At the previous posts, we reviewed the anatomical and neural changes that affect the vision in older adults. Now, we will analyze how these changes influence the quality and features of the of the elderly person's vision. Each reviewed anatomical change, neural change, or combination of the two has an appropriate impact on the human vision.
Older persons experience lower light levels than younger persons as a result of decreased illumination within the eye. This decrease is due to a combination of several anatomical changes previously discussed. The first obstacle, light faces up on its way, is significantly decreased pupil diameter through which the light beam must enter to the eye. The aging variable lens further filters light, which is more yellow and opaque in comparison with younger eyes. Researchers estimate that the elderly retina receives approximately one third of the amount of light that a younger retina would receive. This means that older persons require much more intense lighting than younger persons do. This essential need for more light intensity interacts with a greater susceptibility to glare, causing vision-related problems in everyday living.
Glare is a common complaint among the low-vision elderly. There are several types of glare, each resulting from an interaction between different types of lighting and visual anatomy or anatomical changes. Each ultimately causes a different type of visual impairment.
· Veiling glare occurs when stray light hits the retina uniformly, such as when light from inside a car reflects off the windshield.
· Scotomatic glare occurs when the eye is overloaded with light and often results in an afterimage, like a camera flash may cause.
· Dazzling glare can only be noticed in situations with very bright light, such as when looking at the filament within a light bulb.
The orientation of photoreceptors is an important anatomical issue because it influences both the response to incoming light and the perception of glare. In a normal, healthy eye, the photoreceptors are angled so that light entering the eye through the pupil is most likely to hit directly the top of the photoreceptors. This phenomenon is referred to as the Stiles-Crawford effect and serves to limit the response to light scatter in the healthy eye.
In older eyes, the photoreceptors have become disarranged and abnormally oriented. Photoreceptors that are irregularly oriented are less likely to respond to light than normally oriented receptors. This is because the light is less likely to travel through the long axis of the photoreceptor as in normally oriented cells. When light does not pass through the entire receptor, it is less likely to activate enough of the chemical within the cell to cause it to respond. In older eyes, not only does light entering through the pupil directly hit the top of photoreceptors, but light bouncing off the aging retinal pigment epithelium and light scattering as a result of floaters is also likely to directly hit the top of photoreceptors. Overall, the disorientation of aging photoreceptors causes the photoreceptors to be less responsive to incoming light but more responsive to scattered light within the eye. The result of this situation is that the older eye will perceive more glare than the younger eye in the same conditions.
Another visual effect of age-related changes is the inability to discriminate colors. The older, yellow lens acts as a filter for shorter wavelengths (which correspond to purple and blue colors), meaning that these colors will appear dull or even gray. Pastel shades, not matter what color, are difficult to distinguish from one another in the elderly eye. Decreased color discrimination is caused mostly by cell loss in the fovea. The fovea is the small area of the retina where the retinal image falls when the eye focuses on something. The fovea is responsible for resolving fine details and it contains densely packed color-sensitive photoreceptors, called cones. As the eye ages and the fovea experiences cell loss, important color information is lost. The need for increased intensity also hampers color discrimination. As the eye ages, it requires greater intensities of color for the visual system to perceive stimuli. Even colors on opposite ends of the color spectrum can be difficult to discriminate if they are of the same intensity.
People live in a three-dimensional world. But we must infer the structure of that world from the two-dimensional array of light on our retinas. The construction of the third dimension is accomplished by using a number of cues, such as interposition, shading, and relative height. Only stereopsis sensitivity has been studied among different adult age groups. Stereopsis is the depth cue derived from the different images projected on the retinas by an object. Objects that are less than twenty feet from the observer will fall at slightly different positions on each retina. The disparity of these images is a cue for depth. The greater the disparity, the closer the object to the perceiver. As with the other vision characteristics that we have reviewed, stereopsis peaks in early adulthood with notable decreases in sensitivity after the fourth decade of life.
Poor color discrimination interacts with poor contrast sensitivity to make boundary detection a challenge. The term contrast refers to the amount of luminance difference between various parts of a stimulus, such as text and background, or the edges of objects. Contrast sensitivity is a measurement of the smallest amount of contrast a person can perceive. People with high (good) contrast sensitivity will be able to distinguish two parts of a stimulus with little contrast (i.e. little difference between the foreground and background), such as dark gray text on a slightly lighter gray background, or light gray text on a white background. People with low contrast sensitivity, such as the elderly, need high contrast stimuli, such as black text on a white background. Contrast sensitivity also correlates with size of the objects, meaning poor contrast sensitivity is less of a problem when a stimulus is large.
Objects in motion create a changing pattern of light on our retinas. Our ability to detect and discriminate these shifts of light stimulation is critical for our ability to determine not only the movement of objects but also our body motion and stability. While it is a subject that has generated a lot of interest, few studies of the impact of aging on motion perception have been reported. In one study of individuals from twenty-five to eighty years of age, the investigators reported that there was a linear decline of motion sensitivity with age. As with the decline in light sensitivity, such a pattern of change is suggestive of an age-related neurodegeneration in the visual system. However, several studies comparing the motion sensitivity of young and elderly adults have reported that the deficit in motion sensitivity was restricted to elderly women. That is, these studies reported that only elderly women and not men had poorer motion perception. A reason for such gender effects has not been suggested.
Field of Vision
The combined structural changes in elderly patients, eyes all contribute to decreased visual fields. Due to opacity of the lens, miosis of the pupil, and changes in the cornea, visual fields and peripheral vision are altered with age.
Reduced tear production causes many elderly patients to have dry eyes. Although this age-related change does not affect vision, it may be bothersome for patients.
Decreased acuity is probably the most commonly known effect of the older person's anatomical and neurological changes. Acuity refers to a person's ability to resolve details (i.e. read small print) and is the measurement of vision with which most people are familiar. Decreased acuity results from a combination of changes within the aging eye, including cell loss in the fovea, decreased transparency of the vitreous, and increased light scatter and glare within the eye. Acuity in the aging eye also depends on the individual's contrast sensitivity. Stimuli with higher contrast will lend itself to higher acuity scores while stimuli with lower contrast will result in lower acuity scores among the elderly.
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