Physiology of the eye deals with the function of the eye, its capacities, and its limitations. The actual perception of light takes place in a well-delineated area called the field of vision . What is not seen beyond these boundaries is cataloged and stored in our visual memory center, so that we are not uncomfortable or handicapped by this imposition. Most eyes cannot form a sharp image on the retina without an internal adjustment made by focusing or by some external appliance, such as lenses placed before them. There is a limit to how much detail the eye can resolve, its magnifying abilities being only 15×, considerably less than most microscopes. The spectrum of light to which our retinal receptors are sensitive is confined to specific wavelengths of light; the world of ultraviolet and infrared is invisible to ordinary perception.
Despite these limitations, the human eye is an extremely versatile instrument capable of seeing both in daylight and in dim light, registering colors, appreciating depth, and exercising rapid focusing adjustments. This chapter deals with the mechanisms that enable the eye to carry out these tasks.
Alignment of the eyes
In human beings the two eyes work as though they were one, both projecting to the same point in space and fusing their images so that a single mental impression is obtained by this collaboration. Without this delicate balance we would “see double” because two images would be formed by the independent action of each eye. In other words, stereopsis would be lost because this faculty is totally dependent on the eyes seeing in unison. The ability of the eyes to fuse two images into a single one is called binocular vision .
Binocular vision depends on an exquisite balance of motor and sensory function. The eyes must be parallel when looking straight ahead and they must be able to maintain this alignment when gazing in other positions. Each impulse that directs an eye to move in one direction must be equally received by the other eye. Further, the contraction of an eye muscle pulling the eye in one direction must be accompanied by an equivalent amount of relaxation of its opponent muscle. Without perfectly harmonious eye movement, binocular vision would be impossible because eyes that do not move together do not see together.
Each eye must have good vision because a clear image and a fuzzy image cannot be fused. The brain usually ignores the fuzzy image (suppression). Each macula must have its projection straight ahead, so that the line of vision from each eye intersects at one point in space. Also, the field of vision from each eye must overlap ( Fig. 2.1 ). Although we can see more with two eyes than with one, this difference is not great (~35 degrees) because most of the field of vision from one eye overlaps the field from the other eye. Overlapping visual fields act as a locking device, forging our peripheral vision in place and thereby ensuring central fusion.
Looking straight ahead (fixation)
Fixation involves the simple task of looking straight ahead toward an object in space. Fixation requires stability of the eyes and good monocular function. If the eyes are constantly moving, such as occurs with congenital nystagmus (shaking of the eyes), the eyes can make only scanning motions around an object and never adequately see it in detail. Needless to say, if the ability to fixate becomes compromised by constant eye movements, then the visual acuity of the affected eyes is reduced. If the macula is damaged, then fixation is difficult because anything viewed directly ahead becomes enshrouded in relative darkness.
Fixation can be reduced without organic changes in the eye. Children with strabismus often are found to have poor vision in the turned eye. If a child has crossed eyes, we would think that double vision would occur because the two eyes would not be directed to the same point in space ( Fig. 2.2 ). Children, however, have a wonderful faculty for completely ignoring the image in the turned eye to avoid confusion. It is this constant habit of actively suppressing the image in the turned eye that eventually leads to loss of vision or amblyopia. In some of these children, in whom the suppression mechanism has become profound and the resultant vision very poor, foveal function becomes so depressed that a new point just outside the fovea is used. Such an eye can no longer see straight ahead and the fixation pattern is described as eccentric .
Locking images (fusion)
Fusion is the power exerted by both eyes to keep the position of the eyes aligned so that both foveae project to the same point in space. Because fusion is a binocular act, it is easily disrupted by covering one eye. The eye under cover drifts to its fusion-free position. The amount of movement that the eye makes is a measure of the latent muscular imbalance kept in check by fusion, or the amount of heterophoria. Heterophoria , then, may be defined as the position the eyes assume when fusion is disrupted. The eye under cover may drift in, called esophoria , or drift out, called exophoria . The eye also may drift up and down; this position is called hyperphoria . Fusion also may be disrupted by placing a Maddox rod before one eye. The Maddox rod changes the size and shape of the image presented to the eye under cover so that fusion becomes impossible.
The power of fusion is measurable by prisms (see Ch. 3 ). For example, a four-diopter prism is placed with the base toward the nose of an observer looking at a small letter placed 16 inches (40 cm) from the eye. The prism will displace the image before that eye in a direction toward its apex and the eye moves outward to follow it because of the power exerted by the fusional reflex ( Fig. 2.3A ). Now the prism is removed and the uncovered eye returns to its original position in response to the fusional reflex ( Fig. 2.3B ). Normally, 20- to 40-prism diopters can be exercised by fusional convergence. The amount of fusion exercised with respect to divergence is less, being only 10- to 20-prism diopters. This is measured by using base-out prisms. Vertical imbalances are difficult to overcome because our eyes can overcome only about 2- to 4-prism diopters.
The primary position of the eyes is the straight-ahead position as they look at a point just below the horizon with the head held erect. Movement of the eye from the primary position to a secondary position occurs when the eyes are moved either horizontally or vertically. If the eyes are directed in an oblique position (up and in or down and in), they are said to be in a tertiary position.
The movement of one eye from one position to another in one direction is called duction . In duction, the fellow eye is either covered or patched. The movement of two eyes in the same direction is called a version (dextro-, levo-, sursum-, and deorsumversion) ( Fig. 2.4 ).
