Vision.

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Vision.
I

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.

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.

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.

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.

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
Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.