Interaction of levodopa and cues on voluntary reaching in Parkinson's disease

The bradykinesia associated with Parkinson's disease (PD) can be improved by both levodopa and the use of external cues. We examined the combined effect of levodopa and external cueing on the voluntary reaching movements of individuals with PD. Nine subjects with PD and nine matched controls were studied reaching to a ball target. Subjects with PD were studied after being off levodopa overnight and again on their morning dose. Kinematic data were collected as all subjects made both accurate and fast reaches under two different cue conditions: noncued (self-initiated) and cued (triggered by a light). Subjects with PD reached more slowly than controls under all conditions. PD subjects increased their reach velocity and decreased movement time after taking levodopa and also when moving to a cue. However, the effects of levodopa and cueing were not additive. Instead, levodopa improved reach velocity to a greater extent in the noncued vs. cued condition. We also found that levodopa improved accurate (self-paced) reaches more than fast reaches. These data suggest that levodopa may preferentially improve voluntary reaches that are more internally generated.