Diffuse noxious inhibitory controls (DNICs): psychophysical evidence in man for intersegmental suppression of noxious heat perception by cold pressor pain.

Counterirritation, the phenomenon of one painful stimulus reducing pain caused by a second noxious stimulus, has been recognized clinically for decades. Recently a physiological mechanism to explain counterirritation was described and termed diffuse noxious inhibitory controls (DNICs). Nevertheless, few psychophysical studies have examined systematically the effects of a noxious conditioning stimulus on pain perception. The present study examined the perception of painful heat stimuli on the face before, during and after the subject submerged a hand in painfully cold water (5 degrees C) for 5 min (cold pressor pain). We found that the subjects' ratings of the heat stimuli were significantly, although not completely, reduced during the cold pressor pain; this attenuation of pain perception continued after the noxious conditioning stimulus was withdrawn. Similarly, the pain threshold was significantly increased from 45.7 degrees C to 47.3 degrees C while the hand was in cold water and this threshold remained elevated after the cold water was terminated. Since DNICs have been found to completely and selectively inhibit the activity of only one type of pain transmission neuron (wide dynamic range), our data suggest that these neurons are involved in the perception of pain intensity. However, the persistence of residual pain perception in the presence of noxious conditioning stimuli indicates the importance of other nociceptive pathways.