EFFECTS OF LEVODOPA AND VISCOSITY ON THE VELOCITY AND ACCURACY OF VISUALLY GUIDED TRACKING IN PARKINSONS-DISEASE

Deficits in velocity generation and movement accuracy occur in Parkins on's bradykinesia. In the present study, we attempted to clarify the r elationship between the deficits in velocity generation and movement a ccuracy. Patients with Parkinson's disease and normal controls tracked visually displayed sinusoidal and step targets with the wrist. Perfor mance was evaluated using measurements of velocity and error. Movement velocity was manipulated by two methods: (i) administration of levodo pa; (ii) viscous loading. Dependencies of velocity and error on diseas e state, medication state and viscosity were examined. Visually guided pursuit tracking was characterized by intermittent and frequent veloc ity excursions in both the patients and controls. Fro sinusoidal track ing, levodopa significantly increased velocity in the severely affecte d parkinsonian patients. Prior to the administration of levodopa, step tracking velocity was significantly lower in all patients than in con trols. The 'on' state produced an increase in velocity to control leve ls. Error was significantly greater in the parkinsonian subjects than in controls, but was unchanged by levodopa for both tracking tasks. Ma nipulations of viscosity produced greater changes in velocity than did levodopa, yet a similar independence with respect to accuracy remaine d. Velocity significantly changed by 40-60% in the two tracking tasks from the viscous to antiviscous loads. Error did not change significan tly in 12 out of 14 comparisons of subgroups based on disease and medi cation state. This contradicts the hypothesis that patients with Parki nson's disease primarily reduce velocity during tracking to maintain a cceptable accuracy in the presence of a defective error correction sys tem. Although parkinsonian subjects tracked with reduced accuracy, bot h normal and parkinsonian subjects were able to compensate for signifi cant changes in velocity due to external loading. Thus a propulsion de ficit exists in parkinsonism that may be alleviated with either anntiv iscosity of levodopa. An error correction deficit is also present in p arkinsonism, but is not modified by antiviscosity or levodopa.