The effect of d-amphetamine on performance an two operant timing schedules

Rationale: Previous experiments have shown that d-amphetamine disrupts timi ng behaviour in rats. It has been proposed that d-amphetamine's effects ref lect a reduction in the period of the pacemaker of the hypothetical interna l clock. However, some studies have obtained conflicting results. Objective : To examine the effects of d-amphetamine (0.2, 0.4, 0.8 mg kg(-1) i.p.) on performance on two quantitative timing schedules: a free-operant schedule, in which rats were trained to distribute their responses differentially be tween two levers during the course of a 50-s trial (free-operant psychophys ical procedure), and a discrete-trials schedule, in which rats were trained to discriminate the duration of light stimuli (interval bisection task). M ethods: In experiment 1, rats were trained under the free-operant psychophy sical procedure to respond on two levers (A and B) in 50-s trials in which reinforcement was provided intermittently for responding on A during the fi rst half and on B during the second half of the trial. For one group, repet itive switching between levers was permitted; for another group, it was pre vented. In experiment 2, rats were exposed to press lever A after a 2-s sti mulus and lever B after an 8-s stimulus, and were then tested with stimuli of intermediate group, a poke response' (depression duration. For one group of a central tray flap) was required after stimulus presentation to effect lever presentation; for the other group, this requirement did not operate. In both experiments, quantitative indices of timing were derived from the psychophysical functions (%B responding vs time). Results: In experiment 1, d-amphetamine increased the Weber fraction and displaced the psychophysica l curve to the left in both versions of the schedule, as well as producing rate-dependent suppression of responding. In experiment 2, d-amphetamine in creased the Weber fraction in both versions of the task without displacing the curve. Conclusions: These results confirm the disruptive effect of d-am phetamine on timing. The results of experiment 1 are consistent with the pr oposal that the drug reduces the period of the hypothetical pacemaker, Howe ver, the results of experiment 2 do not support this suggestion. Taken toge ther, the results support the notion that different neural mechanisms may b e involved in timing tasks involving temporal distribution of responding an d discrimination of the duration of exteroceptive stimuli.