An approach to understanding eye movements best begins by considering how they serve vision (Leigh & Zee, 1999). One class of eye movements brings objects of interest onto the fovea and includes saccades and quick phases of nystagmus, which are the fastest of eye movements and allow us to rapidly change our line of sight. A second class of eye movements holds images steady on the fovea and includes pursuit, which allows us to track small objects moving across our visual environment, and vestibular slow phases, which hold images steady on the fovea during head motion. Optokinetic slow phases (OKN, full-field visual following) also help stabilize gaze during head rotation. Vergence eye movements rotate the eyes in opposite directions; they bring the images of an object of interest onto both foveae at once, and then keep them there.

Head motion is of two types: angular (rotations), sensed by the semicircular canals, and linear (translations) sensed by the otoliths. For head rotations, horizontal (yaw), vertical (pitch) and, roll (ear to shoulder), compensatory slow phases in the orbit must be equal and opposite to the angular motion of the head. For head translations, fore–aft, up–down or side-to-side, slow phases must be scaled to the location of the point of regard; the closer the target the greater the amplitude of the compensatory slow-phase for a given amount of translational motion. During natural movements of the head, rotational and translational vestibular reflexes work together with the visual-following reflexes, OKN, pursuit and vergence, so that subjects can maintain their line of sight on the particular location of interest in three-dimensional space.

Oculo-motor control signals

To interpret abnormal ocular motility it is helpful to understand the way the central nervous system controls eye movements under normal circumstances (Leigh & Zee, 1999). Here, we review the normal patterns of innervation for moving the eyes to change gaze accurately, and for holding the eyes steady to maintain gaze on a stationary object of interest. The major hindrance to rotation of the globe is orbital viscosity because the moment of inertia of the globe is relatively small. For rapid eye movements (saccades and quick phases of nystagmus), a powerful contraction of the extraocular muscles is necessary to overcome viscous drag.