Loss of visual suppression of vestibular nystagmus after flocculus lesions.

Summary Eye movements of rhesus monkeys were studied after flocculus and paraflocculus lesions. Positional nystagmus, gaze nystagmus, optokinetic nystagmus (OKN) and visual suppression of caloric nystagmus was present to the operated side after bilateral flocculus lesions. Gaze nystagmus was present to the operated side after unilateral flocculus removal, and in all directions after bilateral lesions. Optokinetic nystagmus (OKN) was altered after unilateral and bilateral flocculus destruction. Animals were unable to follow OKN drum rotation at velocities higher than 45°/sec after bilateral flocculectomy. Caloric nystagmus in darkness was unaffected by floccular destruction. Initially after unilateral flocculus removal there was ipsilateral directional preponderance but this later disappeared. Visual suppression of caloric nystagmus, a reproducible phenomenon in the monkey, was lost or significantly reduced by flocculus destruction. After unilateral flocculus removal visual suppression of caloric nystagmus was lost or reduced when quick phases were directed to the ipsilateral side. After bilateral flocculus destruction suppression of caloric nystagmus was lost or reduced in both directions. Suppression of spontaneous nystagmus and PAN was also reduced after flocculus destruction. These data suggest that the flocculus exerts an inhibitory influence on mechanisms responsible for production of nystagmus with ipsilateral quick phases and contralateral slow phases. They support the hypothesis that the flocculus is a medium through which the visual system can alter or modify vestibular reflexes.

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