Effect of drugs on response-duration differentiation VII: Response-force requirements

Rats were trained to press a lever for at least I s but for less than 1.3 a . The force required to press the lever was then increased or decreased by 10, 15, or 20 g. Increases in the force requirements for lever pressing dec reased timing accuracy, but decreases in the force requirement had the oppo site effect. Accuracy decreases at increasing force requirements were chara cterized by an increase in the relative frequency of responses that were to o short to meet the reinforcement criterion. In contrast, increases in accu racy when the force requirements were decreased were characterized by incre ases in response durations that met the reinforcement criterion and decreas es in the relative frequency of responses that were too short to produce th e reinforcer. Phencyclidine (PCP) and methamphetamine produced dose-depende nt decreases in accuracy that were associated primarily with increases in t he relative frequency of short response durations, although methamphetamine also produced increases in long response durations at some doses. When the effects of PCP were determined with the force requirement increased by 10 g or decreased by Ii g, the cumulative response-duration distribution shift ed toward even shorter response durations. When the effects of methamphetam ine were determined with the force requirement on the lever increased by 10 g, the cumulative frequency distribution was shifted toward shorter respon se durations to about the same extent as it had been before force requireme nts increased; however, when the force required to press the lever was decr eased by 15 g, these shifts toward shorter response durations almost comple tely disappeared. These results show that increases and decreases in the fo rce requirements for lever pressing have different effects on the accuracy of temporal response differentiation.