NOXIOUS AND INNOCUOUS COLD DISCRIMINATION IN HUMANS - EVIDENCE FOR SEPARATE AFFERENT CHANNELS

The present study evaluated the ability of humans to discriminate temp erature decreases in the noxious and innocuous cold range. Two groups of five subjects detected changes in cold stimuli applied to the maxil lary face. For five subjects, adapting temperatures of 22 degrees, 16 degrees, 6 degrees and 0 degrees C were used, and thresholds for detec ting temperature decreases were determined using an adaptive psychophy sical paradigm. Visual analogue scale (VAS) ratings of cold and pain s ensation were also recorded at 5-min intervals throughout each session . A second group of five subjects performed a similar detection task, but in this case using classical psychophysical techniques (method of constant stimuli) and adapting temperatures of 22 degrees, 16 degrees, 6 degrees and 0 degrees C. These subjects described the quality of th e detected change in sensation, in addition to rating overall cold and pain sensation throughout the session. Detection thresholds were 0.27 degrees, 0.48 degrees, 4.8 degrees, 8.0 degrees and > 10.0 degrees C for baselines of 22 degrees, 16 degrees, 10 degrees, 6 degrees and 0 d egrees C, respectively, indicating that discrimination was better in t he innocuous cool (22 degrees and 16 degrees C) than in the noxious an d near-noxious cold (10-0 degrees C) range (P < 0.05). Tonic adapting temperatures of 22 degrees and 16 degrees C were always rated as cool but not painful, whereas adapting temperatures of 10 degrees and 6 deg rees were sometimes and 0 degrees C usually rated as painful. Phasic t emperature decreases from 22 degrees and 16 degrees C always produced cooling sensations, whereas decreases from baselines of 10 degrees and 6 degrees C produced primarily sensations of painful and non-painful prickle. These data suggest that different afferent channels mediate c ool and noxious cold perception and add support to the hypothesis that noxious cold sensation is mediated by subdermal nociceptors.