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Although later life is noted for the occurrence of visual defects, the average 45- or 50-year-old does not simply wake up one morning and discover that the dials on the television set or the stove are not clear, or that the newspaper must be held at arm's length to be read. Young children have relatively poor visual acuity, but it improves until about age 20 and then remains fairly steady until the early forties. During the forties or early fifties, and 3-5 years earlier in women than in men, presbyopia ("old-sightedness") gradually manifests itself. The result is that reading glasses or bifocals must be worn for close work and brighter lighting becomes necessary.

The loss of visual acuity during middle- and later adulthood is due in part to the decreasing diameter of the pupils of the eyes and to the obstruction of vision caused by sagging eyelids. The major cause of the problem, however, is the thickening and hardening of the lenses and the consequent decrease in their flexibility. The lenses are unable to accommodate, or change their focal length, as quickly and as effectively as before, making it difficult to see things close up.

In addition to reduced acuity and presbyopia (senile miosis), "normal" age-related changes in vision include the following:

1. Decreased ability to differentiate among colors. The yellowing lenses of the eyes filter out greens, blues, and violets, making these colors more difficult to distinguish than yellows, oranges, and reds.

2. Glare produced by increased scattering of light within the eyes due to reduced transparency of the lenses.

3. Poorer peripheral vision due to glare and reduced sensitivity to light.

4. Slower adaptation to changes in ambient illumination—from light to dark (darkadaptation) or dark to light (light adaptation).

Other common eye complaints in later adulthood include tiny spots or specks that float across the visual field (floaters), light flashes, and too much or too little tearing.

Aging is accompanied not only by changes in visual sensations but also in the perception (sensation plus meaning) of visual stimuli. For example, there is a deterioration in the ability to perceive objects in depth. In addition, the critical fusion frequency (CFF)—the highest frequency of a flickering light at which they flashes appear to fuse into a continuous beam—becomes lower. Rather than being caused by changes in the eyes, the lower CFF for older adults appears to be due to the dynamics of the central nervous system. According to stimulus persistence theory, the lower CFF and the general tendency of older adults to react more slowly than younger adults to a series of stimuli presented in rapid succession is due to the longer time required for an older nervous system to recover from stimulation (Axelrod, Thompson, & Cohen, 1968). This theory may also explain why older adults find it more difficult to detect contrasting patterns involving small differences between light and dark. In any event, the greater cautiousness of older adults may also contribute to their slower responsiveness to visual stimuli.

Senile miosis (presbyopia) is certainly a visual defect but not a visual disorder or disease in the same sense as cataracts, glaucoma, or retinal degeneration or detachment. Cataracts are a condition in which the opacity of the lens of the eye obstructs the passage of light to the retina. Cataracts increase with age, but the affected lens can be surgically removed and a special lens inserted in its place.

Also increasing with age and even more serious than cataracts is glaucoma. This disorder, which is caused by improper drainage of the intraocular fluid and hence increased intraocular pressure, leads to a reduction in the size of the visual field and eventually damage to the optic nerve and blindness if not treated. The pressure screening test for glaucoma is a part of a routine eye examination. When glaucoma is diagnosed, it is treated by means of prescription eyedrops, oral medications, laser treatments, and surgery.

Three other eye diseases that increase in frequency with aging are macular degeneration, diabetic retinopathy, and retinal detachment. In macular degeneration, the macula, a yellowish area in the center of the retina, stops functioning efficiently and central vision is affected. In diabetic retinopathy, the blood vessels to the retina fail to supply it properly with blood. In retinal detachment, the outer retinal layer becomes separated from the inner layer and must be treated with laser surgery.

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