The pain inhibiting pain effect: an electrophysiological study in humans.

This study examines the counterirritation phenomenon of experimental pain in human subjects. Phasic pain induced by intracutaneous electrical stimuli was simultaneously applied with tonic pain induced by ischemic muscle work. Pain ratings, spontaneous EEG and evoked potentials were measured. We found a significant reduction of phasic pain ratings during and 10 min after tonic pain. The late somatosensory evoked potentials as neurophysiological correlates of phasic pain sensation were attenuated until 20 min after tonic pain offset. The extent of phasic pain relief due to concomitant tonic pain was small but significant, comparable to the effect of a regular systemic dose of a narco-analgesic drug in this experimental pain model. On the other hand, no modulations in the late components of the auditory evoked potential and the power spectrum of the spontaneous EEG were observed. These variables reflect the attention and vigilance of the subject and are well-known to be affected by opioids. The only exception was an increase of beta power, which might reflect hyperarousal during tonic pain. These results support the suggestion, that the analgesic effect of heterotopic noxious stimulation in humans is based on the activation of a specific inhibitory pain control system. Systemic release of endogenous opioids is unlikely to be involved, because the typical effects of opioids on the EEG were not observed.