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.

Rapid ventricular tachycardia in patients with tetralogy of Fallot and implantable cardioverter-defibrillator: Insights from the DAI-T4F nationwide registry

BACKGROUND

In repaired tetralogy of Fallot (TOF), little is known about characteristics of patients with rapid ventricular tachycardia (VT). Also, whether patients with a first episode of nonrapid VT may subsequently develop rapid VT or ventricular fibrillation (VF) has not been addressed.

OBJECTIVES

The objectives of this study were to compare patients with rapid VT/VF with those with nonrapid VT and to assess the evolution of VT cycle lengths (VTCLs) overtime.

METHODS

Data were analyzed from a nationwide registry including all patients with TOF and implantable cardioverter-defibrillator (ICD) since 2000. Patients with ≥1 VT episode with VTCL ≤250 ms (240 beats/min) formed the rapid VT/VF group.

RESULTS

Of 144 patients (mean age 42.0 ± 12.7 years; 104 [72%] men), 61 (42%) had at least 1 VT/VF episode, including 28 patients with rapid VT/VF (46%), during a median follow-up of 6.3 years (interquartile range 2.2-10.3 years). Compared with patients in the nonrapid VT group, those in the rapid VT/VF group were significantly younger at ICD implantation (35.2 ± 12.6 years vs 41.5 ± 11.2 years; P = .04), had more frequently a history of cardiac arrest (8 [29%] vs 2 [6%]; P = .02), less frequently a history of atrial arrhythmia (11 [42%] vs 22 [69%]; P = .004), and higher right ventricular ejection fraction (43.3% ± 10.3% vs 36.6% ± 11.2%; P = .04). The median VTCL of VT/VF episodes was 325 ms (interquartile range 235-429 ms). None of the patients with a first documented nonrapid VT episode had rapid VT/VF during follow-up.

CONCLUSION

Patients with TOF and rapid VT/VF had distinct clinical characteristics. The relatively low variation of VTCL over time suggests a room for catheter ablation without a backup ICD in selected patients with well-tolerated VT.

Dopamine Precursor Depletion in Healthy Volunteers Impairs Processing of Duration but Not Temporal Order

Studies in animals and humans have implicated the neurotransmitter dopamine in duration processing. However, very few studies have examined dopamine's involvement in other forms of temporal processing such as temporal order judgments. In a randomized within-subject placebo-controlled design, we used acute phenylalanine/tyrosine depletion (APTD) to reduce availability of the dopamine precursors tyrosine and phenylalanine in healthy human volunteers. As compared to a nutritionally balanced drink, APTD significantly impaired the ability to accurately reproduce interval duration in a temporal reproduction task. In addition, and confirming previous findings, the direction of error differed as a function of individual differences in underlying dopamine function. Specifically, APTD caused participants with low baseline dopamine precursor availability to overestimate the elapse of time, whereas those with high dopamine availability underestimated time. In contrast to these effects on duration processing, there were no significant effects of APTD on the accuracy of discriminating the temporal order of visual stimuli. This pattern of results does not simply represent an effect of APTD on motor, rather than perceptual, measures of timing because APTD had no effect on participants' ability to use temporal cues to speed RT. Our results demonstrate, for the first time in healthy volunteers, a dopaminergic dissociation in judging metrical (duration) versus ordinal (temporal order) aspects of time.