Body site variation of cool perception thresholds, with observations on paradoxical heat.

Thresholds for the perception of coolness and heat pain were determined in sessions that randomly intermixed temperature increases and decreases. Four body sites were tested bilaterally: thenar eminence of the hand, plantar surface of the foot, dorsolateral forearm, and lateral calf. Coolness thresholds were lowest for the hand, intermediate for the forearm, and highest for the leg and foot. Laterality differences were not statistically significant. In 34% of the sessions, subjects did not consistently report cool or cold sensations with detectable temperature decreases. When they did not report cool or cold, they most often reported heat or pain, thus exhibiting the phenomenon of "paradoxical heat." There were significantly more paradoxical heat responses when cooling stimuli were intermixed with painfully hot stimuli than when they were intermixed with only warm stimuli. There was no significant 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 sensitivity (i.e., thermal properties of the epidermis, innervation density) are differentially important for cool versus heat pain perception.