Mechanism of the silent period following transcranial magnetic stimulation- Evidence from epidural recordings

We investigated the nature of the silent period (SP) following transcranial magnetic stimulation by recording corticospinal volleys in a patient with implanted cervical epidural electrodes. Single suprathreshold test stimuli and paired stimuli at interstimulus intervals (ISIs) of 50-200 ms were deli vered while the subject maintained a constant background contraction. The s ilent period duration from a single test stimulus was 357+/-62 ms. The test motor-evoked potentials were markedly reduced at all the ISIs tested. The I (indirect) waves induced by the test stimulus were largely unchanged at a n LSI of 50 ms, suggesting that there was little change in motor cortex exc itability. However, the corticospinal volleys, especially the late I waves, were substantially reduced at ISIs of 100 ms, 150 ms, and 200 ms. Our find ings suggest that the early part of the SP is mainly due to spinal mechanis ms, while the late part of the SP is related to reduced motor cortex excita bility.