P 81. Effects of motor cortical quadripulse transcranial magnetic stimulation (QPS) on the contralateral primary motor cortex

Abstract

Corpus callosum connects the bilateral primary motor cortices (M1s) and plays an important role in motor control. Repetitive transcranial magnetic stimulation (rTMS) is known to induce long-term potentiation (LTP) and long-term depression (LTD) like effect on the stimulated M1. Reported effects of rTMS on the contralateral non-stimultaed M1 are variable between the rTMS techniques and their mechanisms are not fully understood. Quadripulse transcranial magnetic stimulation (QPS) over M1 is known to induce LTP/LTD like plasticity in the stimulated M1. We tested whether QPS over M1 would induce plasticity on the contralateral non-stimulated M1 by measuring motor evoked potentials (MEPs). We also measured several parameters for intracortical and interhemispheric excitability changes in the contralateral M1. Subjects were twelve normal volunteers. We applied QPS over the left M1. The QPS protocol consisted of trains of 4 monophasic TMS pulses separated by inter-stimulus intervals (ISIs) of 5 ms (QPS-5) or 50 ms (QPS-50) with an inter-train interval of 5 s for 30 min. We recorded MEPs to single pulse TMS over the left or right M1 from the relaxed first dorsal interosseous muscles on both sides, before, 0 min and 30 min after QPS. We also measured the resting and active motor thresholds (RMT and AMT), short-and long-interval intracortical inhibition (SICI and LICI), intracortical facilitation (ICF), short-interval intracortical facilitation (SICF), interhemispheric inhibition (IHI), and interhemispheric facilitation (IHF) before and after QPS. After QPS-5, MEPs from the left FDI significantly increased as well as the right FDI. After QPS-50, MEPs from the left FDI did not change significantly, whereas the right FDI decreased significantly. Neither RMT nor AMT changed from the baseline after any conditions. Intracortical inhibition (SICI and LICI) and facilitation (ICF and SICF) did not change after QPS-5 or QPS-50. Both of IHI at an ISI of 10 ms and IHF at an ISI of 4 ms from the left to right hemisphere significantly increased after QPS-5 over the left M1. The QPS-5 over left M1 could also induce LTP in non-stimulated right M1. Increase in the interhemispheric connections (IHI and IHF), without any intrinsic excitability changes in the M1, suggests that the contralateral M1 excitability increment after QPS-5 is associated with the functional potentiation of transcallosal connection from the stimulated to the non-stimulated hemisphere.