INFLUENCE OF DRUGS ON BEHAVIOR CONTROLLED BY INTERNAL AND EXTERNAL STIMULI

¹ Departments of Radiation Biology and Biophysics, Pharmacology, and Brain Research, School of Medicine and Dentistry, University of Rochester, Rochester, New York

Three pigeons were trained to peck an illuminated disk for a food reinforcer which was delivered according to a fixed interval (FI) schedule of reinforcement. Under one set of conditions, the stimulus projected on the disk remained constant during the 5-min FI. Under a second set of conditions, the symbol projected on the disk changed once per minute during the interval, thus providing the birds with a "clock." These two conditions were chosen to represent behavior controlled by internal and external stimuli respectively, and were presented to the bird in an irregular order. Greater changes in response distribution were produced by amphetamine, scopolamine and pentobarbital when no "clock" was available; with the added clock, the virds responded mostly near the end of the interval, and drug-induced changes in response rate were for the most part confined to the end of the interval. This confirms for these drugs the hypothesis that behavior largely controlled by externally based discriminative stimuli is more resistant to drugs than behavior controlled by internal stimuli. Chlorpromazine and promazine, however, did not display such a pronounced differential effect on response distribution.

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