Single-trials analyses demonstrate that increases in clock speed contribute to the methamphetamine-induced horizontal shifts in peak-interval timing functions

Abstract

Drugs that increase dopamine (DA) transmission have been shown to produce an overestimation of time in duration production procedures as exhibited by horizontal leftward shifts of the psychophysical functions. However, the generality of these results has been inconsistent in the literature. The present report evaluates the effects of five doses of methamphetamine (MAP) (0.5-1.5 mg/kg, i.p.) on two duration production procedures, the single duration peak-interval (PI) procedure and the multiduration tri-peak procedure in rats. We replicated and extended prior results by showing a dose-dependent proportional overestimation of time that was equivalent on both procedures (i.e., subjects behaved as though they expected reinforcement to be available earlier in real time). Single-trials analyses demonstrated that the reduction in peak rate that is often observed after MAP administration is due to an increase in the proportion of trials in which responding occurred at very low rates and without temporal control. However, these low-rate trials were not the source of the leftward shift in the temporal estimates. Rather, we found that the leftward shift of the PI functions was due to proportional changes in the placement of temporally controlled high-rate responding, which is consistent with a DA-mediated alteration in clock speed.