Head and eye movements following vestibular stimulus in squirrel monkeys.

Head and eye movements induced by vestibular stimuli (rotational velocity steps) were studied under several experimental conditions in squirrel monkeys. Monkeys showed prominent head nystagmus under a free head condition. Slow phase gaze velocity (SPGV) was calculated by summing the slow phase speed of coupled head and eye nystagmus. A statistical test showed no significant difference between slow phase eye velocities in a fixed head versus SPGV in a free head condition. This indicates that an accurate negative feedback control of eye movements is performed by the vestibular system when sensing active head movements. Bilateral lateral semicircular canal block eliminated these responses in subjects without amphetamine injection. However, in amphetamine-injected animals, 14-21% of preoperative response could be found even after this operation. Since the plane of rotation was not parallel to that of the lateral semicircular canals, the result was considered to be due to vertical semicircular canal stimulation.