Effect of vestibular deafferentation upon positional nystagmus in the squirrel monkey.

Seventeen of eighteen squirrel monkeys, under a restrained condition, showed positional nystagmus in different body positions. Good repeatability and consistency of the nystagmus were found, especially at the left-lateral, right-lateral, and head-hanging positions. Positional nystagmus was not observed after the subject underwent bilateral labyrinthectomy. However, one subject in which part of the crista ampullaris posterior remained, continuously showed a positional nystagmus. Therefore, the existence of a minimal vestibular imput from the crista ampullaris could provoke the nystagmus. Some changes occurred in positional nystagmus after bilateral macular ablation; however, the nystagmus did not completely disappear. Even though imput from the crista ampullaris is essential to provoke positional nystagmus in squirrel monkeys, the positional nystagmus probably results from a central dyscoordination between vestibulo-oculomotor function, spino-oculomotor function, vestibular and brain circulation, and psychological condition.