Initiation and inhibitory control of saccades with the progression of Parkinson's disease – Changes in three major drives converging on the superior colliculus
The cardinal pathophysiology of Parkinson's disease (PD) is considered to be the increase in the activities of basal ganglia (BG) output nuclei, which excessively inhibits the thalamus and superior colliculus (SC) and causes preferential impairment of internal over external movements. Here we recorded saccade performance in 66 patients with PD and 87 age-matched controls, and studied how the abnormality changed with disease progression. PD patients were impaired not only in memory guided saccades, but also in visually guided saccades, beginning in the relatively early stages of the disease. On the other hand, they were impaired in suppressing reflexive saccades (saccades to cue). All these changes deteriorated with disease progression. The frequency of reflexive saccades showed a negative correlation with the latency of visually guided saccades and Unified Parkinson's Disease Rating Scale motor subscores reflecting dopaminergic function. We suggest that three major drives converging on SC determine the saccade abnormalities in PD. The impairment in visually and memory guided saccades may be caused by the excessive inhibition of the SC due to the increased BG output and the decreased activity of the frontal cortex-BG circuit. The impaired suppression of reflexive saccades may be explained if the excessive inhibition of SC is “leaky.” Changes in saccade parameters suggest that frontal cortex-BG circuit activity decreases with disease progression, whereas SC inhibition stays relatively mild in comparison throughout the course of the disease. Finally, SC disinhibition due to leaky suppression may represent functional compensation from neural structures outside BG, leading to hyper-reflexivity of saccades and milder clinical symptoms.