Abstract

Drug effects on pigeons working on two schedules of reinforcement were compared. Both schedules demanded that at least eight consecutive responses be made on one key before a peck on a second key would be reinforced; if the bird switched before the count of eight, the series of responses had to be started again. On one schedule, no external stimulus change occurred with the eighth response; on the other, such a change invariably occurred. The addition of the cue modified performance considerably, with the subjects becoming much more efficient in just meeting the required minimum count before switching to the reinforcement key. Response rate, however, remained about the same. This ensured that comparisons of how drug effects were modulated by the addition of the discriminative stimulus would not be complicated by rate differences in baseline performance. When a discriminative stimulus was not present, chlorpromazine, promazine, d-amphetamine and scopolamine led to pronounced increases in premature switching to the reinforcement key. The addition of the external discriminative stimulus attenuated the effects of scopolamine and d-amphetamine most; chlorpromazine and promazine least. Haloperidol had less effect on these performances even though it, like the other drugs, could produce dose-related decreases in rate measures.