The horizontal optokinetic reflex (HOKR) and the horizontal vestibuloocular reflex (HVOR) were tested in 21 rabbits before and after unilateral lesions were made in the left cerebellar flocculus. The immediate effect, observed within 15 min following placement of a floccular lesion, was a conjugate nystagmus with the slow phase toward the side opposite to the lesion when the animal was placed in total darkness. This spontaneous nystagmus lasted from several hours to 2 days depending on the extent of damage to the flocculus. It was reversed in sign if the subjacent vestibular nuclei or vestibular nerve were damaged by the operation, and it was totally absent if the unilateral floccular lesions were made in rabbits that had been bilaterally labyrinthectomized. The spontaneous drift of the eyes observed immediately postoperatively caused a bias in measurement of the HVOR that was dependent on the frequency of vestibular stimulation. When measured 50 days postoperatively the HVOR had a normal gain and normal bias. When measured 50 days postoperatively the monocular HVOR (posteroanterior stimulation of the left eye) was significantly reduced in gain at stimulus velocities below 5°/s. A quantitative anatomical analysis of the degeneration of inferior olivary neurons caused by lesions of the flocculus demonstrated contralateral cell loss of as much as 65% of the dorsal cap neuronal population. These data reveal a permanent deficit in the HOKR, but not the HVOR, following unilateral floccular lesions and are consistent with the idea that the flocculus contributes to the regulation of the low-velocity eye movements through the inhibitory modulation of the activity of the subjacent vestibular nuclei.
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