Evaluation of rate-dependency and internal clock effects of d-amphetamine

Dopamine precursor depletion affects performance and confidence judgements when events are timed from an explicit, but not an implicit onset

Dopamine affects processing of temporal information, but most previous work has tested its role in prospective tasks, where participants know in advance when the event to be timed starts. However, we are often exposed to events whose onset we do not know in advance. We can evaluate their duration after they have elapsed, but mechanisms underlying this ability are still elusive. Here we contrasted effects of acute phenylalanine and tyrosine depletion (APTD) on both forms of timing in healthy volunteers, in a within-subject, placebo-controlled design. Participants were presented with a disc moving around a circular path and asked to reproduce the duration of one full revolution and to judge their confidence in performance. The onset of the revolution was either known in advance (explicit onset) or revealed only at the end of the trial (implicit onset). We found that APTD shortened reproduced durations in the explicit onset task but had no effect on temporal performance in the implicit onset task. This dissociation is corroborated by effects of APTD on confidence judgements in the explicit task only. Our findings suggest that dopamine has a specific role in prospective encoding of temporal intervals, rather than the processing of temporal information in general.

Pharmacological studies of performance on the free-operant psychophysical procedure

In the free-operant psychophysical procedure (FOPP), reinforcement is provided intermittently for responding on lever A in the first half and lever B in the second half of a trial. Temporal differentiation is measured from the psychometric function (percent responding on B, %B, versus time from trial onset, t), the index of timing being T50, the value of t at %B=50. T50 is reduced by acute treatment with 5-hydroxytryptamine (5-HT1A, 5-HT2A) and dopamine (D1-like, D2-like) receptor agonists. The effects of the agonists can be reversed by the respective antagonists of these receptors. Evidence is reviewed suggesting that the effect of endogenous 5-HT is mediated by 5-HT2A receptors and the effect of endogenous dopamine by D1-like receptors. Data are presented on the effects of lesions of the prefrontal cortex and corpus striatum on the sensitivity of performance on the FOPP to D1-like and D2-like receptor agonists. Lesions of the nucleus accumbens, but not the dorsal striatum or prefrontal cortex, attenuated the effects of a D1-like receptor agonist, 6-chloro-2,3,4,5-tetrahydro-1-phenyl-1H-3-benzazepine [SKF-81297], but not a D2-like receptor agonist, quinpirole, on T50. The results indicate that a population of D1-like receptors in the ventral striatum may contribute to the control of timing performance on the FOPP.