Eye movements
From Freepedia
Eye movements are the voluntary or involuntary movements of the eye.
Contents |
Types
Eye movements are typically classified as either ductions, versions, or vergences.
- Ductions
- Versions
- Vergences
Yoked movements vs. antagnoistic movements
- Hering's law of equal innervation
- Sherrington's law of reciprocal innervation
Eyes are the structural organs that contain the retina, a specialized type of brain tissue that contains the photoreceptors and interneurons that convert light into electrochemical signals that travel along the fibers of the optic nerve to the brain. 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 (Westheimer and McKee, 1954). 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 complication for vision 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 one hertz, 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.
Having two eyes is an added complication, because the brain must point both of them accurately enough that the object of regard falls on corresponding points of the two retinas; otherwise, we would see double. Before dealing with this problem, we shall discuss the movements of one eye alone, and restrict our discussion to primates (monkeys, apes and humans). The movements of different body parts are controlled by striated muscles acting around joints. The movements of the eye are no exception, but they have special advantages not shared by skeletal muscles and joints, and so are considerably different. First, the eye is not rigidly attached to anything, but is held in the orbit by six extraocular muscles. The muscle tension pulls the eye against a pad of fat at the back of the eye, so that when the muscles exert different tensions, 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 joint in the center of the eye.
Four of the extraocular muscles have their origin in the back of the orbit in a fibrous ring called the zonule of Zinn. They then course forward through the orbit and insert onto the globe on its anterior half (i.e., in front of the eye's equator). These muscles are named after their straight paths, and are called the four rectus muscles, or four recti. They insert on the globe at 12, 3, 6, and 9 o'clock, and are called the superior, lateral, inferior and medial rectus muscles. (Note that lateral and medial are relative to the subject, with lateral toward the side and medial toward the midline, thus the medial rectux is the muscle closest to the nose). The names are often abbreviated as the SR, LR, MR, and IR muscles, respectively. The other two extraocular muscles follow more complicated paths. The superior oblique (SO) muscle originates at the back of the orbit and courses forward to a rigid pulley, called the trochlea, on the upper, nasal wall of the orbit. The muscle passes through the pulley, turning sharply across the orbit, and inserts on the lateral, posterior part of the globe. Thus, the SO goes backward for the last part of its path, and even though it goes over the top of the eye, it pulls it downward and lateralward. The last muscle is the inferior oblique (IO), which originates at the lower front of the nasal orbital wall, and passes under the LR to insert on the lateral, posterior part of the globe. Thus, the IO pulls the eye upward and lateralward.
References
- Roger H.S. Carpenter (1988); Movements of the Eyes (2nd ed.). Pion Ltd, London. ISBN: 0850861098.
- Westheimer Gerald, McKee Suzanne P (1975); "Visual acuity in the presence of retinal-image motion".
- Journal of the Optical Society of America 65(7), 847-50.
