Dopaminergic effects on the implicit processing of distractor objects in Parkinson's disease.

The aim of the present study was to assess the effects of dopaminergic medication on the selection-for-action mechanisms in Parkinson's disease (PD). PD subjects were tested after not having taken medication for at least 12 h ("Off' state) and then retested 1-2 h after medication ("On" state). A three-dimensional kinematic system (ELITE, BTS, Italy) was used to record reach-to-grasp movements to a target object placed at a reaching distance of 30 cm. The target was presented alone or in the presence of distractor objects, which could be of either the same size (compatible distractor) or a different size (incompatible distractor). PD subjects in the Off state were significantly more affected by the presence of the incompatible distractor than in the On state. These results indicate that dopaminergic medication is of benefit in reducing interference effects when distractor objects evoke motor programs that differ from the motor program elicited by the target. Results are discussed in light of the role played by the striatal and mesocortical dopaminergic systems for response selection in basal ganglia disorders.