Introduction
The visual system is one of the primary means by which humans are aware of and monitor their environment. The visual system provides information on the form, color, size, movement, and distance of any object in sight range. Its importance is seen in the fact that sight loss (blindness) is much more debilitating than any other sensory deprivation.
The anatomy of the eye is quite complex. Each eye sits in a protective, bony skull cavity, an eye socket. The human eye is roughly spherical and about one inch in diameter. Six muscles, attached at one end to the eyeball and at the other end to the eye socket, control the directional movements of each eye.
The semiliquid eyeball interior is surrounded by three tissue layers. Outermost is the tough and protective sclera, made up of fibrous tissue. The sclera, or “white of the eye,” has at its front a circular cornea. This sclera segment is modified to allow light rays to enter the eye and to aid in the focusing of light reflected from objects seen. At the front of each eye, paired eyelids protect the sclera’s outer surface, removing dirt and lubricating with tears by blinking. The eyelids also close reflexively for protection when an object comes close to an eye.
The middle and inner tissue layers of the eye are the choroid and the retina. The choroid holds all the blood vessels that feed the eye and a muscular ciliary body that alters the shape of the eye lens to help to focus light. The retina lines most of the eyeball interior, except at its front. Retinal tissue converts light energy to nerve impulses carried to the brain. Choroidal blood vessels extend throughout the retina, except at its front. There, a hole, the pupil, allows light entry into the eye. A circular iris around the pupil gives each eye its color.
The retina translates light energy into nerve impulses, using rod cells, cone cells, bipolar cells, and ganglion cells. Rods and cones are light-sensitive, yielding nerve impulses when they are struck by light. Bipolar cells transfer the acquired information to the brain via ganglion cell fibers in the optic nerve at the rear of the eye. The rods, sensitive to tiny amounts of light, enable dim light vision. Cones enable the perception of color and detail. Rods and cones hold the pigment rhodopsin (RO, or visual purple). When it interacts with light, RO decomposes to a protein (opsin) and a form of vitamin A called retinol1. More RO must be made before a rod’s next operation. Unless the diet provides vitamin A (as retinol1) in amounts enabling this, an afflicted person has night blindness (nyctalopia). Nyctalopics cannot see well in dim light.
A small region in the retina’s center, the fovea centralis, contains cones but no rods, and cone number per unit of retinal area decreases as the front edge of the retina—near the pupil—is approached from the fovea. In contrast, the relative number of rods increases as the number of cones diminishes. Humans see most clearly in daylight, using the fovea almost exclusively. At night, vision is accomplished mostly by using a retina region at the side of each eye.
A blind spot in the visual field occurs when objects cast images on the retina’s optic disk. This disk, the point where the optic nerve leaves the eye, lacks both rods and cones. The optic nerves from the two eyes pass through the optic chiasma. Fibers from the inner half of each retina cross to the opposite side of the brain. Those from the outer half remain on the same side of the brain. This causes the right visual field, which stimulates the left half of each retina to activate the left half of the thalamus and visual cortex. The left visual field affects the right half of the brain, a situation similar to that of the other human sensory systems.
The visual cortex includes the occipital lobe of each cortical hemisphere, and there is a point-for-point correspondence between the retinas and the cortex. This yields a “map,” whose every point represents a point on the retina and visual space seen by each eye. Vision simultaneously depicts object color, shape, location, movement, and orientation in space. Seeking a model to explain the overall brain action in vision, neurophysiologists have identified various cortical cell types, each involved selectively in these features. Retinal maps from each eye merge in a cortical projection area, which allows images from the two eyes to yield stereoscopic vision. Other brain regions also participate in vision; for instance, the cortex appears to be involved in perceiving form and movement.
Lenses and Vision Types
Just behind the cornea, a transparent, elastic lens is attached to a ligament that controls its shape. Lens shape focuses light reflected from an object and forms on the retina the sharpest possible visual image. The eyes regulate the amount of light reaching retinal rods and cones by contracting or expanding the pupil by means of the iris. These involuntary responses are controlled by brain reflex pathways. The lens also divides the eye into two compartments. The small compartment in front of the lens holds watery aqueous humor. The much larger rear compartment between lens and retina is full of jelly-like vitreous humor. This humor maintains the eye’s shape.
Usable visible (380- to 730- nanometer) light enters the eye through the pupil and excites color sensations by interacting with retinal cones. Light reflected from an object passes through the lens to form a focused retinal image, similar to what a camera forms on film. Focusing a visual image also requires regulation of the amount of light passing through the pupil by making the iris larger or smaller. The eye lens produces an inverted retinal image, interpreted in the brain right-side-up. Binocular vision enables accurate depth perception
. Each eye gets a slightly different view of any object, and the two retina images are interpreted by the brain as a three-dimensional view.
Nocturnal animals see in low-light environments with black-and-white (scotopic) vision. Diurnal (day-living) animals have photopic vision, which needs much more light to perceive colors and textures. Humans have both photopic and scotopic vision. Scotopic vision uses the rods as well as photosensitive RO. RO is bleached by bright light, so scotopic animals are almost blind by day. Humans suffer brief blindness on walking indoors on bright days. Then, by dark adaptation, scotopic vision quickly begins to function. Faulty dark adaptation (night blindness) occurs in humans who lack rods or are vitamin A deficient. Afflicted individuals cannot find their way around at night without artificial light. Photopic vision mostly uses the fovea, so it is due to cones, the only foveal visual cells. In the central fovea there are approximately 100,000 cones per square millimeter of retinal surface. Each cone associates with nerve cells that process the incoming visual data, convey them to the brain cortex, and provide detailed information on objects whose images fall in the fovea.
Peripheral vision occurs outside of the fovea, as may be seen by looking directly at one letter on a page. That letter and a few others nearby look very sharp and black because they are seen by foveal vision. The rest of the page, seen by peripheral vision, blurs. The clarity of foveal vision versus peripheral vision is due to the increasing scarcity of cones in retinal areas farther and farther from the fovea. Also, the nerve connections in the retinal periphery result in each optic nerve fiber being activated by hundreds of rods. This shared action is useful in detecting large or dim objects at night. However, it prevents color vision, which requires the brain to differentiate among many signals.
Visual Defects and Their Symptoms
Human eyes can have numerous vision defects. The most common of these defects are small, opaque bodies (floaters) in the eye humors. Usually, floaters are only an inconvenience. Much more serious are lens opacities called cataracts. They develop for several reasons, including advancing age and diabetes. Opacity of the cornea can also cause obscured vision. It can be repaired through the transplantation of a section of clear cornea from another person. Three very dangerous eye diseases that can cause blindness are detached retinas (retina rips), glaucoma (eye pressure buildup due to blocked tear ducts), and macular degeneration (destruction of the retinal areas responsible for sight).
Six serious, but relatively easily treated, vision problems are myopia, hyperopia, presbyopia, astigmatism, diplopia, and strabismus. They are due to incorrect eyeball length, to lens defects, or to external eye muscle weakness. In myopia (nearsightedness) the eyeball is too long. Light from nearby objects will focus well on the retina. Distant rays focus before it, yielding blurry images. Conversely, farsightedness (hyperopia) occurs because the eyeball is too short. As a result, the light from distant objects focuses on the retina, but that from nearby objects focuses behind it and makes them blurry. An eye can also lose the ability to adapt quickly from far vision to near vision. This problem, presbyopia, usually happens after age forty. In an astigmatism, uneven curvature of an eye lens causes retinal images to be made up of short lines, not sharp points. Weakness or paralysis of external eyeball muscles may cause diplopia (double vision) and strabismus (crossed eyes).
Blindness, the most serious vision problem, is much more debilitating than any other sensory deprivation. It occurs in many forms. Some are temporary, mild, and readily treated. Others are severe and untreatable. Color blindness is an incurable lack of some or all color vision. This congenital form of blindness is attributed to genetic defects in the retina or some part of the optic tract. It is mild, causing, at worst, no color perception and a life spent in a black-and-white world. Amblyopia, weak vision without apparent structural eye damage, is another type of acquired blindness, due to toxic drugs, alcoholism, or hysteria. Blindness may also be caused by diseases such as iritis and trachoma.
Blindness varies in extent, from inability to distinguish light from darkness (total blindness) to inability to see well enough to do any job requiring use of the eyes (economic blindness) to vocational and educational blindness: inability to work in a job done before becoming blind, and inability to become educated by methods commonly used in school, respectively. Most severe blindness is permanent and incurable. There are about two blind people per thousand in industrialized nations and two per hundred in underdeveloped countries. The causes of blindness include genetic abnormalities of components of the eye or brain, pressure on the optic nerve from a brain tumor, detachment of the retina from the choroid, damage to the eyes or brain by excess light, or severe head trauma.
Treatment Options for Vision Problems
Visual defects are most often identified by ophthalmologists who prescribe eye treatments such as the use of eyeglasses, contact lenses, or surgery. Detached retinas and glaucoma can both be repaired by surgery, and most night blindness is cured by adding sufficient vitamin A to the diet to allow optimal cone and rod operation. Amblyopia is treated by psychologists or psychiatrists who identify its basis and work with afflicted individuals to reassure them or apply pharmacological treatment of the problem. Some kinds of structurally based blindness may be cured by surgery (such as removing brain tumors). However, in a great many cases the blindness is incurable. When blindness is acquired by sighted people (adventitious blindness), it can be important to receive the help of a mental health professional to gain the ability to live with it successfully. This is most crucial early in the adventitious blindness, when afflicted individuals are least likely to be able to cope with being cut off from a major source of their contact with the world.
The adjustments that must be made on occurrence of adventitious blindness are so extensive that the blind person eventually becomes a different individual from the sighted person he or she once was. Usually, the initial response to adventitious blindness is apathy and severe depression. These symptoms are followed by the return of interest in living and coping with the practical problems caused by blindness. The function of a psychologist or a psychiatrist is the careful combination of psychotherapy and pharmacological treatment—varied, individually, in its length and scope—to ready each afflicted individual to function well in a sighted world. After an adventitiously blind individual is capable of coping well with being blind, there are several federal and private agencies aimed at teaching such individuals to operate well and to engage in training so as to allow them to reachieve gainful employment.
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