Evidence that tactile stimulation inhibits nociceptive sensations produced by innocuous contact cooling.

It was recently shown that stinging, pricking or burning is reliably perceived by some individuals when the skin is cooled to temperatures as mild as 25-30 degrees C. These seemingly paradoxical sensations, which have been termed innocuous-cold nociception (ICN), were significant only when cooling was produced by a thermode resting statically on the skin (static contact); touching an already cooled thermode to the skin (dynamic contact) produced reports of only coolness and cold. The present study investigated the hypothesis that ICN is inhibited by tactile stimulation produced when a thermode contacts the skin. Experiment 1 pitted the tactile hypothesis against an alternative explanation that inhibition results from higher rates of skin cooling during dynamic contact. ICN was measured at three different cooling rates (-1.0, -2.5, -5.0 degrees C/s) when the thermode was resting on the skin or was touched to the skin at the moment cooling began. The results supported the tactile hypothesis: faster cooling rates during static contact led to stronger rather than weaker nociceptive sensations, and ICN was suppressed even when dynamic contact was coincident with the onset of cooling, and thus could not affect cooling rate. Experiment 2 confirmed the latter result and showed that suppression was greatest at 28 degrees C, less at 24 degrees C, and not significant at 18 degrees C. We conclude that dynamic tactile stimulation produced by contact with a surface inhibits the nociceptive component of innocuous but not noxious cooling. The implications of this conclusion for the role of cold perception in behavioral thermoregulation versus haptic perception, and for theories of cold perception in general, are discussed.