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 Encyclopedia - Peripheral Vision

What makes peripheral vision work?
How is peripheral vision tested?

Peripheral vision, or side vision, is that part of vision that detects objects outside the direct line of vision. For instance, when you read a word on a page, you are using your central vision, but its your side vision that tells you if the word is at the beginning or end of a sentence, or at the top or bottom of a page. Your peripheral vision also tells you where to look if someone enters the room or if a car is approaching from the side. Like most people, you are probably not aware of the limitations that would exist without peripheral vision, because you are constantly moving your eyes in order to focus with your central vision.

The difference between central and peripheral vision becomes apparent when you understand the visual function of the eye. The eye works like a camera with two lenses -- the cornea at the front of the eye and the natural crystalline lens behind the pupil. The cornea is responsible for about 70 percent of the eyes focusing power, while the natural lens fine-tunes the image before it is focused on the retina at the back of the eye. The retina works like the film in a camera, receiving light rays and sending them through the optic nerve to the brain where they are converted into images (See Anatomy of the Eye).

What makes peripheral vision work?

There is a small area in the center of the retina called the macula, which is less than inch in diameter, that is responsible for sharp, clear central vision and ability to perceive color. The densely packed photoreceptor (light-sensitive) cells in the macula control the eyes central vision and are responsible for the ability to read, drive a car, watch television, see faces, and distinguish detail. The rest of the retina handles peripheral vision that enables the eyes to see objects off to the side while looking forward.

There are two types of photoreceptor cells in the retina -- rods and cones. The rods specialize in work at low light levels, and the cones provide sharp vision and discrimination. The macula contains a high concentration of cones, which accounts for the sharper focus of straight-ahead vision, particularly in bright light. Most of the rods are located in the periphery of the retina. This is why you can often see faint objects more clearly if you dont look directly at them. A dim star, for instance, is best seen when your eyes are not aimed directly at it.

When you see something out of the corner of your eye, its image focuses on the periphery of your retina and you are unable to distinguish the color of what you are seeing. And, because there are fewer rods, the ability to resolve the shapes of objects at the periphery of vision is limited. Normal peripheral vision, called visual field, for one eye is approximately 150 degrees from side to side. For both eyes, it is approximately 180 degrees.

Loss of peripheral vision results in a condition called tunnel vision, which is like looking through a tunnel. Tunnel vision can be caused by several disorders including glaucoma, retinitis pigmentosa, and strokes.

How is peripheral vision tested?

Most eye examinations include a perimetry or visual field test to check peripheral vision. The perimetry test is used to detect and monitor damage from glaucoma and other conditions that may affect the visual pathway from the eye to the brain. In medical terms, perimetry is a systematic measurement of visual field function. An instrument called a perimeter is used to plot the central or peripheral field of vision.

Loss of peripheral vision can actually be more difficult than the loss of visual acuity. However, many low vision aids are available for people who have lost all or a portion of their peripheral vision. Training and optical devices, such as prisms, mirrors, reverse telescopes, and minus lenses, can improve awareness of the environment and independent travel ability.

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