We have recently demonstrated that humans report heat stimuli as less painful when presented concurrently with a second noxious stimulus applied to another part of the body. Previous neurophysiological studies have shown that similar heterotopically applied noxious stimuli selectively and completely inhibit the activity of wide-dynamic-range (WDR) neurons in the dorsal horn — a phenomenon termed diffuse noxious inhibitory controls (DNICs). Taken together, these 2 lines of evidence suggest that activation of WDR cells may be necessary for normal perception of pain. Recent studies in the behaving monkey have additionally shown that WDR neurons respond to small changes in noxious heat stimuli better than do high threshold neurons, thus indicating a more specific role for WDR neurons in sensory-discriminative aspects of pain perception. If DNICs do indeed selectively and completely inhibit the activity of WDR neurons, then a heterotopically applied noxious stimulus should selectively interfere with a subject's ability to discriminate noxious stimuli. This hypothesis was tested using a noxious heat discrimination task and a cold water (5°C) diffuse noxious stimulus. We found that the ability to detect small changes (0.4–0.8 °C) in painful heat stimuli applied to the face decreases when the person's hand is submerged in painfully cold water (P = 0.005) and that this effect persists, to a lesser extent, after the hand is removed from water. Control tasks, using visual stimuli, demonstrated that the modulation of nociceptive discrimination was not a generalized effect on sensory perception; other control measures indicated that the results could not be attributed to distraction, fatigue or changes in response bias. Data from the present psychophysical study thus strengthen the argument that WDR neurons are important for sensory-discriminative aspects of pain perception.
*Correspondence to: Dr. Gary H. Duncan, Facultéde Médecine Dentaire, Universitéde Montréal, Montreal, Que. H3C 3J7, Canada.
Submitted April 25, 1988; revised and accepted August 25, 1988.
© Lippincott-Raven Publishers.