Modification of the silent period by double transcranial magnetic stimulation

Objectives: To study the time course of the changes of the inhibitory network of the human motor system, we investigated the silent period (SP) in 7 healthy subjects by double suprathreshold transcranial magnetic stimulation (TMS). Methods: SPs and motor evoked potentials (MEPs) were recorded from the voluntarily activated right abductor digiti minimi muscle. Conditioning and test stimuli were delivered with equal intensity, which was set to yield a baseline SP duration of 130 ms by a single pulse, and with various interstimulus intervals (ISIs). In addition, a control experiment with adjustment of the intensity of single stimuli was performed. Results: At ISIs of 20 and 30 ms the test pulse SP duration was prolonged, without increasing the MEP amplitude. The SP duration shortened at longer ISIs and showed a significant depression between ISIs of 60–110 ms. The shortened SP was accompanied by a diminished MEP. The control experiment revealed that the SPs evoked by the adjusted pulses were significantly shorter than the test pulse SPs. Conclusions: A conditioning stimulus can prolong and shorten the test pulse SP duration at different ISIs. The prolongation is probably cortically generated, whereas the shortening is likely to occur at a cortical and spinal level.