Effects of levodopa and viscosity on the velocity and accuracy of visually guided tracking in Parkinson's disease

Deficits in velocity generation and movement accuracy occur in Parkinson's disease and are postulated to contribute to the characteristic bradykinesia. In the present study, we attempted to clarify the relationship between the deficits in velocity generation and movement accuracy. Patients with Parkinson's disease and normal controls tracked visually displayed sinusoidal and step targets with the wrist. Performance was evaluated using measurements of velocity and error. Movement velocity was manipulated by two methods: (i) administration of levodopa; (ii) viscous lending. Dependencies of velocity and error on disease state, medication state and viscosity were examined. Visually guided pursuit tracking was characterized by intermittent and frequent velocity excursions in both the patients and controls. For sinusoidal tracking, levodopa significantly increased velocity in the severely affected parkinsonian patients. Prior to the administration of levodopa, step tracking velocity was significantly lower in all patients than in controls. The 'on' state produced an increase in velocity to control levels. Error was significantly greater in the parkinsonian subjects than in controls, but was unchanged by levodopa for both tracking tasks. Manipulations of viscosity produced greater changes in velocity than did levodopa, yet a similar independence with respect to accuracy remained. Velocity significantly changed by 40-60% in the two tracking tasks from the viscous to antiviscous loads. Error did not change significantly in 12 out of 14 comparisons of subgroups based on disease and medication state. This contradicts the hypothesis that patients with Parkinson's disease primarily reduce velocity during tracking to maintain acceptable accuracy in the presence of a defective error correction system. Although parkinsonian subjects tracked with reduced accuracy, both normal and parkinsonian subjects were able to compensate for significant changes in velocity due to external loading. Thus a propulsion deficit exists in parkinsonism that may be alleviated with either antiviscosity or levodopa. An error correction deficit is also present in parkinsonism, but is not modified by antiviscosity or levodopa.