Paradoxical heat sensations during moderate cooling of the skin

Paradoxical heat sensations during cooling of the skin were examined in two experiments. In Expt. I the number of occurrences of sensation was studied in 19 naive test subjects (Ss) when cooling from thermal indifference both without and with preceding heating. Without preceding heating 13 Ss reported sensations of paradoxical heat (9.8% of all stimulations). Preheating markedly facilitated the occurrence of the sensations (35% of all stimulations). In Expt. II the effects of cooling velocity (velocities 0.4, 0.7 and 2.0 degrees C/s) and the type of skin area stimulated (hairy or glabrous skin of the hand) on the thresholds of paradoxical sensations were studied in 4 Ss without and with preheating. Cooling velocity, type of skin area and preheating had significant effects on the sensation thresholds, the thresholds being the higher (i.e. the sensation appearing at lower stimulation temperatures) the higher the cooling velocity, if the stimuli were applied to the glabrous skin, or if no preheating was used. The results confirm the existence of paradoxical heat sensations during cooling of the skin and suggest that the sensation is mediated by polymodal units supplied by C-fibers.