Differential effects of dopaminergic medication on basic motor performance and executive functions in Parkinson's...

Patients with Parkinson's disease (PD) often show deficits in the self-initiation and selection of movements, which can be partly compensated for by external cues. We here investigated impairments in the initiation and selection of self-initiated or externally cued movements in PD. Specifically, we assessed how behavioral changes relate to medication, disease severity, and basic motor or cognitive deficits.

Seventeen akinetic-rigid PD patients and 16 healthy controls (HC) performed a computerized motor task assessing differences between internally and externally triggered movements and reaction times. Patients performed the task twice in a randomized fashion, once with their regular dopaminergic medication and once 12h after withdrawal of medication. Additionally, all subjects underwent comprehensive neuropsychological and motor assessments.

Compared to HC, patients showed a significant slowing across all tasks. Furthermore, patients showed a selective deficit of movement initiation as indexed by longer reaction times when movement lateralization was internally chosen as opposed to being externally cued. This deficit correlated significantly with motor scores of the Unified Parkinson's Disease Rating Scale (UPDRS). Notably, there was no main effect of dopaminergic medication ("ON"/"OFF") on internally and externally triggered movements despite significant improvement of UPDRS and maximum finger tapping frequency in the "ON" state.

Our results suggest that disease severity in PD patients is related to disturbances in internal action initiation, selection and simple decision processes. Moreover, the data add further support to the notion that dopaminergic medication differentially affects motor and cognitive performance in PD. These findings imply that disturbances in executive functions in PD are also influenced by factors other than reduced dopaminergic activity.