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 Encyclopedia - Visual Field

What is visual field?
How is visual field examined?
How is peripheral vision lost or diminished?

What is visual field?

The visual field is the total area in which perception is possible while an individual is looking straight ahead. For a person with normal vision, the visual field usually extends outward over an approximately 90-degree angle on each side of the vertical midline of the face. But the angle is smaller above and below the midline, especially for a person whose eyes are deep-set or who has prominent eyebrows. Because the visual fields of the two eyes overlap to a large extent, a defect in the field of one eye may not be evident when both eyes are open. Thus, the visual field can be divided into the area that is visible only to the right eye, the area that is visible to the left eye, and the middle region of binocular vision.

All the light from the visual fields that are left of center of both eyes falls on the right sides of the retinas of both eyes. This information is transmitted by the optic nerves to the right visual cortex of the brain. Information about the right fields of vision is transmitted to the left cortex, and information about the region of binocular vision is transmitted to both the right and left cortex.

How is visual field examined?

An examination of the visual field measures the responsiveness of the peripheral area of the retina. That area does not have as many information-carrying nerve fibers as the center of the retina. The photoreceptors in the peripheral part of the retina are rods, which are less sensitive to light than the cones in the center of the retina, and so only larger and brighter objects are seen on the periphery of the field of vision. It is impossible to read fine print that is as little as 5 degrees to one side of a point on which the vision is fixed. The visual field examination generally is done by using electronic equipment that prints a computerized reading of an individualís field of vision and that can measure an individualís attention span and the consistency, accuracy and pattern of his or her responses to visual stimuli.

How is peripheral vision lost or diminished?

Peripheral vision can be lost because of glaucoma or a stroke. Although the loss caused by a stroke is sudden and obvious, the damage done by chronic glaucoma usually is gradual. Therefore, it is not apparent to the affected individual until considerable vision has been lost. Periodic tests of the visual field thus are important as people mature, because the incidence of glaucoma increases with age.

Loss of peripheral vision is obvious to anyone with macular degeneration, a progressive condition in which deterioration of the macula, the central part of the retina, produces a growing blind spot in the center of the field of vision.

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