Psychophysical study of noxious and innocuous cold discrimination in monkey

Psychophysical evidence shows that humans are better able to distinguish differences in the intensity of cutaneous temperature in the cool range than in the noxious cold range. In order to compare these human perceptual findings with physiological data from non-human primates, we performed similar psychophysical experiments of cold perception in monkeys. Two adult male rhesus monkeys were trained to detect cooling shifts from baseline temperatures between 0 degrees and 22 degrees C applied to the face with a 1-cm2 contact thermode. Detection thresholds were determined using the method of constant stimuli for one monkey and an adaptive psychophysical algorithm which insured constant behavioral performance for the other monkey. Results showed that both monkeys detected significantly smaller temperature decreases from innocuous cool baselines (i.e., 22 degrees and 16 degrees C) than from noxious and near-noxious baselines (10 degrees, 6 degrees, 0 degrees C). Similarly, the latencies for detecting the cooling shifts were shorter and less variable in the innocuous cool range than in the noxious cold range. The observation of more precise discrimination of innocuous cool than noxious cold temperatures in monkeys is consistent with human psychophysical data. Thus, these data suggest that differential patterns of neuronal activity evoked by cool and noxious cold cutaneous stimuli, observed in peripheral afferents as well as in the central nervous system of monkey and cat, probably also exist in the human.