When comparing our findings to the literature, it is important to note that we tested a small and relatively high-functioning cohort of patients with no cognitive impairment and mild to moderate motor symptoms (Hoehn and Yahr stage between 1 and 2); all patients were on dopaminergic medications. Whereas eye movement deficits are highly characteristic of PD and other Parkinsonian disorders
17,19,21–28 and neurodegenerative disorders in general,
18,20 we observed impairments in fixational stability only, and these were limited to the rate of small microsaccades. We did not observe a significant increase in the proportion of saccades classified as SWJ. Notably, more severe deficits in contrast sensitivity and eye movement control are usually found with more advanced disease.
5 Decreased contrast sensitivity at 6 and 12 cyc/deg—that is, in the high-frequency range that is found to be spared in may studies, including ours—has been observed with symptom progression, when patients' performance in static tasks was retested 20 months after the first testing.
54 Sensitive longitudinal eye movement assessments, concurrent with perceptual testing, could reveal the time course of oculomotor impairments in PD and clarify the role of microsaccades and saccadic intrusions such as SWJ in known perceptual impairments in these patients. Our findings indicate that frequent small-amplitude microsaccades occur even at the earliest stage of the disease, while smooth pursuit is still relatively unimpaired.
53 Dopaminergic medications have been found to have little effect on contrast sensitivity with moving stimuli.
49 They have, however, been found to improve ocular motility, which may explain preserved pursuit in our patients.
22 To conclude, our results suggest that specific spatiotemporal contrast sensitivity profiles may represent an easily measurable metric as a component of a broader combined biometric
55 for nonmotor features observed in PD. The simultaneous assessment of eye movements and perceptual contrast sensitivity in PD patients can enhance our understanding of the mechanisms underlying sensorimotor deficits in these patients.