Dissociation of smooth pursuit and vestibulo-ocular reflex cancellation inSCA-6

Objective: To study gaze in SCA-B patients during pursuit and passive whole -body rotation. Background: Smooth pursuit and vestibularly induced eye mov ements interact to maintain the accuracy of eye movements in space (i.e., g aze). Previous studies have implicated the cerebellum, particularly the flo ccular lobe and dorsal vermis, in the control of gaze velocity during pursu it and vestibule-ocular reflex (VOR) cancellation. SCA-6 has recently been identified genetically and characterized as pure cerebellar ataxia that aff ects the cerebellar cortex selectively. Methods: Using infrared oculography , eye movements of five SCA-6 patients and five age-matched normal control subjects were recorded during sinusoidal pursuit and passive whole-body rot ation in the horizontal plane (amplitude, +/- 10 deg; frequency, 0.2 Hz). E ye and gaze gain (eye and gaze velocity/stimulus velocity) were calculated after deleting saccades. Results: Eye gain of all SCA-B patients during pur suit was significantly lower than those of the control subjects (mean +/- S D, 0.26 +/- 0.06 versus 0.91 +/- 0.07). In contrast, eye gain of the patien ts was not significantly different fr om that of the control subjects eithe r during VOR cancellation, when the subjects tracked a target that moved wi th tale same amplitude and phase, like a chair (0.21 +/- 0.05 versus 0.12 /- 0.07), or during visually enhanced VOR (x1), when the target remained st ationary in space (0.85 +/- 0.06 versus 0.95 +/- 0.05). Moreover, there was no significant difference in mean VOR gain in total darkness between the t wo groups. Gaze gain of patients (0.26 +/- 0.06 versus 0.81 +/- 0.06) but n ot control subjects (0.91 +/- 0.07 versus 0.88 +/- 0.08), was significantly different during pursuit and VOR cancellation. Conclusion: SCA-6 patients show dissociation in the control of gaze tracking during smooth pursuit and VOR cancellation.