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When the eyes move to bring a new object into fixation, the movement is not smooth; it is a series of short, abrupt jumps called saccades. A typical test of saccade function might consist of having a subject fixate on a light in the center of the visual field and immediately move his eyes to any new light that appears. Such saccades have been called ''reflexive'' since, although not true reflexes, they occur in an almost involuntary response to the appearance of a target. A second type of saccade test might be called voluntary. In such a test, the subject might be asked not to move his eyes on the appearance of a target light, but after the target has disappeared to move his eyes to the spot where it had been. Researchers investigating the physiology of Parkinson's disease have found that comparing these two types of saccades, reflexive and voluntary, reveals subtle deficits in the performance of Parkinson's patients. The velocity, duration, and latency of saccades were not significantly different between Parkinson's patients and healthy subjects, nor were the patients deficient in reflexive saccades. However, the patients had significantly poor performance in voluntary saccades, when they were required to move their eyes to the spot where the target had been. This deficit can not be explained by a loss of spatial information by the Parkinson's patients, since after a few corrective saccades their eyes came to rest on the target area just as accurately as normal subjects. Therefore, without the visual input, the final result is just as accurate, but the Parkinson's patients can only achieve this result through an erroneous saccade, followed by corrections. The authors interpret this result to mean that the neuronal pathways necessary for reflexive saccades are not impaired by Parkinson's disease. However, when the visual stimulus is removed, alternate pathways, presumably involving the basal ganglia, come into play. These alternate pathways are impaired by the pathologic processes of Parkinson's disease. (Consumer Summary produced by Reliance Medical Information, Inc.)