Abstract

Drug effects on operant behavior are often characterized by their effects on rate of responding, usually expressed as the number of responses per unit of time. The time between two consecutive responses constitutes an interresponse time (IRT), and this measure has been used also to characterize the effects of drugs on operant behavior. IRTs which occur during a session can be classified on a statistical basis as: 1) short-IRT, an IRT(s) of short duratio generated by high-frequency responses; 2) pause, an IRT of long duratio generated by low-frequency responses; and 3) post-reinforcement pause, an IRT which immediately follows reinforcement. This investigation used three schedules of water reinforcement (fixed-ratio 20, fixed-interval 90-seconds and variable-interval 20-seconds) to examine how these IRT classes are influenced by changes in water deprivation conditions or amphetamine administration. Base-line IRT distributions depended upon the schedule of reinforcement. Changes induced by doses of amphetamine or alterations in level of water deprivation were compared and contrasted. Short-IRTs that characterized fixed-ratio 20 performance were resistant to change with increasing doses of amphetamine, but were increased in duration with decreasing water deprivation. Animals responding on a fixed-interval 90-second schedule showed a decreased postreinforcement pause after amphetamine, but an increased postreinforcement pause after access to water. An additional experiment studied the combined effects of presession water consumption and d-amphetamine administration on variable interval performances. Making water available before the session lowered the amphetamine dose-response curve along the vertical axis, suggesting that amphetamine did not mimic satiation. In most cases the effect of amphetamine and changing levels of water deprivation were dissimilar in their effects on IRT distributions, suggesting that amphetamine does not exert its major action on behavior through its adipsic effect.