Oculomotor related interaction of vestibular and visual stimulation in vestibular nucleus cells in alert monkey

The activity of single cells in the vestibular nuclei in alert, behaving monkey was studied by extracellular recording. A majority of the neurons found in the superior and the rostral medial vestibular nuclei can be divided into two classes on the basis of their discharge relationship to eye movements evoked during head rotation, visual target pursuit, or visual suppression of the vestibulo-ocular reflex. The firing rate of the first unit type is proportional to head rotational velocity (and the resulting compensatory eye velocity) but is not modulated during slow eye movements of pure visual origin. During visual suppression of the vestibulo-ocular reflex, the relationship of this type of unit discharge to head velocity remains unchanged, although the eye velocity is now zero. The second type of unit more closely resembles oculomotor neurons in that its discharge pattern is proportional to eye position and velocity during eye movements of both visual and vestibular origin. However during suppression of the vestibuloocular reflex this type of unit continues to show a greatly reduced but consistent modulation of discharge rate proportional to head velocity. Thus the direct projection of vestibular neurons to oculomotor neurons cannot by itself account for the ability of the monkey to completely suppress its vestibulo-ocular reflex.