Physiology & Behavior, Vol. 66, No. 1, pp. 109–112, 1999 © 1999 Elsevier Science Inc. Printed in the USA. All rights reserved 0031-9384/99/$–see front matter PII S0031-9384(98)00257-1 109 Time Course of Burn to Repeated Applications of Capsaicin CAREY D. BALABAN,* 1 DONALD H. MCBURNEY† AND MINDY STOULIS† *Departments of Otolaryngology and Neurobiology, †Department of Psychology, University of Pittsburgh, Pittsburgh, PA 15213 Received 2 June 1998; Accepted 17 September 1998 BALABAN, C. D., D. H. MCBURNEY AND M. STOULIS. Time course of burn to repeated applications of capsaicin. PHYSIOL BEHAV 66(1) 109–112, 1999.—Human subjects rated the burn of 100 ppm capsaicin applied to the tongue via fil- ter paper and replaced every minute. Capsaicin was presented at a constant concentration for 10 min, followed by 10 min of blank stimuli; this sequence was then repeated once. Both sensitization and desensitization were seen. A dynamic model com- posed of a level detector and change detector predicted the results and also previous data. Thus, the model provides a parsimo- nious description of responses to constant and intermittent capsaicin stimulation. © 1999 Elsevier Science Inc. Capsaicin Burning Irritation Adaptation Sensitization Desensitization Tongue Psychophysics SENSORY responses to stimuli, like all biological processes, are dynamic events that are characterized poorly by a single static measurement, such as a magnitude estimate. Our previ- ous study examined responses to a constant concentration of capsaicin applied to the human tongue (6). These responses were modeled mathematically as a sum of three underlying processes, a change detector (phasic mechanism), a level de- tector (tonic mechanism), and a rising function that may rep- resent cumulative response to a painful, potentially injurious stimulus. Two parameters, a gain and time constant, charac- terize each component. The level detector and change detec- tor components of the model are adequate to describe the time course of psychophysical judgments in vision, audition, taste and self-ratings of alcohol intoxication with modification of only four parameters (McBurney and Balaban, unpub- lished observations). A minority of subjects displayed the ris- ing component in our previous study. Data to be described in the present study do not require use of this component, leav- ing four parameters. The previous article showed that adaptation to capsaicin over days could be explained as a reduction in the gain of the tonic mechanism (6). However, the model has only been ap- plied to one temporal pattern of stimulation, a step increase from no stimulus to a constant capsaicin concentration. Previ- ous studies have used intermittent capsaicin stimulation (1– 5). Sometimes, the response to a later stimulus is greater than to the initial response, and sometimes it is less than the origi- nal response. These cases have been termed sensitization and desensitization, respectively. This study tests whether our phasic-tonic model is adequate to explain psychophysical judgments during intermittent capsaicin stimulation. The re- sults will address the issue of whether sensitization and desen- sitization can be predicted by the temporal properties of a model developed for responses to constant stimulation. METHODS Twenty-nine undergraduate students participated in the experiment for course credit. The recruitment material re- quested subjects who did not find the taste of foods containing capsaicin aversive. Capsaicin (Sigma, 98%) was dissolved in 95% alcohol to 100 ppm and stored at -10°C. Twenty-five mi- croliters of the solution was pipetted onto 1.27-cm diameter filter paper disks (S&S) and allowed to dry. The filter papers were rewetted with 50 L of water just before being placed on the tongue. Standard, nonmodulus, magnitude estimation instructions were read to the subjects and any questions answered. Then subjects were given practice judging the distance between the experimenter’s hands. Subjects wrote their responses to the capsaicin on index cards, which they turned over after each trial. Filter papers were laid out in a matrix array before the experiment to prevent the subject from surmising the pattern of stimulation. There was an excess of both filter papers and 1 To whom requests for reprints should be addressed. E-mail: cbalaban@vms.cis.pitt.edu