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Effects of d-amphetamine on
performance on two operant timing schedules The disruptive effect of d-amphetamine on timing behaviour has been attributed to a reduction of the period of the pacemaker of the hypothetical internal clock. We examined the effects of d-amphetamine (0.2-0.8 mg/kg) on performance on two timing schedules. In Experiment 1, rats were trained to respond on two levers (A and B) in 50-s trials in which reinforcement was provided intermittently for responding on A in the first half, and B in the second half, of the trial (free-operant psychophysical procedure). In Experiment 2, rats were trained to press lever A after a 2-s stimulus and lever B after an 8-s stimulus, and were then tested with stimuli of intermediate durations (interval bisection task). In both experiments, quantitative timing indices were derived from the psychophysical functions (%B responding vs time). In Experiment 1, d-amphetamine increased the Weber fraction and displaced the psychophysical curve to the left. In Experiment 2, d-amphetamine increased the Weber fraction without displacing the curve. The results of Experiment 1 are consistent with the proposal that d-amphetamine reduces pacemaker period. However, the results of Experiment 2 do not support this suggestion. Taken together, the results suggest that different neural mechanisms are involved in timing tasks involving temporal distribution of responding and discrimination of the durations of exteroceptive stimuli. Keywords: timing, amphetamine, free-operant psychophysical procedure, interval bisection task. |
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