Microsaccades in Parkinson's disease
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Individuals with Parkinson’s disease (PD) display deficits in voluntary saccade generation but improved automatic, visually-triggered saccade performance. This can be tested using prosaccades, saccades to visual stimuli, and antisaccades, saccades in the opposite direction from the visual stimuli. Voluntary saccade impairments resulting in antisaccade direction errors and longer saccadic reaction times (SRTs) are thought to be due to insufficient presetting of neural circuitry during saccade preparation in complex tasks involving suppression and selection. The basal ganglia, a major site of PD pathology, might be the cause of abnormalities in preparing for action selection in PD patients. Recently, microsaccade rates have been hypothesized to reflect the dual preparatory signals of saccade facilitation and suppression. In this thesis, we investigated the microsaccade behaviour of PD patients as they performed prosaccades and antisaccades. We hypothesized that deficits in voluntary movements in PD would result in impaired suppression of involuntary movements as reflected by increased microsaccade rates. Our findings demonstrate consistently elevated microsaccade rates in PD subjects compared to age-matched controls. Furthermore, positive correlations were found between antisaccade direction error rate and microsaccade rate as well as microsaccade rate and Hoehn-Yahr score, an indicator of disease severity in PD patients. We conclude that microsaccades reflect the impaired suppression of involuntary movements caused by voluntary movement deficits in PD pathology. Our findings indicate that microsaccades provide insight into action preparatory mechanisms and BG dysfunction. Therefore, measuring microsaccades in PD may provide a useful biomarker to follow disease progression and effectiveness of treatment therapies.