The Image Stabilization System

Ironically, one of the principal reasons for the eyes to move is to keep the direction of gaze stationary—that is, fixed in space. The underlying objective is to keep the visual image of the external world stationary on the retina despite movements of the head and body because even slight movements of this image across the retina (i.e., retinal slip) cause considerable visual blur. For this reason, seeing animals need image-stabilization strategies and reflexes, and for vertebrates this principally involves extraocular muscles that rotate the eyeball within its bony orbit so as to cancel

Table I

Principal Functions of Eye Movements

Image stabilization: Keep the visual image stationary on the retina

Purpose: To prevent blurred vision caused by movements of the head and body

Movements

Vestibuloocular reflex in response to vestibular signals Optokinetic reflex in response to whole-field visual motion Quick phases of vestibuloocular and optokinetic nystagmus Foveation: Capture and keep particular stimuli on the foveal part of retina

Purpose: To facilitate fine, detailed viewing and visual analysis of important objects

Movements

Saccades to rapidly foveate new of different stimuli Smooth pursuit to track moving visual stimuli Fixation to maintain foveation of stationary visual stimuli Binocular alignment: Foveate the same object/point in both retina

Purpose: To facilitate single vision and steropsis in order to extract relative depth (3D) perception from fine disparities between the 2D images in the two eyes Movements

Disjunctive eye movements, both convergent and divergent (other eye movements are conjugate/conjunctive, i.e., equal in the two eyes)

or minimize the retinal slip consequent to movements of the head. Furthermore, the anatomical and physiological substrates of image-stabilization eye movements are remarkably similar across vertebrate species, from primitive fish to modern apes.

Three distinctive types of eye movements are associated with image stabilization: the vestibuloocular reflex (VOR), the optokinetic reflex (OKR), and the extremely high-speed resetting movements called the "quick phase'' (of nystagmus). The VOR is a very short latency reflex instigated by the vestibular system in response to body/head rotation. The OKR, on the other hand, is triggered by visual motion. In the laboratory, VOR and OKR can be separately studied; however, in the real world both are activated whenever the head moves, and they synergistically sum to compensate for most head movements and thereby minimize movement of the visual image on the retina. In contrast, the quick phase is an anticompensatory movement that very rapidly returns the eye to a more central position whenever it is taken to eccentric orbital positions by the vestibulooptokinetic reflexes.

These image-stabilization eye movements are reflexive and automatic. From an evolutionary perspective, they are the most basic, primary eye movements. Indeed, for most vertebrate species VOR, OKR, and the quick-phase movements of nystagmus are the only eye movements.

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