On the estimation of silent period thresholds in transcranial magnetic stimulation

Objective: We evaluated the induction of corticospinal silent period (SP) using transcranial magnetic stimulation (TMS) at stimulation intensities normalized to resting motor threshold (rMT) or silent period thresholds (SPTs). The aim was to reduce the characteristic inter-individual variation in SP measurements in healthy population to improve the sensitivity of such measurements. Methods: The cortical representation area of the right hand musculature of 12 healthy subjects was stimulated with navigated TMS with varying stimulating intensities. Subsequently, the individual SPTs for eliciting SPs of 20, 30, and 50. ms in duration were determined from the input-output characteristics. Results: While SPT for 20 and 50. ms SPs differed from rMT, the SPT for 30. ms was similar to rMT. Nevertheless, the inter-individual variation in SP duration was reduced significantly at 120% of SPT30 when compared with SP durations obtained at 120% of rMT. Conclusions: Inter-individual variation in the SP duration decreases when applying TMS at stimulation intensities normalized to the individual SPTs instead to the rMT. This makes the SP duration more specific to inhibition and less affected by changes in cortical excitability. Significance: Use of individual SPTs may improve the sensitivity of the SP measures in studies with inter-individual design.