Tonic pain inhibits phasic pain: evoked cerebral potential correlates in man.

In a sample of 10 healthy volunteers phasic pain ratings and evoked cerebral potentials (EPs) elicited by brief electrical skin stimuli were investigated in periods before, during, and after contralateral tonic ischemia pain. In all subjects the phasic pain ratings and the late EP components P80-N150 and N150-P260 were depressed under concurrent tonic pain. The magnitude of the mean reduction (31%, 40%, and 26%) is comparable to morphine analgesia. The early EP components with latencies below 80 ms, which are considered to be correlates of mechanosensitivity, were not influenced. The findings of tonic pain inhibiting phasic pain are discussed on the basis of changes in attention as well as of pain-specific physiological mechanisms like diffuse noxious inhibitory control.