Abnormalities of tracking behavior in Parkinson's disease.

Tracking behavior in Parkinsonian patients and in a control group of elderly subjects was studied using a one-dimensional pursuit tracking task. The most obvious abnormality in patients with Parkinson's disease is a tendency to restrict movements to a limited range, thus "clipping" the extreme deviations of the target, reflected by movement arrests, reduced amplitude gain, and decreased velocity gain. Most of the tracking parameters were selectively sensitive to parkinsonian as opposed to age-related impairment of motor functions, the only exception being directional errors, which were more frequent in the old age group. Movement range and amplitude gain were related to general disability as reflected by the Hoehn and Yahr stage. Amplitude gain was also the single factor with the strongest influence on tracking error and was smaller for large target deviations in both groups. A reduction in target velocity caused a significant increase in amplitude gain, suggesting a compensatory strategy (amplitude/lag trade-off) as one possible cause of low-amplitude gain. Decreased peak velocity did not contribute significantly to tracking error. These findings suggest that reduced movement amplitude and slowness are independent phenomena, of which only the former has a significant influence on tracking performance in the task used. Patients with Parkinson's disease showed no impairment of motor learning on this task.