The first component of polyphasic motor evoked potentials is resistant to suppression by paired transcranial magnetic stimulation in humans.

The suppressive effect of paired transcranial magnetic stimulation on the different components of motor evoked potentials (MEPs) from the left first dorsal interosseous muscle (FDI) was investigated in ten healthy human subjects. Conditioning stimuli were delivered via a round coil centered on the vertex, and test stimuli via a figure-eight coil over the right motor area. The threshold stimulus intensity of the minimally contracted FDI was determined with each coil. The stimulus intensities of conditioning and test stimuli were 80% and 130-140% of the threshold intensity, respectively. The coil current direction at the intersection of the figure-eight coil was medio-lateral. The coil position for test stimuli was adjusted to evoke discretely polyphasic MEPs in the contracted FDI. Conditioning stimuli were followed by test stimuli at intervals of 1-10, and 15 ms. When the interstimulus intervals were less than 5 ms, the second negative phase of MEPs was suppressed compared to that of the unconditioned responses, whereas the first phase was not suppressed. The present results imply that the first phase reflects direct corticospinal volleys, which are resistant to cortico-cortical inhibition.