Psychophysical detection and pain ratings of incremental thermal stimuli: A comparison with nociceptor responses in humans

The capacity of humans to detect and scale the magnitude of pain elicited by small increments in temperature, delivered by a contact thermal stimulator to localized areas of the arm or leg, was measured on non-painful and painful adaptation temperatures. Subjects continuously rated the magnitude of any pain sensation elicited by heat increments superimposed on base temperatures of 38, 44, 47 or 48 degrees C. Detection threshold was also measured using a two-alternative forced choice method. The increment detection thresholds were lower for a continuously painful base of 47 degrees C than for a non-painful base of 38 degrees C in normal skin, and likewise were lower for a base of 38 degrees C following hyperalgesia induced by a mild burn. Incremental pain thresholds were nearly equal to detection thresholds on the base of 47 degrees C. The sensitivity with which subjects could scale the magnitude of pain was 2-7 times better for increments delivered on a 48 degrees C as opposed to a 38 degrees C base. Evoked responses in 6 single C-fiber mechanoheat nociceptive afferents (CMHs) were recorded percutaneously from the peroneal nerves of 3 humans, who were simultaneously judging pain magnitude. For a base of 38 degrees C, both the pain and the neural response thresholds were an order of magnitude higher than corresponding thresholds on a base of 48 degrees C. For a base of 47 degrees C, response thresholds of the CMHs ranged from 0.1 to 0.5 degrees C and were comparable to detection thresholds of 0.1 to 0.3 degrees C. The sensitivity with which most nociceptors could signal increment size was 3-4 times better on a 48 degrees C than a 38 degrees C base. Incremental pain sensitivity was not altered by a compression block of activity in myelinated afferents that eliminated the sense of cool and touch. Thus, activity in unmyelinated fibers alone could account for the sensitivity to incremental thermal stimuli that were superimposed on a painful base temperature. Further, it is likely that CMH nociceptors alone could provide the peripheral information necessary to detect and to make magnitude judgments of pain elicited by these stimuli.