Primates and many other invertebrates use two types of voluntary eye movement to track objects of interest: smooth pursuit and saccades.[1] These movements appear to be initiated by a small cortical region in the brain's frontal lobe.[2][3] This is corroborated by removal of the frontal lobe. In this case, the reflexes (such as reflex shifting the eyes to a moving light) are intact, though the volutary control is obliterated.

Yoked movement vs. antagonistic movement

The visual system in the brain is too slow to process that information if the images are slipping across the retina at more than a few degrees per second.[6] Thus, to be able to see while we are moving, the brain must compensate for the motion of the head by turning the eyes. Another specialisation of visual system in frontal-eyed animals is the development of a small area of the retina with a very high visual acuity. This area is called the fovea, and covers about 2 degrees of visual angle in people. To get a clear view of the world, the brain must turn the eyes so that the image of the object of regard falls on the fovea. Eye movements are thus very important for visual perception, and any failure to make them correctly can lead to serious visual disabilities. To see a quick demonstration of this fact, try the following experiment: hold your hand up, about one foot (30 cm) in front of your nose. Keep your head still, and shake your hand from side to side, slowly at first, and then faster and faster. At first you will be able to see your fingers quite clearly. But as the frequency of shaking passes about 1 Hz, the fingers will become a blur. Now, keep your hand still, and shake your head (up and down or left and right). No matter how fast you shake your head, the image of your fingers remains clear. This demonstrates that the brain can move the eyes opposite to head motion much better than it can follow, or pursue, a hand movement. When your pursuit system fails to keep up with the moving hand, images slip on the retina and you see a blurred hand.

When the muscles exert differential tensions (contractions in synergistic muscles and relaxation of antagonist muscles), a torque is exerted on the globe that causes it to turn. This is an almost pure rotation, with only about one millimeter of translation (Carpenter, 1988). Thus, the eye can be considered as undergoing rotations about a single point in the center of the eye.

Neuroanatomy

The brain exerts ultimate control over both voluntary and involuntary eye movements. Three cranial nerves carry signals from the brain to control the extraocular muscles. They are: