Introduction | The Mechanics of Vision | Visual Acuity | Vision Disorders | Low Vision

INTRODUCTION

Vision, ability to see the features of objects we look at, such as color, shape, size, details, depth, and contrast. Vision is achieved when the eyes and brain work together to form pictures of the world around us. Vision begins with light rays bouncing off the surface of objects. These reflected light rays enter the eye and are transformed into electrical signals. Millions of signals per second leave the eye via the optic nerve and travel to the visual area of the brain. Brain cells then decode the signals into images, providing us with sight.

Almost all animals respond to light. The one-celled amoeba responds to light by turning in its direction. Bees have complicated eyes that contain many lenses for sensing colors and shapes of flowers. However, it is the vertebrates (animals with backbones) that have eyes and a brain that work together to process light into true images. Human vision is particularly unique in that the human brain can process visual images and use them to create language and pictures and to store information for future use.

The eyes of many vertebrates are specialized for certain situations. Bats see best at night. Birds of prey, such as hawks and eagles, are able to see extremely small details, such as tiny rodents viewed from high in the air.

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II THE MECHANICS OF VISION

Light rays reflected from any object we look at enter the eye and are focused by the eye's optical structures: cornea, iris, pupil, and lens. The final destination of the light rays is the retina, a layer of nerve tissue that lines two-thirds of the back of the eye. In the center of the retina is the macula, an area that is only 1.5 mm (0.06 in) in diameter. The macula is responsible for the clearest, most detailed vision.

The retina is made up of two types of cells: cones and rods. Cones are nerve cells that are sensitive to light, detail, and color. Millions of cone cells are packed into the macula, aiding it in providing the visual detail needed to scan the letters on an eye chart, see a street sign, or read the words in a newspaper.

Cones also produce the sensation we call color. Cones contain three different pigments, which respond either to blue, red, or green wavelengths of light. Cones mix the color signals to produce the variety of colors we see. If a person is missing one or more of the pigments, that person is said to be color-blind and has difficulty distinguishing between certain colors, such as red from green.

Rods are designed for night vision and the detection of motion and objects. They also provide peripheral vision, but they do not see as acutely as cones. Rods are insensitive to color. When a person passes from a brightly lit place to one that is dimly illuminated, such as entering a movie theater during the day, the interior seems very dark. After some minutes this impression passes and vision becomes more distinct. In this period of adaptation to the dark the eye becomes almost entirely dependent on the rods for vision, which operate best at very low light levels. Since the rods do not distinguish color, vision in dim light is almost colorless.

Light rays that reflect from the upper half of any object we look at are focused on the lower half of the retina. Rays from the lower half of the same object are focused on the upper half of the retina. This would seem to give us an upside-down picture of the world. Fortunately, these signals are rearranged when the brain processes them into an image that is right side up.

Another feature of eyesight is stereoscopic or binocular vision, the ability of both eyes to look straight ahead but see the same scene from a slightly different angle. The eyes' visual fields overlap in the center, and the brain merges these images to create a sense of depth important for judging distance. Humans and other mammals have stereoscopic vision. Birds, fish, and snakes have monocular vision in which each eye sees a separate image covering a wide area on each side of the head.

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III VISUAL ACUITY

Visual function is described in terms of visual acuity and visual field. Visual acuity is a measurement of the ability to distinguish details and shapes. One way to measure visual acuity is with a standardized chart of symbols and letters known as the Snellen chart, invented in 1862 by Dutch ophthalmologist Herman Snellen. He derived a simple formula that determines the relation between the distance at which a letter is read by the patient to the distance at which that same letter is read by a normal eye. Normal vision is designated as 20/20. Visual acuity that is less than normal is designated with a larger second number, such as 20/200. An individual with a visual acuity of 20/200 must stand at 6 m (20 ft) to see objects that a person with normal sight can see at 60 m (200 ft).

Visual field indicates the ability of each eye to perceive objects to the side of the central area of vision. A normal visual field is said to be 180 degrees in diameter, or half a circle. An individual with a visual field of 20 degrees or less who stands at a distance from a large clock and looks at the number 12 is unable to see the numbers 11 and 1 to either side of it.

In the United States, legal blindness is defined as a visual acuity of 20/200 or worse in the better eye with the best optical correction, such as eyeglasses or contact lenses. In the legally blind, the visual field is not better than 20 degrees in the better eye.

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IV VISION DISORDERS

Eye examinations are performed by an ophthalmologist, a medical physician trained to diagnose and treat eye disorders, or an optometrist, an eye-care specialist trained to examine the eye and prescribe eyeglasses or contact lenses. One part of an eye examination tests if there is a reduction in the ability to see. These vision tests measure visual acuity using the Snellen chart. Refraction tests determine if a patient has distorted vision that can be corrected with eyeglasses or contact lenses, such as nearsightedness, farsightedness, and astigmatism. In visual field tests, which indicate problems with peripheral vision, one eye is covered while the other eye is directed to a point straight ahead. Lights are projected onto a screen at various locations in the periphery of vision and the person indicates when a light is seen (see Ophthalmology).

Children often do not know that they have faulty vision. Parents may notice that their child does not seem to see distant objects clearly or that an eye appears crossed. An eye examination can uncover the cause of the problem. The child may only need corrective eyeglasses to see things far away, such as the blackboard in school. Crossed eyes, known as strabismus, may be corrected with eyeglasses. Other children with this condition may be given eye exercises to strengthen weak eye muscles. They may wear a patch over the eye with better vision to force the weaker eye to see well. If exercises and eyeglasses do not correct the problem, a surgeon may operate to strengthen the weak muscles and straighten the crossed eye.

Another common cause of damaged vision in young people is injury from sharp objects, such as scissors, or head injuries from automobile or bicycle accidents. Eyes can be protected from injuries by the use of safety equipment—such as wearing a helmet for cycling or wearing shatterproof goggles during fast-moving ball games.

Some vision problems are the result of illnesses, such as high blood pressure or diabetes mellitus. Glaucoma is an eye disease caused by faulty drainage of normal eye fluid from inside the eye. The pressure in the eye slowly rises and over many years may cause damage to the optic nerve, eventually resulting in blindness. Macular degeneration is a serious eye condition that is usually associated with aging. The macula is vital for clear, sharp sight. In people with macular degeneration, deteriorating cells or abnormal blood vessel growth in the macula cause blurred vision in the central area of focus. Vision loss associated with macular degeneration cannot be corrected with standard eyeglasses or contact lenses.

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V LOW VISION

Faulty vision that cannot be corrected by eyeglasses or contact lenses is called low vision. A variety of tools are available to help a person with low vision read and work. Some people with low vision are helped with magnifier glasses in the form of eyeglasses or hand-held glasses that enlarge type in books and newspapers. Computer users can benefit from closed-circuit television-reading machines that use a zoom lens to enlarge print on a computer monitor. Printers also create text in large type that is easier to read. And some computers are equipped with a voice output that reads the text aloud.

Contributed By:
Eleanor E. Faye

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