The primate as a model for the human temperature-sensing system: 1. Adapting temperature and intensity of thermal stimuli

The thermal sensitivity of three humans and two rhesus monkeys was measured behaviorally, using the "yes-no" paradigm of the Theory of Signal Detection. The aim was to evaluate the monkey's thermal-sensing system as a model for that of humans. Three of the principal variables of human thermal sensations-rate of the temperature change, area of stimulation, and site of stimulation-were held constant. The other three variables-adapting skin temperature (AT), intensity, and direction of the temperature change-were varied systematically. Systematic differences between species were not evident tor warming or cooling stimuli, lsodetectability curves (d′_e = 1) for small cooling stimuli plotted as a function of the AT were isomorphic, and the points for the human and monkey subjects were frequently superimposed, lsodetectability curves for warming stimuli, on the other hand, had similar shapes for ATs between 33° and 40°C, but the points for the different subjects were not superimposed. At ATs below 30° C., one of the two humans in the warming series and the two monkeys continued to show similarly shaped curves, but the other human was markedly different. Qualitative descriptions of the thermal sensations obtained during threshold measurements of human subjects, reported previously, suggest that this unusual subject probably adopted a criterion qualitatively different from that used by the other subjects. The data presented here and in combination with previously published work from this laboratory (Kenshalo, 1970) suggest that thermal stimuli produce similar sensations in rhesus monkeys and humans, and that the neural systems responsible for coding AT and temperature change in the two species are fundamentally similar.