Dissociable Effects of Dopamine on the Initial Capture and the Reactive Inhibition of Impulsive Actions in Parkinson's Disease

Abstract

Dopamine plays a key role in a range of action control processes. Here, we investigate how dopamine depletion caused by Parkinson disease (PD) and how dopamine restoring medication modulate the expression and suppression of unintended action impulses. Fifty-five PD patients and 56 healthy controls (HCs) performed an action control task (Simon task). PD patients completed the task twice, once withdrawn from dopamine medications and once while taking their medications. PD patients experienced similar susceptibility to making fast errors in conflict trials as HCs, but PD patients were less proficient compared with HCs at suppressing incorrect responses. Administration of dopaminergic medications had no effect on impulsive error rates but significantly improved the proficiency of inhibitory control in PD patients. We found no evidence that dopamine precursors and agonists affected action control in PD differently. Additionally, there was no clear evidence that individual differences in baseline action control (off dopamine medications) differentially responded to dopamine medications (i.e., no evidence for an inverted U-shaped performance curve). Together, these results indicate that dopamine depletion and restoration therapies directly modulate the reactive inhibitory control processes engaged to suppress interference from the spontaneously activated response impulses but exert no effect on an individual's susceptibility to act on impulses.

Figures

Figure 1

Mean RTs (A) and accuracy rates (B) on corresponding (Cs) and noncorresponding (Nc) trial types for HCs and PD participants in off and on dopamine medication states. Error bars reflect SEMs.

Figure 2

CAFs for corresponding (Cs) (A) and noncorresponding (Nc) trial (B) types for HCs and PD participants in the dopamine off and dopamine on state. Errors are predominantly associated with the fastest RTs on noncorresponding (Nc) trials, a pattern that does not differ between HCs and PD participants in the off dopamine state. Error bars reflect SEMs.

Figure 3

RT delta plots for HCs and PD participants in the dopamine off and dopamine on state. HCs show initial increase in interference followed by a drastic suppression of interference (i.e., large negative delta slope) at the slow end of the distribution. PD participants in off dopamine state show markedly less proficient suppression of interference from action impulses. The reduced proficiency of suppressing interference in the off dopamine state is significantly improved in the on dopamine state (i.e., a steeper negative-going delta slope at the slow end of the distribution). Error bars reflect SEMs.