7.7: Age Changes in Vision

Light Intensity

As people age, brighter lighting must be present if they are to see as much as they did when they were younger. The main reason for this is that the eye allows less of the light striking the conjunctiva to reach the retina. Less light can pass through the eye partly because of the declining transparency of the conjunctiva, cornea, lens, and vitreous humor. The decreasing size of the pupil also blocks the passage of a substantial amount of light. With less light reaching the retina, fewer cones and rods are stimulated enough to send impulses in response, and the photoreceptors that do respond send fewer impulses.

Brighter lighting is also needed to compensate for age changes in the cones and rods. These changes reduce the sensitivity of the retina to light, causing an additional decrease in the number of impulses being produced for a given amount of light. Aging of the processing neurons of the retina may further reduce the number of impulses that leave the eye. Since fewer impulses go to the brain, it is less able to perceive clearly and accurately what is being viewed. Aging of neurons in the visual pathways and processing areas of the brain seems to add to the problem.

The deficit in retinal impulses is probably greatest in the macula region because that region contains cones. Recall that cones need bright light to be stimulated and that they and the processing neurons for the macula are more affected by aging than are the rods and their processing neurons. Therefore, aging causes a decline in the ability to distinguish colors. Since age‑related decreases in transparency preferentially affect short wavelengths, the perception of violet, blue, and green decrease the most.

The need for brighter lighting causes vision problems in aging individuals whenever the field of view is lit dimly. This is especially apparent when they move from a brightly lit environment to a dimly lit one or when the lighting consists mostly of light with shorter wavelengths, such as violet and blue light.

Aging individuals also see less well when light intensity changes rapidly, because the pupil adjusts to such changes more slowly. Rapidly interspersed periods of dim light may produce alternating periods of inadequate lighting and excess lighting. Such conditions may be encountered during the day when one is driving through patchy shade (e.g., wooded areas) and at night when one is driving past bright lights (e.g., oncoming traffic, electric lights in roadside signs).

Quality of Light

Vision in old age is also affected by the quality of light. This phenomenon occurs because more of the light entering the eye is scattered. Age changes in the cornea, the vitreous humor, and especially the lens contribute significantly to light scattering.

Since much of the scattered light strikes the retina in the wrong places, the retina produces scattered impulses, which cause glare. Depending on the degree of aging of the eye and the conditions present, the amount of glare may range from being barely a nuisance to obscuring objects completely.

As age increases, external conditions that intensify problems from glare include having brightly lit objects or bright lights against a dark background (e.g., driving at night); having bright light strike the eye at an angle (e.g., multiple bright lights); and viewing light with shorter wavelengths, such as blue light. Sunlight causes glare because it contains much short‑wavelength light. Yellow light produces less glare. Aging also slows recovery of good vision after being exposed to glare.

Visual Acuity

Even when light is bright and glare is minimized, there is a decrease in visual acuity with aging. Almost all the decrease for close objects is due to stiffening of the lens, which causes a great reduction in the refractive power of the eye. Other age changes that contribute to declining refractive power include flattening of the cornea and lens and the diminishing ability of the ciliary body to move. Close visual acuity is decreased to a lesser degree by reductions in the sensitivity to contrast caused by changes in the neuron pathways. Since most of the decline in close visual acuity is caused by a decrease in the refractive power, the use of eyeglasses that increase refractive power can restore much of the lost acuity.

Recall that the decline in near visual acuity caused by aging is usually not noticed until about age 40. Most of the deterioration of close visual acuity is completed by age 60 because by then the lens has become completely stiff. However, since the neuronal changes and flattening of the cornea and lens continue for many years, close visual acuity may continue to decline gradually after age 60. By contrast, in some individuals the continued thickening of the lens and the resulting increase in its refractive power may restore some close visual acuity that has been lost.

Visual acuity for distant vision also declines with aging. However, distant visual acuity does not begin to decline until about age 45, after which the decline is usually steady but slow. The deterioration of distant visual acuity is so slow that more than 50 percent of those who reach age 80 have fairly good distant vision as indicated by scores of 20/40 or better on a Snellen eye chart.

Most of the decline in distant visual acuity is caused by age changes that reduce the amount of light that passes through the eye. Since only a small proportion of the decline is caused by focusing problems, eyeglasses and contact lenses can compensate for only a limited amount of the loss. Thus, unlike near visual acuity, distant visual acuity declines steadily even if corrective lenses are used.

Declining close and distant visual acuity is noticed whether a person is observing stationary or moving objects. However, the decline for moving objects is much greater than that for stationary ones because viewing moving objects involves additional processes that also undergo age changes.

One of these processes is smooth movement of the eyes to keep the object being viewed centered in the field of vision. Recall that with aging, the movement of the eyes becomes less smooth, causing the image of a moving object to move irregularly on the retina. The brain has difficulty interpreting the increasing complexity of the resulting retinal impulses.

Another relevant process is the rapid recovery of each neuron every time it is stimulated. This is needed because moving objects produce rapid changes in the light striking the photoreceptors. Since aging causes slowing of the neurons involved in vision, neuron recovery lags behind changes in the moving images. The result is a lengthening of afterimages.

An afterimage is a lingering perception that an image is present even though the image in the eye has changed or disappeared. For example, a light that is flickering rapidly may be perceived as emitting a steady light because the individual images blend into one continuous image. Another effect of long afterimages is the apparent blurring of an object as it moves faster. This effect is easily observed by watching a fan as it begins to spin.

As aging lengthens afterimages, blurring of moving objects increases and therefore visual acuity for moving objects decreases. Detecting small movements in nearly stationary objects also becomes more difficult. These results adversely affect a person's performance in walking and driving, recreational and occupational activities, and even spectator activities.

Depth Perception (Binocular Vision)

A person's eyes usually do not undergo age changes at the same rate. As the eyes become more different from each other, they send increasingly different impulses to the brain. These differences, together with the declining ability of the brain to interpret them, result in a decrease in binocular vision, as was discussed previously.

Field of view

The width of the field of view (i.e., peripheral vision) decreases gradually from 170 degrees to 140 degrees. Also, there is an age-related decline in the ability to notice a specific object in a field of view. This latter change is more evident in the periphery of the field of view and with increasing complexity of the items in the field of view.

Optimizing Vision

Though age changes that affect vision cannot be prevented, the limitations and disabilities they cause can be minimized by providing adequate lighting; reducing sources of glare; enlarging items; using magnifying lenses; increasing contrast; positioning objects at greater distances from the eyes; slowing the motion of moving objects; and using eyeglasses or contact lenses. These steps can help prevent accidents and injuries while preserving much of an aging person's activity, independence, and quality of life.