Evidence for independent feedback control of horizontal and vertical saccades from Niemann-Pick type C disease

We measured the eye movements of three sisters with Niemann-Pick type C disease who had a selective defect of vertical saccades, which were slow and hypometric. Horizontal saccades, and horizontal and vertical pursuit and vestibular eye movements were similar to control subjects. The initial movement of oblique saccades was mainly horizontal and most of the vertical component occurred after the horizontal component ended; this resulted in strongly curved trajectories. After completion of the horizontal component of an oblique saccade, the eyes oscillated horizontally at 10–20 Hz until the vertical component ended. These findings are best explained by models that incorporate separate feedback loops for horizontal and vertical burst neurons, and in which the disease selectively affects vertical burst neurons.