Speed pressure in conflict situations impedes inhibitory action control in Parkinson's disease

•Parkinson's disease (PD) disrupts certain aspects of cognitive control.•The current study investigated the effects of PD on the ability to resolve response conflict under speed and accuracy instructions by means of a Simon task and ERPs.•PD patients were less proficient at suppressing incorrect response impulses compared to HC under speed instructions.•Likewise, the ERPs showed reduced suppression of the incorrect response alternative under speed instructions for PD patients compared to HC. The current study investigated the effects of Parkinson's disease (PD) on the ability to resolve conflicts when performance emphasized speed vs. response accuracy. PD patients and healthy controls (HC) completed a Simon task, and a subset of participants provided movement-related potential (MRP) data to investigate motor cortex activation and inhibition associated with conflict resolution. Both groups adjusted performance strategically with speed or accuracy instructions. The groups experienced similar susceptibility to making fast errors in conflict trials, but PD patients were less proficient compared to HC at suppressing incorrect responses, especially under speed pressure. Analysis of MRPs showed attenuated inhibition of the motor cortex controlling the conflicting response in PD patients compared to HC. These results confirm the detrimental effects of PD on inhibitory control mechanisms with speed pressure and also suggest that a downstream effect of inhibitory dysfunction in PD might be due to diminished inhibition of the motor cortex.