Sustained increase of somatosensory cortex excitability by 5 Hz repetitive transcranial magnetic stimulation studied by paired median nerve stimulation in humans

Abstract

Repetitive transcranial magnetic stimulation (rTMS) has been shown to alter cortical processing within primary motor cortex dependent on the choice of stimulation variables. However, little is known about the effects of TMS in other cortical areas such as the primary somatosensory cortex (SI). Here we asked whether high-frequency (5 Hz) rTMS applied over the left SI evokes sustained changes in cortical excitability. To assess excitability changes, we applied a paired-pulse protocol consisting of paired electrical stimulation of the median nerve using an interstimulus interval of 30 ms and recordings of somatosensory evoked potentials. For ipsilateral SI we found that 1 h after termination of 5 Hz rTMS applied over the left SI with a figure-of-eight coil there was a sustained suppression of the normally present paired-pulse inhibition. Latencies and N20 amplitudes of the first peak remained unchanged. No changes of paired-pulse behavior were observed in the contralateral SI that was not TMS stimulated. The sustained excitability enhancement in SI is discussed in respect to previous findings regarding an improvement of tactile discrimination behavior by rTMS.