only in responses to I3-waves, whereas no inhibition was elicited in responses to I1-waves or magnetic D-waves. In contrast, at an ISI of 1 ms, responses to I3-waves and I1-waves were moderately suppressed. Moreover, even magnetic D-waves were slightly suppressed, whereas anodal D-waves were unaffected. At an ISI of 2 ms, none of the descending volleys were inhibited. We propose that we should use ISIs of 3–5 ms for estimating function of the GABAergic inhibitory system of the motor cortex by paired-pulse transcranial magnetic stimulation (TMS). Our results support the idea of Fisher et al. that the mechanism responsible for the inhibition at an ISI of 1 ms is not the same as that responsible for suppression at ISIs of 3–5 ms (GABAergic inhibitory circuits in the motor cortex). At an ISI of 2 ms, we suggest that the inhibitory influence evoked by the first stimulus (S1) should collide with or be occluded by the second stimulus (S2), which leads to the lack of inhibition when the subjects make a voluntary contraction of the target muscle.