Eyes right: dextroversion
Eyes left: levoversion
Eyes up: sursumversion
Eyes down: deorsumversion
An outline of the functions of the extraocular muscles is given in Table 2.1 . The medial and lateral rectus muscles have only one action: to move the eye horizontally. The other four muscles of the eye have auxiliary functions. When these secondary roles are used, assisting the lateral or medial rectus muscles to abduct or adduct, these muscles are called synergists ( Fig. 2.5 ).
|Turns eye inward toward nose or adducts eye
|Turns eye outward toward temples or abducts eye
The main function of the oblique muscles is to rotate the globe either inward (intorsion) or outward (extorsion) . Intorsion occurs when the eye rotates on its long axis so that the 12 o’clock position on the cornea moves toward the nose. For example, if a point on the cornea of the right eye moves inward from 12 to 1 o’clock, then intorsion is said to occur because of the primary action of the right superior oblique muscle or secondary action of the right superior rectus muscle. Similarly, if the point on the right cornea moves outward from 12 to 11 o’clock, then extorsion is said to occur because of the primary action of the right inferior oblique muscle or secondary action of the right inferior rectus muscle.
Control centers for eye movements
The eyes move in response to our own volition or in a passive manner, such as in following a slow-moving target. Volitional eye movements usually are rapid, starting at high speeds and ending just as abruptly. Such movements occur with reading, when words or phrases are quickly scanned, with an abrupt halt coming at the end of a section or a line. These voluntary eye movements are controlled from centers in the frontal lobe of the brain.
Whereas voluntary eye movements tend to be short and choppy, following or pursuit eye movements are rather slow, smooth, and gliding. The velocity of a following movement depends entirely on the speed of the object the eye is tracking. If the fovea is fixed on a moving target with an angular velocity (<30 degrees per second), the eye follows the target almost exactly. With greater speeds, following movement becomes difficult and the smooth, gliding movement is replaced with an irregular, jerky movement. Pursuit movements are controlled from centers in the occipital lobe of the brain.
Looking toward a close object
Vergence is the term applied to simultaneous ocular movements in which the eyes are directed to an object in the midline in front of the face. The term is usually applied to convergence , in which the eyes rotate inward toward each other, or to divergence , in which they rotate outward simultaneously ( Fig. 2.6 ).
Convergence is invariably accompanied by narrowing, or constriction, of the pupils and by accommodation. The triad of convergence, pupillary constriction, and accommodation is often called the accommodative reflex , although in the true sense these movements are merely associated reactions (synkinesis) rather than a true reflex. Each component of the triad facilitates fixation at near. The constriction of the pupil is the attempt by the eye to form a pinhole camera device so that a clearer image is seen. Accommodation enables the object to be focused on the retina; convergence brings the eye inward toward the object of regard.
Seeing in depth
The ability to see in depth enables us to travel comfortably in space. Without it, we could not judge distances, estimate the size of objects beyond us, or avoid bumping into things. Without depth perception, even the simplest of tasks would be difficult. We would be unable to reach accurately for our morning coffee, and passing a car on the highway would be tantamount to suicide. Fortunately, everyone has some depth perception, whether the person has one eye or two. Those with only one eye learn to estimate depth with monocular clues ( Figs. 2.7 and 2.8 ). They know that the speck in the distance that becomes a huge train standing beside them in the station has not grown larger but has merely come closer. There are other clues in addition to changes in object size. The train tracks spread from a point and become parallel, the color of the train changes from a misty blue-gray to dark green, the sound increases, and when the train is alongside, one can feel the heat.
There are many monocular clues that facilitate depth perception, including the following: magnification: well-recognized objects, if they become larger, are deemed to be nearer
Confluence of parallel lines to a point (e.g., railway tracks)
Interposition of shadows
Blue-gray mistiness of objects at a great distance
Parallax: if two objects situated at different points in space are aligned and the head of the observer is moved in one direction, the nearer object will appear to move in the opposite direction
A monocular person, however, if removed from familiar surroundings, would have great difficulty in judging distances because of a lack of any intrinsic depth-perception mechanism. For example, a one-eyed pilot would create a hazard because of the difficulty he or she would experience in maneuvering in space without the normal monocular clues.
Stereopsis is a higher quality of binocular vision. Each eye views an object at a slightly different angle, so that fusion of images occurs by combining slightly dissimilar images. It is the combination of these angular views that yields stereopsis. The same method is used in photography in making three-dimensional pictures. The stereoscopic picture is taken at slightly different angles and later viewed that way.
Focusing at near (accommodation)
Any object can be moved from a distance to about 20 feet in front of an observer and still be seen clearly without accommodation. This distance is called the range of focus . As the object is brought closer than 20 feet, however, the eye must continuously readjust to keep the image of the object clearly focused on the retina. This readjustment requires an increase in the power of the eye and is brought about by an automatic change in the shape of the lens in response to a blurred image ( Fig. 2.9 ). This zoom-lens mechanism in the eye is very active in children; they are able to see a small letter in clear focus only 7 cm from the eye, whereas an adult of 55 years can focus no closer than 55 cm. The range of accommodation is the distance in which an object can be carried toward an eye and be kept in focus. The power of accommodation of an eye is the dioptric equivalent of this distance. By age 75 years, this power is zero.