The effect of circulating levels of plasma levodopa on reaction time performance was studied in patients with Parkinson's disease and untreated normal controls when instructed to move either a shorter or longer distance. On half the movements, subjects were pre-cued on the direction and amplitude of an impending movement. On the remaining movements, only the direction was pre-specified, and the amplitude was determined only when the cue to move was presented. Reaction time performance of patients was evaluated at three infusion levels of levodopa so that the patients were optimally, moderately, or minimally medicated. Parkinsonian patients were always slower to react and move than normal subjects. Clinical state correlated with movement time, but not with reaction time. These results contrast with those in which reaction time was related to plasma levodopa levels when movement direction and initiation were processed concomitantly, but the movement amplitude was pre-cued. It is possible that specification of the amount of muscle activity is partially independent of dopaminergic transmission.
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