Noxious and innocuous cold discrimination in humans: evidence for separate afferent channels.

The present study evaluated the ability of humans to discriminate temperature decreases in the noxious and innocuous cold range. Two groups of five subjects detected changes in cold stimuli applied to the maxillary face. For five subjects, adapting temperatures of 22 degrees, 16 degrees, 6 degrees and 0 degrees C were used, and thresholds for detecting temperature decreases were determined using an adaptive psychophysical paradigm. Visual analogue scale (VAS) ratings of cold and pain sensation 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, 10 degrees and 6 degrees C. These subjects described the quality of the 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 degrees C, respectively, indicating that discrimination was better in the innocuous cool (22 degrees and 16 degrees C) than in the noxious and 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 degrees were sometimes and 0 degrees C usually rated as painful. Phasic temperature 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 cool and noxious cold perception and add support to the hypothesis that noxious cold sensation is mediated by subdermal nociceptors.