Influence of stimulus intensity on the TMS induced ipsilateral silent period – Comprehensive findings from a large healthy cohort

Introduction: Transcranial magnetic stimulation (TMS) of the primary motor area induces a brief suppression of voluntary muscle activity in electromyography (EMG), known as the ipsilateral silent period (iSP). The iSP reflects transcallosal inhibitory interaction between motor cortices. However, protocols vary, and systematic studies on how stimulation intensity influences the iSP are lacking. Methods: In 100 healthy young adults, we applied line based neuronavigated single-pulse TMS to both primary motor hand areas at 90–230% of resting motor threshold (RMT). iSPs were recorded at each intensity, and summary statistics were calculated for common parameters. A multilevel mixed-effects model identified variables contributing to differences in outcomes. Results: We obtained 197 complete datasets (2116 iSPs). iSP duration and depth increased non-linearly with intensity, while onset remained unchanged. Consistently detectable iSPs (>80% occurrence) were observed at 120% RMT. Variability increased at higher intensities, interacting with pre-stimulation EMG. Conclusion: iSP parameters show a non-linear relationship with intensity, except for onset, with greater variability at higher levels. To minimize variability, we recommend 120–140% RMT. Significance: This first systematic evaluation of stimulation intensity on iSPs in the largest cohort to date provides recommendations for future TMS studies.