Effect of repetitive transcranial magnetic stimulation applied over the premotor cortex on somatosensory-evoked potentials and regional cerebral blood flow.

Somatosensory-evoked potentials (SEPs) are attenuated by movement. This phenomenon of 'gating' reflects sensorimotor integration for motor control. The frontal N30 component after median nerve stimulation was shown to be reduced in amplitude prior to hand movement. To investigate the mechanism of this sensory gating, we recorded median SEPs immediately before and after application of monophasic very low-frequency (0.2 Hz) repetitive transcranial magnetic stimulation (rTMS) of 250 stimuli over motor cortex (MC), premotor cortex (PMC), or supplementary motor area (SMA) in 9 healthy volunteers. The stimulus intensity for MC or PMC was set 85% of the resting motor threshold for the hand muscle, and that for SMA was at the active motor threshold for the leg muscle. SEPs showed significant increases in amplitudes of the frontal N30 component after PMC stimulation, but not after SMA or MC stimulation. Low-frequency (1 Hz) biphasic stimulation over PMC showed no significant N30 changes in 6 out of 9 subjects tested, indicating the effect being specific for 0.2 Hz monophasic stimulation. To examine the functional anatomy of the N30 change, single photon emission computed tomography was performed immediately before and after monophasic 0.2 Hz rTMS over PMC in all the 9 subjects. Regional cerebral blood flow showed significant increases mainly in PMC and prefrontal cortex, indicating the involvement of these cortical areas in sensory input gating for motor control.