Effects of stimulus–response compatibility on inhibitory processes in Parkinson’s disease

Parkinson's disease (PD) is a neurodegenerative basal ganglia disorder accompanied by deficits in cognitive functions. One important executive function is the inhibition of responses. Due to basal ganglia damage, processes related to the selection of response are also dysfunctional. However, the relevance of deficits in response selection to processes related to response inhibition in PD is not clear. In this study we examined these processes by means of event-related potentials (ERPs) in two Go/Nogo tasks. In one task the stimulus-response mapping was compatible and in the other task it was incompatible with the meaning of the stimuli. The behavioural results show that PD patients were unaffected in the compatible response inhibition task but encountered problems in the incompatible one. In the ERPs the N2, generally reflecting response selection, was delayed for the PD compared to the control group. This suggests that response selection is delayed in PD. Moreover, the N2 was specifically enhanced in Nogo trials. This indicates that premotor inhibition, which is probably reflected by the Nogo-N2, is intensified in PD. The P3 was specifically attenuated and delayed after Nogo stimuli in the incompatible condition for PDs. Assuming that the Nogo-P3 reflects the evaluation of successful motor inhibition, our data show that this process is attenuated and delayed in PD but mainly in the incompatible task. The results suggest that inhibitory deficits in PD are only evident in complex (incompatible) stimulus-response mappings. These effects are probably due to an overstrain of striatal processes.