Oculomotor Systems and Control

A plethora of motor control challenges are presented to the oculomotor system whose primary purpose is to point the eyes to hold images stably on the fovea, the region of the retina with the highest visual acuity. The eyes can rotate in horizontal, vertical, and torsional (around the axis of gaze) directions and require the coordinated action of a total of 12 extraocular muscles. In addition, the eyes are mounted in a head that has the ability to move in roll, pitch, and yaw axes in addition to the ability to translate from side to side, and fore and aft. Positioned on the top of the body, the head is carried through three-dimensional space. Therefore, if the oculomotor system cannot solve this litany of stabilization problems, blurred, moving, or double images are presented to the visual system. Many portions of the cerebral cortex and brain stem have been committed to carry out this complex system control problem. We use a layered approach to understand the oculomotor system starting with the extraocular muscles, and working backward into the brain stem and eventually the cerebral cortex. Particular attention is devoted to control of voluntary saccades, smooth pursuit, and vergence. The longstanding dispute that was raised more than 150 years ago between Hermann von Helmholtz and Ewald Hering on whether the central nervous system synthesizes signals that control the eyes together or individually is addressed. A number of clinical applications are provided to amplify the understanding of the anatomy and neurophysiology of eye movement control.