/ eh08 2487 Mp 181 Wednesday Jan 08 04:18 PM EL–PB (v. 61, No. 2) 2487 181 Physiology & Behavior, Vol. 61, No. 2, pp. 181–190, 1997 Copyright  1997 Elsevier Science Inc. Printed in the USA. All rights reserved 0031-9384/97 $17.00 / .00 PII S0031-9384( 96 ) 00366-6 Adaptation to Capsaicin Within and Across Days DONALD H. MCBURNEY, 1 CAREY D. BALABAN, DALE E. CHRISTOPHER AND CAMERON HARVEY University of Pittsburgh, Pittsburgh, PA 15260 USA Received 14 September 1995; Accepted 19 July 1996 MC BURNEY, D. H., C. D. BALABAN, D. E. CHRISTOPHER AND C. HARVEY. Adaptation to capsaicin within and across days. PHYSIOL BEHAV 61 (2) 181–190, 1997.—Subjects judged the time-course of the burn caused by 100 ppm capsaicin applied to the tongue on Day 1 and Day 5. On Days 2–4, they tasted hard candy containing capsaicin. Most subjects did not show adaptation within Day 1, but either plateaued after about 16 min or rose monotonically for the entire 34 min. Intensity was less on Day 5 and levelled off or declined for most subjects. Data were fit to a mathematical model of adaptation. Adaptation across days was accounted for by changes in the gains of the three processes. Copyright  1997 Elsevier Science Inc. Capsaicin Burning Irritation Adaptation Sensitization Desensitization Tongue Psychophysics Human 1 Requests for reprints should be addressed to Donald H. McBurney, 420 Langley Hall, University of Pittsburgh, Pittsburgh, PA 15260 USA. E- mail: mcburney@vms.cis.pitt.edu THIS study concerns the temporal properties of the burning sen- sation caused by capsaicin, the ingredient that gives chili peppers their characteristic piquancy. It is well known that repeated stim- ulation with capsaicin reduces the effect of irritants, including capsaicin itself, for periods up to days, a phenomenon known as desensitization ( e.g., 8,11 ) . On the other hand, capsaicin can also have the opposite effect of increasing the response to irritants, including itself, a phenomenon called sensitization ( e.g., 8,20 ) . Green ( 9 ) studied these two phenomena in the same experiment, and found that the presence of desensitization depends on the temporal relations between stimuli. In addition, it is well known that there are wide individual differences in response to capsaicin (16,17): Some people regularly choose to consume concentra- tions of capsaicin that would cause others intense discomfort. Although it is believed that the differences may result to some extent from adaptation caused by repeated exposure (11), the sources of individual differences are not well understood. Our interest in capsaicin arose from our model of sensory adaptation (13,14). The model is comprised of two main pro- cesses: a phasic component and a tonic component, such as one sees in a simple resistor-capacitor electrical circuit. The phasic and tonic processes may be thought of as corresponding to the rapidly and slowly adapting fibers, respectively, that are common in sensory systems. The phasic process is responsible for the decline in response to a steady stimulus that is usually considered the defining characteristic of adaptation. The tonic process shows a steady response to steady stimulation. Each process has 2 pa- rameters: gain and time constant. The weighting of the 4 param- eters accounts for differences between sensory systems. This sim- ple mathematical model predicts a number of temporal properties of sensory systems, including some, such as memory and noise filtering, that are not usually associated with sensory adaptation. In addition, the model suggests to us that the widely accepted distinction between sensory adaptation and habituation may be artificial. Relevant here is the distinction on the basis of time- course: It is generally held that sensory adaptation is rapid and reversible, whereas habituation is slower and more permanent. It can be shown, however, that the time-courses of phenomena called adaptation overlap widely with those called habituation. Olfactory thresholds in humans increase over a 2-week chronic exposure to the test odorant (5). On the other hand, the slow- phase velocity of the vestibulo-ocular reflex adapts to zero with a time-constant on the order of 10s of s (12). The long time-constants associated with adaptation to capsa- icin suggested to us that it would provide a good test of our model. We looked to see if the model accounted for the time- course of the burning sensation. Specifically, we tested whether or not there were changes in the gains and time-constants of the tonic and phasic processes across days of stimulation with cap- saicin. METHOD Thirty-two undergraduate students, 22 men and 10 women, participated in the experiment for course credit. The experiment was conducted in 2 phases: The recruitment material for Phase 1 requested subjects who use chili peppers, etc., less than once per month. The results of Phase 1 led us to conduct Phase 2, in which we requested subjects who used products containing cap- saicin almost every day. Subjects in both phases were quizzed about their use of products containing capsaicin as part of their briefing for the experiment. Subjects were included, however, regardless of their reported use of capsaicin. After both phases of the experiment were run, subjects were categorized on the