Temporal and qualitative properties of cold pain and heat pain: a psychophysical study

Dorsal horn neurons that respond to noxious cold also respond to noxious heat, suggesting the hypothesis that pain evoked by temperature extremes, whether hot or cold, may be processed similarly in the CNS. In this study, we tested perceptual consequences of this hypothesis by comparing characteristics of heat and cold pain, as well as of innocuous warm and cool. Eight healthy subjects performed psychophysical tasks involving hot and cold cutaneous stimuli. Using a 9-cm2 contact thermode, temperatures from -5 degrees to 48 degrees C were each applied for 30 s to the thenar eminence. Subjects gave continuous ratings of perceived temperature and pain intensity, using an electronic VAS. After each stimulus, subjects also reported the maximum stimulus intensity and unpleasantness, and chose appropriate words from a list of qualitative verbal descriptors. We found that larger temperature differences were needed in the noxious cold than in the noxious heat range to produce equal perceptual differences. Further, in the heat range, stimulus-response functions were steeper for noxious than for innocuous temperatures, whereas in the cold range, the opposite held true. The relative unpleasantness of heat pain did not differ from that of cold pain, but subjects used a wider range of qualitative words to describe cold pain. Perceived stimulus intensity was compared to temperature recordings from intradermal and skin surface thermocouples. Heat pain, cool and warmth appeared to depend on surface temperature, whereas cold pain was related to subcutaneous temperature, suggesting different receptors for noxious heat and noxious cold. These data, combined with results of human brain imaging and primate electrophysiological studies, suggest that the unpleasantness associated with both heat pain and cold pain is processed similarly in the CNS, whereas differential information about stimulus quality is preserved in the cerebral cortex.