Nystagmus slow-phase velocity during vestibular, optokinetic, and combined stimulation in the monkey.

In Rhesus monkeys the slow-phase velocity of nystagmus was measured during optokinetic, vestibular, and combined stimulation. Accelerations and decelerations of 2.5--40 degrees/s2, and rotation at constant velocities of 70-160 degrees/s were applied. During combined visual-vestibular stimulation, nystagmus slow-phase velocity is a function only of the instantaneous stimulus velocity: It has a gain near unity and is independent of the duration and value of the acceleration. The limited linear working range of the vestibular or optokinetic system is thus extended. During deceleration an inappropriate nystagmus response is elicited only when the previous constant velocity rotation was above the saturation velocity of optokinetic nystagmus (OKN). These results are related to single neuron activity recorded in the vestibular nuclei and the flocculus under identical stimulus conditions.