Shared velocity feedback for saccades and smooth pursuit: A control system explanation for eye movement abnormalities in SCA3

Spinocerebellar ataxia type 3 (SCA3), also known as Machado Joseph disease, is a rare genetic neurodegenerative disease. Among various eye movement abnormalities, slow saccades and low gain smooth pursuit have been well described in SCA3. Using the scleral search coil system, we found that saccadic overshoots are also frequent in SCA3. Herein, we use models from engineering control systems in order to link the saccadic and smooth pursuit abnormalities. Saccades and smooth pursuit have different functions and are controlled by diffrernt brain regions. Nonetheless, there is evidence that they do have shared mechanisms (Liston and Krauzlis, 2003. J. Neuroscience, 23:11305; Keller and Missal 2003. Ann NY Acad. Sci. 1004:29). Saccades can be viewed as a step input to a control system. In designing a control system there is a trade-off between increasing reaction time and minimizing overshoot. For example, in a second order system, high gain will reduce the response time but will introduce an over-shoot. One technique to overcome this tradeoff is to add a velocity feedback. In the current presentation we will argue that the lack of velocity feedback in SCA3 patients can explain the measured abnormalities in smooth pursuit as well as the slow saccades and frequent overshoots.