Localization of chemical stimulation: capsaicin on hairy skin

The ability to localize a chemical stimulus applied to the skin of the forearm was compared to the ability to localize a punctate tactile stimulus. The chemical stimulus was a single, 6-microliters drop of a 1.0% solution of capsaicin in an ethanol vehicle; the tactile stimulus was a polyester monofilament that exerted 7.5 g of force. Subjects attempted to localize the stimuli at 30-sec intervals for a period of 13.5 min, and rated the perceived intensity and quality of the chemogenic sensations. To avoid generating potentially confounding tactile sensations, localization attempts were made by pointing to the area of sensation with a focused light beam. The results showed that overall, chemical localization was inferior to tactile localization: The absolute error of localization averaged 2.5 cm for capsaicin compared to 1.4 cm for the monofilament. The experiment also revealed that chemical localization (1) varied significantly across arms, (2) exhibited a relatively strong bias toward the elbow, and (3) appeared to be unaffected by the perceived intensity of the sensation. The dominant sensation quality reported was itch. The results are discussed in the context of cutaneous localization in general and localization in the nociceptive system in particular.