Evaluation of rate-dependency and internal clock effects of D-amphetamine.

The impact of two doses of d-amphetamine on rats' peak-interval performance was evaluated at two different points of training: with minimum training, 20 sessions, and with extended training, 120 sessions. At both points of training, none of the doses changed the location of the peak time; however, both doses caused a significant increase in the standard deviation of the response distribution during peak trials. Both results are incompatible with some previous empirical results, and with timing accounts that assume that dopamine modulates the pacemaker rate, but are compatible with a rate-dependent effect.