An 'interference" approach to the study of somatosensory evoked potentials in man

Somatosensory evoked potentials (SEPs) were recorded from the shoulders, neck and scalp in response to electrical stimulation of the median nerve, with and without various interfering stimuli delivered to the ipsilateral hand. Vibration applied to the fingertips, like active movement of the fingers, caused a reduction in the amplitude of peripheral and cervical evoked potentials, with a more marked attenuation of the initial negativity and positivity (N18 and P21) recorded over the somatosensory cortical hand area. Light touch on the palm and first three digits had little effect on peripheral and cervical potentials, but caused attenuation of N18 and P21 and marked enhancement of a negative wave at 25-35 msec latency. This was shown to be a composite effect due to wave form alterations at the hand area electrode and the midfrontal reference site. Similar changes occurred to a lesser degree when the interfering stimulus was applied to other parts of the ipsilateral arm and the face, but not with stimulation of the contralateral arm. The altered wave form did not resemble that which is recorded at low median nerve stimulus intensities, but was similar to that recorded at high stimulation frequencies. The absence of any interfering effect of painful or thermal stimulation supports the hypothesis that these changes may reflect build-up of inhibition at synapses of the dorsal column/medial lemniscal pathway, due to input from cutaneous touch receptors and proprioceptors.