The physiology of eye movement

Why do our eyes move? One basic reason is that the visual system functions optimally only under certain constraints of positioning and velocity of the retinal image. Limited slip velocity is a basic requirement: images moving at more than a few deg/sec across the retina are seen blurred. A primary source of such slip is rotation of the head. The vestibulo-ocular reflex (VOR) and optokinetic reflex (OKN) generate a counterrotation of the eye in the head which largely compensate the head movement, with a relatively stable direction of gaze (position of eye in space) as a result. The compensatory eye movement is in principle continuous and smooth, and has a velocity approximating that of the head, with opposite direction. This smooth component is interrupted by discontinuous steps (fast phases or saccades), which rapidly displace the eye in the direction of the head rotation and at the same time prevent mechanical saturation of the eye-in-head motion. A-foveate animals such as the rabbit only generate these combined eye and head movements, in which gaze is kept almost stable or displaced stepwise over rather large angles.