Optokinetic nystagmus (OKN) and optokinetic after-responses after bilateral vestibular neurectomy in the monkey.

The superior branch of the vestibular nerve containing peripheral axons of primary afferents originating in the lateral and anterior semicircular canals was cut bilaterally in three monkeys (vestibular neurectomy). Vertical and horizontal components of eye position were monitored by electro-oculography (EOG) during different stimulus and behavioral paradigms. Postoperatively, monkeys were unable to hold their eyes in eccentric lateral positions in complete darkness. The eyes drifted slowly back to the primary position where eye drift was minimal (null-zone). After vestibular neurectomy the time constant of the eye position integrator in darkness was 4-8 s. Constant velocity optokinetic stimuli produced peak velocities of horizontal OKN that were similar to those before operation. Consistent optokinetic after-responses could not be observed after neurectomy for stimulus durations of less than 60 s. However, with stimulus periods greater than 60-120 s a drift near the primary position of the eyes appeared in darkness which had the same direction as the slow phases of the preceding OKN. Drift velocity was too high to be explained by drift due to the imperfect eye position integrator alone. We assume that drift after prolonged optokinetic stimulation is a combination of an after-response similar as it can be observed after smooth pursuit and of drift due to an imperfect eye position integrator. Secondary optokinetic after-nystagmus was not observed after neurectomy.