BODY SITE VARIATION OF COOL PERCEPTION THRESHOLDS, WITH OBSERVATIONS ON PARADOXICAL HEAT

Thresholds for the perception of coolness and heat pain were determine d in sessions that randomly intermixed temperature increases and decre ases. Four body sites were tested bilaterally: thenar eminence of the hand, plantar surface of the foot, dorsolateral forearm, and lateral c alf. Coolness thresholds were lowest for the hand, intermediate for th e forearm, and highest for the leg and foot. Laterality differences we re not statistically significant. In 34% of the sessions, subjects did not consistently report cool or cold sensations with detectable tempe rature decreases. When they did not report cool or cold, they most oft en reported heat or pain, thus exhibiting the phenomenon of ''paradoxi cal heat.'' There were significantly more paradoxical heat responses w hen cooling stimuli were intermixed with painfully hot stimuli than wh en they were intermixed with only warm stimuli. There was no significa nt correlation observed between thresholds for coolness and heat pain, either across body sites or across subjects at any single body site. This result implies that the various factors relevant to thermal sensi tivity (i.e., thermal properties of the epidermis, innervation density ) are differentially important for cool versus heat pain perception.