Influence of short - interval intracortical inhibition on short - interval intracortical 1

27 Using the paired-pulse paradigm, transcranial magnetic stimulation (TMS) has 28 revealed much about the human primary motor cortex (M1). A preceding 29 subthreshold conditioning stimulus (CS) inhibits the excitability of the motor cortex, 30 which is named short-interval intracortical inhibition (SICI). In contrast, facilitation is 31 observed when the first pulse (S1) is followed by a second one at threshold (S2), 32 named short-interval intracortical facilitation (SICF). SICI and SICF have been 33 considered to be mediated by different neural circuits within M1, but more recent 34 studies reported relations between them. In this study, we performed triple-pulse 35 stimulation consisting of CS-S1-S2 to further explore putative interactions between 36 these two effects. Three intensities of CS (80%–120% of active motor threshold: AMT) 37 and two intensities of S2 (120% and 140% AMT) were combined. The SICF in the 38 paired-pulse paradigm exhibited clear facilitatory peaks at ISIs of 1.5 and 3 ms. The 39 second peak at 3 ms was significantly suppressed by triple-pulse stimulation using 40 120% AMT CS, although the first peak was almost unaffected. Our present results 41 obtained using triple-pulse stimulation suggest that each peak of SICF is differently 42 modulated by different intensities of CS. The suppression of the second peak might be 43 ascribed to the findings in the paired-pulse paradigm that CS mediates SICI by 44 inhibiting later I waves such as I3 waves and that the second peak of SICF is most 45