Dopaminergic effects on simple and choice reaction time performance in Parkinson's disease.

The present study examined whether premovement central neural processing in Parkinson's disease was related to functional motor disability and plasma L-dopa concentration. Reaction time (RT) performance in simple and choice RT tasks was assessed while plasma L-dopa levels were controlled by continuous IV L-dopa infusion in five parkinsonian patients. Five age-matched controls performed the same RT tasks for comparison. Simple RT for the patients was longer than the normal control RT at all infusion levels (p less than or equal to 0.005). However, choice RT was normal when the patients were "on," but became prolonged as plasma L-dopa levels decreased (p less than or equal to 0.01). The results show that there are abnormalities of premovement central neural processing in Parkinson's disease, and that simple and choice RTs are differentially affected by L-dopa replacement. This suggests that different neural mechanisms may be involved in the processing of these tasks.