Effects of baseline skin temperature on pain ratings to suprathreshold temperature-controlled stimuli

Variations in baseline skin temperature can be encountered in experimental and clinical pain states. Such variations have been shown to greatly alter the response to radiant heat stimuli when the temperature of the stimulus is not controlled. We carried out a psychophysical investigation to examine the influence of baseline skin temperature on pain ratings to temperature-controlled heat stimuli. A CO₂ laser thermal stimulator was used to deliver heat stimuli under radiometer feedback temperature control to the volar forearm. Each stimulus consisted of a 30 s controlled baseline interval (at 34 or 38°C) followed by a stepped increase in temperature (to 46 or 47°C for 1, 2 or 4 s). A run comprised one presentation of each of these 12 different stimuli to different locations. Each experiment contained three runs. In runs 2 and 3, the stimulus intensity and duration at a given location were not changed, but the baseline temperature was alternated between 34°C and 38°C. The intensity of pain was rated using the technique of magnitude estimation. Mean normalized pain ratings for suprathreshold stimuli applied from the higher base temperature (1.03 ± 0.03) were slightly greater than from the lower base temperature (0.96 ± 0.03). In contrast, pain ratings to the 47°C stimuli (1.11 ± 0.03) were substantially greater than to the 46°C stimuli (0.88 ± 0.03). Thus a 4°C change in baseline temperature has a smaller affect (about 8%) on pain ratings than a 1°C change in stimulus temperature (about 27%). This suggests that variations in baseline skin temperature encountered in experimental and clinical pain states have only a minor impact on pain sensitivity to suprathreshold temperature-controlled stimuli.