Visual Neuroscience (1994), 11, 1077-1082. Printed in the USA. Copyright © 1994 Cambridge University Press 0952-5238/94 $5.00 + .00 Behavioral estimates of absolute threshold in rat CARMEN MUNOZ TEDO, 1 PILAR HERREROS DE TEJADA, 1 AND DANIEL G. GREEN 2 1 Departamento de Psicobiologia, Universidad Complutense de Madrid, Madrid 28023, Spain 2 Department of Ophthalmology, University of Michigan, Ann Arbor (RECEIVED August 16, 1993; ACCEPTED April 25, 1994) Abstract Dark-adapted thresholds of albino and pigmented rats were estimated using behavioral methods. Albino and pigmented rats who had been water deprived learned to bar press for water reinforcement when a light stimulus was presented. Absolute threshold was defined to be the light intensity at which bar pressing behavior was significantly modified by the presence of the light stimulus. Albino rats had an average threshold of —5.23 log cd/m 2 and the pigmented rats had a threshold of —5.0 log cd/m 2 . These values are close to —5.3 log cd/m 2 , the psychophysical threshold of human observers in the same apparatus. Consistent with our earlier electrophysiology, these behavioral experiments provide no evidence for an albino/pigmented sensitivity difference. Comparisons are made between behavioral and electrophysiological determinations of absolute threshold in albino and pigmented rats. Thresholds determined behaviorally agree remarkably well with those derived from visual evoked potentials. Keywords: Absolute threshold, Behavior, Albino, Pigmented, Rat Introduction Behaviorally determined dark-adapted thresholds of albino and pigmented rats are of interest for several reasons. The first has to do with establishing links between physiology and behavior. While electrophysiology is frequently used to obtain objective estimates of visual sensitivity in animals, if one compares abso- lute thresholds determined with a variety of electrophysiologi- cal measurements, there is a surprising lack of agreement in the values obtained from recordings made in the same structures by different investigators and in the estimates obtained at dif- ferent places in the visual pathway (for a review of literature see Green et al., 1991). Consequently, we wanted to obtain behavioral estimates of absolute threshold which could be com- pared with physiological data. Another reason for doing this study is the controversy about whether hypopigmented rodents are considerably less sensitive to light than normal pigmented animals (Balkema et al., 1981; Balkema, 1988; Balkema & Drager, 1991; Hayes & Balkema, 1993a,b). The evidence for an albino/pigmented difference is based on single-unit recordings in mice and rats and behavioral experiments in mice. We, and others, who have used electro- physiological techniques to examine this question, have been unable to confirm the existence of significant strain differences (Dodt & Echte, 1961; Green, 1971; Green et al., 1991; Ratto et al., 1991; Herreros de Tejada et al., 1992; Green et al., 1994). Behavioral estimates of dark threshold in rats are therefore of considerable interest. Reprint requests to: Daniel G. Green, 3002 Neuroscience Bldg., 1103 E. Huron St., Ann Arbor, MI 48104-1687, USA. In this paper, we report behavioral thresholds for fully dark- adapted albino and pigmented rats. What we will report is that the behavioral dark-adapted sensitivity of albino animals is not significantly different from that of pigmented animals. Both strains have dark-adapted sensitivities that are comparable to the thresholds of normal human observers. In addition, the rat behavioral thresholds agree reasonably well with the physiolog- ical thresholds we previously derived from visual evoked poten- tial (VEP) recordings in rat (Herreros de Tejada et al., 1992). Methods In this study, we have used a free-operant single-response pro- cedure (Blough & Yager, 1972) and looked for light-induced changes in rate of bar pressing for water reinforcement. One of the reasons for using this methodology was we wanted to develop methods for determining rat absolute thresholds that could also be used with mice. Statistically significant differences in behavior induced by the presence of the light stimulus pro- vided clear and unequivocal evidence that a particular stimu- lus had been detected. Three-month-old rats, four albino Charles River Albino (CD) and four hooded Long-Evans, were obtained from the Charles River Laboratories (Boston, MA). On arrival in our facility in Madrid, the animals were housed in cyclic lighting (8-h light/16-h dark cycle) and progressively water deprived to 85% of "ad libitum" consumption. Every 4 weeks the animals were allowed free access to water for a week in order to let them recover and to adjust for possible changes in an individual animal's ad libi- tum consumption of water. Each Saturday the animals were allowed to drink freely and on Sunday the supply was limited to 85%. All procedures were in compliance with the Journal 1077