Smooth pursuit in patients with downbeat nystagmus.

Smooth pursuit and gaze holding in light and darkness was investigated in 19 patients with downbeat nystagmus (DBN) and 17 control subjects. Vertical downward smooth pursuit was selectively impaired in patients, and gaze holding with a visible target was only slightly better than in darkness. The selective impairment of downward pursuit and the poor visual suppression of ocular drift in DBN support the involvement of the floccular lobe, which participates in gaze holding and contains gaze-velocity Purkinje cells with a downward on-direction for smooth pursuit.