Effects of a high-frequency, low-intensity, biphasic conditioning train of TMS pulses on the human motor cortex.

Intracortical circuit excitability of the human motor cortex has been studied by measuring effects of some conditioning TMS stimulus on the succeeding test TMS stimulus in the motor cortex, such as short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF). A single-pulse TMS was used as a conditioning stimulus (CS) in these techniques, but a train of several TMS pulses might induce some intracortical changes in the motor cortex more effectively. For nine healthy volunteers, we compared the SICI and ICF induced by a single conditioning biphasic TMS pulse with those induced by a train of 10 biphasic TMS pulses of the same intensity. As a conditioning stimulus, we delivered a subthreshold single biphasic pulse (CS1) or 10, 10-Hz biphasic pulses (CS10) before a suprathreshold monophasic test stimulus at several interstimulus intervals (ISIs) of 3-40 ms over the hand motor area. The CS intensity was 50-100% of the active motor threshold (AMT). We compared the motor cortical excitability after the conditioning stimulus (single pulse or a train of ten pulses) at the intervals for SICI and ICF. A train of ten 10-Hz pulses elicited greater inhibition at short ISIs than a single conditioning pulse did. The facilitation at ISIs around 10 ms corresponding to the ICF was evoked by CS1 only at an intensity of 80% AMT; CS10 evoked no ICF. Furthermore, CS10 evoked MEP inhibition at longer intervals. Results show that a train of high-frequency, low-intensity, biphasic TMS pulses can have a strong inhibitory effect on the motor cortex.