Ultrastructure of projections to the oculomotor nucleus and inferior olive from vestibular and cerebellar neurons involved in compensatory eye movements

Early in the evolution of vertebrates eye movements were strictly primitive reflexes that were predominantly controlled by vestibular and visual sensory stimuli. Later during phylogeny, along with the development of the fovea of the retina, vertebrates acquired the ability to make voluntary eye movements (BOttner and BUttner-Eonever, 1988). In short, five types of eye movements can be distinguished. The first two are slow eye movements that compensate for movements of the head and the visual surround; these are named the vestibuloocular reflex (VOR) and the optokinetic reflex (OKR), respectively. Extended rotatory or visual stimulation results in a so-called vestibular or optokinetic nystagmus with a slow (compensatory) and a fast (reset) phase. The eye movements that operate predominantly under voluntary control are saccades, smooth pursuit, and convergence. Saccadic eye movements are fast conjugate eye movements which reset the eye position; smooth pursuit movements are used to follow a small moving visual target; and convergence movements are slow disconjugate eye movements enabling frontal-eyed animals to foveate near objects and establish stereoscopic vision.