Just as there is a mechanism which pulls the eyes away from the center of gaze, so must there be a mechanism which brings the eyes back to the central position. The validity of a hypothetical mechanism of eye centering can be discerned in the following clinicopathologic and experimental studies of man and monkey.
In normal man and monkey all eye movements are bilateral, whereas motor acts in the rest of the body may be unilateral or bilateral. Discrete unilateral as well as bilateral movements can occur in the face, neck, trunk, and extremities, but it is quite unusual for man to move one eye at will. Both eyes move synchronously for any given performance. Not only do they move at will rapidly and with great precision in any desired direction, but they coordinate in such a way that the requirements of binocular vision are met without the occurrence of