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

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 PD patients completed the task twice, once withdrawn from dopamine medications and once while taking their medications.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.