Short Interval Intracortical Inhibition Responses to Low‐Frequency Repetitive Transcranial Magnetic Stimulation Under Multiple Interstimulus Intervals and Conditioning Intensities

Background The extent to which short interval intracortical inhibition (SICI) responds to low‐frequency repetitive transcranial magnetic stimulation (rTMS) remains inconclusive with reports of increased, decreased and unchanged response following modulation. The aim of this study was to systematically investigate if the variability of SICI following rTMS is explained by the interstimulus interval (ISI) and/or the conditioning stimulus intensity (CSI). Methods Two experiments with pretesting/posttesting and an rTMS session (1 Hz, 90% RMT, 900 pulses) were done. Experiment I (N = 15): SICI with multiple ISIs (1.0–4.0 msec, 0.2 msec increment). Experiment II (N = 15): SICI with CSIs (50–95% of RMT, 5% increment). In both experiments, the cortical silent period (cSP) was also collected. Results After low‐frequency rTMS, no significant change (p > 0.10) in SICI at any specific ISI or CSI was observed, nor did the optimal ISI or CSI change. However, a significant decrease was observed in SICI responses when assessed under the range of ISIs (p = 0.0001), but not CSIs. cSP inhibition increased significantly (p