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M. F. Walker, J. Tian, X. Shan, H. Ying, R. J. Tamargo, D. S. Zee; Ablation of the Cerebellar Nodulus and Uvula Selectively Impairs Downward Pursuit. Invest. Ophthalmol. Vis. Sci. 2007;48(13):943.
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To determine the role of the cerebellar nodulus and uvula in ocular pursuit.
In two rhesus monkeys, we used the scleral search coil method to record eye movements during step-ramp pursuit, before and after surgical ablation of the cerebellar nodulus and uvula (Nod/Uv). Monkeys sat in a primate chair with the head immobilized. Each pursuit trial began with fixation of a red target that was back-projected onto a tangent screen in front of the animal. After a random delay, the target stepped horizontally or vertically by 4 deg, then moved smoothly in the opposite direction (20 deg/s) for 1 sec. The animal was rewarded for maintaining fixation of the target. For each experimental session, we calculated pursuit gain by determining the ratio of median slow-phase (desaccaded) eye velocity during the period from 200 to 600 ms after the onset of eye movement to target velocity (all trials pooled for each direction of motion).
Results were similar in the two monkeys. Median steady-state pursuit gains before the lesions were > 0.7 for horizontal and upward pursuit, and were somewhat lower for downward pursuit (Figure). After the lesion, the gain of downward pursuit decreased by 67 %, of upward pursuit by 2 %, and of horizontal pursuit by 14% (mean of data from both monkeys). After the lesion, both monkeys had downbeat nystagmus in the dark (mean 5.5 deg/s) but not in the light.
Similar to many humans with cerebellar disease, monkeys with Nod/Uv lesions have a selective deficit of downward pursuit, suggesting a specific role for the Nod/Uv in the facilitation of downward pursuit. A vertical pursuit asymmetry alone does not cause downbeat nystagmus in the light.
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