Pain relief by various kinds of interference stimulation applied to the peripheral skin in humans: pain-related brain potentials following CO2 laser stimulation.

Pain perception is changed by various kinds of interference stimulation applied to the peripheral skin in humans. We investigated pain-related somatosensory evoked brain potentials (pain SEPs) following CO2 laser stimulation applied to the hand or foot in normal subjects, to elucidate the underlying mechanisms. A pain visual analogue scale (VAS) was also scored to determine the degree of subjective feeling of painful sensation. The following stimulations were applied as the interference: (1) vibration, (2) active and passive movements of the hand or foot, (3) noxious warming by hot water (46 degrees C) and (4) noxious cooling by ice water (0 degrees C). These interference stimulations were applied not only to the same hand or foot as the laser stimuli but also to the contralateral hand or foot. Significant changes in the amplitude of pain SEPs and VAS score were observed to some degree for each type of interference, and we concluded that gate control theory and diffuse noxious inhibitory control were the most appropriate hypotheses to account for this particular phenomenon of pain relief. Some movement-related cortical activities were also considered to be an important factor. These findings could not be accounted for by simple changes in the subjects' attention. Pain relief was more prominent at the second pain ascending through C fibers than that of the first pain ascending through Adelta fibers. The responsible sites for this phenomenon are considered to be the dorsal horn of the spinal cord, the brainstem and some parts of the brain such as the second sensory cortex and the cingulate cortex.