Mesencephalic chronic electrodes in pain patients. An electrophysiological study.

Early components of lemniscal potentials after contralateral median nerve or mechanical stimulus are due to lemniscal pathways, whereas later components, after 70 msec appearing bilaterally and at higher stimulus intensities probably express extralemniscal activity. Evoked potentials in the central gray matter show much smaller amplitudes compared with somatosensory cortical evoked potentials (SSEP). The strongest component is a negative wave after 70--100 msec. Longer conditioning stimulation of the lemniscal system inhibits late components in the median nerve evoked cortical potentials. On the contrary, stimulation of the nonspecific periaqueductal gray matter produces inhibition of early components of cortical SSEP together with facilitation of late components.