Segmental inhibition of laser-evoked brain potentials by ipsi- and contralaterally applied cold pressor pain.

An experiment was designed to quantify the inhibitory effect of cold pressor pain (ice water immersion) on brain potentials evoked by acute laser-induced pain. One hand (including the C7 dermatome) was immersed into ice water and simultaneously the laser-evoked brain potentials were elicited consecutively from the right and left lower forearms within the C7 dermatomes. The cold pressor pain was continuously rated on a visual analogue scale (VAS). The size of the averaged brain potentials evoked from the ipsi- and contralateral C7 dermatomes were reduced significantly during cold pressor pain. The largest decrease was obtained for the potentials evoked within the C7 dermatome ipsilateral to the cold pressor pain. The size (power) of the brain potentials to individual stimuli could be measured and was correlated to the intensity of the cold pressor pain (VAS score). During the onset phase of cold pressor pain, there was a decrease in power of the individual brain potentials evoked from the C7 dermatome contralateral to the cold pressor pain but the potentials, evoked from the C7 dermatome ipsilateral to cold pressor pain, were decreased more. The overall rating of the laser-induced pain intensity showed that the perceived intensity was lowest when evoked from the dermatome concurrently exposed to cold pressor pain.