Supervised by Dr. Lauren
Scharff
Stephen F. Austin State University
This entire paper contains several sections: Introduction, Changes in Vision and Their Effects, Impact of Low Vision, Available Services, Obstacles to Services, the Need for Additional Services and an Interdisciplinary Approach, and References.
The visual system changes in many ways as the human body ages. Many visual changes are considered normal while others are considered pathogenic, or disease-related. While many normal age-related changes occur in all healthy eyes, many people also suffer from disease-related changes that further impair vision. In general, older eyes are more susceptible to common age-related diseases, such as macular degeneration, cataracts, and glaucoma. Health problems may also make the elderly even more at risk for eye disease, such as the case of diabetics who develop diabetic retinopathy. Unfortunately, most of the elderly population cannot distinguish between normal vision loss and disease-related vision loss, so many do not seek professional care (Silverstone, 1993).
Although both disease-related and normal age-related impairments in visual function are important to understand, this project will only address the issues concerning normal visual changes.
This section will discuss both the anatomical changes and the neural changes that occur within the visual system with age, as well as the visual effects of these age-related changes.
This subsection will discuss the major anatomical changes that affect the vision of senior citizens. Anatomical changes in the older visual system include changes in the lenses, the pupil, the vitreous humor, the retina, and the retinal pigment epithelium. These structures are labeled in the diagram below.
Figure 1: Major structures of the human eye. Although pictured in red, the vitreous humor is a clear fluid. This image is used with permission from The Colorado Health Site.
The fixed lens, or the cornea, is the first lens that focuses the light and, as the most powerful lens in the eye, is responsible for focusing most of the incoming light. The fixed lens does not change shape like the variable lens, but it does go through changes with age similar to the variable lens. The cornea thickens over time and adds to the amount of extra light scatter already inside the aging eye (Oberlink, 1997). This thickening can also influence the focusing power of the fixed lens.
The variable lens (simply called the lens in the diagram above) is a second lens within the eye and can adjust its shape to further focus incoming light. The variable lens begins to change at birth. As we age, the center of this variable lens loses flexibility and becomes more rigid (Spector, 1982, p. 34). Over time, the variable lens loses so much flexibility that it can no longer focus on detailed objects at close range. Although this change occurs throughout the lifespan, adults most often notice it around the age of forty when the lens can no longer focus on average sized print at a normal reading distance. The chemical composition of the lens also changes with time as proteins are produced in different proportions (Spector, 1982, pp. 33-34). These chemical changes cause the lens to yellow over time. This yellowing of the lens reduces its transparency, causing it to become more opaque with time.
The pupil is the opening in the iris inside the eye that allows light to enter. As we get older, the pupil gets smaller in diameter (Ordee, Brizzee, & Johnson, 1982, p. 86). This, along with the change in variable lens transparency, reduces the amount of light that the older adult eye receives.
Changes also occur in the vitreous humor, which is the fluid that fills the back of the eye. In young and healthy eyes, the vitreous is a clear gel-like substance with the consistency of egg whites. With age, the vitreous becomes thinner and more water-like (Balazs & Denlinger, 1982, p. 49). Pockets of liquid vitreous develop within the eye (Balazs & Denlinger, 1982, p. 46). This liquefied vitreous makes floaters more visible to older individuals. Floaters are clumps of cellular debris that accumulate within the eye and settle at the bottom of the eye in normal eyes. When the fluid thins, the floaters do not settle and, while seeing the floaters does not significantly impair vision, they can be a source of irritation to seniors already dealing with changing vision. Another fairly common change that occurs within the vitreous is referred to as posterior vitreous detachment. In older individuals with this condition, the thinning vitreous begins to pull away from the retina at the back of the eye (Balazs & Denlinger, 1982, p. 53). This detachment does not cause any serious impairment alone, but the symptoms can impact vision. Symptoms include flashes of light, distorted and blurred images, and increased floaters.
The retinal pigment epithelium is a layer of darkly pigmented tissue behind the retina. This layer provides the retina with rich nutrients as well as serving to absorb excess light and prevent light scatter within the eye. In the older human eye, the cells of the pigment epithelium become irregular (Marmor, 1982, p. 67). This change means that the pigment epithelium is less able to absorb excess light and less able to help control light scatter in the elderly eye. Controlling light scatter in the elderly eye is important because light scatter causes glare (Oberlink, 1997).
The term "neural" refers to components of the nervous system, which includes the brain, spinal cord, and nerves. The nervous system is made up of specialized cells called neurons. Neurons are unique because they are the only cells in the body that communicate through chemical and electrical signals. Perception relies heavily on the nervous system because neurons pass the information to the brain, which then processes the information.
The retina is an extension of the brain at the back of the eye and is made up of several layers of neurons. The back layer of the retina is made up of photoreceptors, special receptor cells that turn light energy into a neural signal. The loss of retinal cells with age causes the retina to thin, especially in the periphery. The cells that survive often exhibit irregular orientations. This change in orientation means that light entering the eye no longer hits the photoreceptors at the same angle as in younger, healthier eyes (Sekuler, Kline, & Dismukes, 1982, p. 118). This contributes to increased glare in elderly eyes, which will be discussed further in the subsection, Visual Effects of Changes.
Other parts of the brain experience cell loss with age as well. Many different neural pathways make up the human brain's visual system. Each of these pathways sends sensory information to a specific part of the brain responsible for a specific aspect of vision. All of these pathways are likely to lose neurons due to age (Ordy, Brizzee, & Johnson, 1982, p. 86). Because neurons in the brain do not regenerate, this cell death results in diminished abilities to perceive different aspects of visual stimuli. Another result of neural cell death is a general slowing of response time, meaning that the visual systems of older persons respond to light much more slowly than younger persons.
Along with changes in the retina, changes in the brain affect the way the aging eye responds to light stimuli. Studies using electroencephalograms have provided evidence that reaction times to stimuli increase with age (Ordy, Brizzee, & Johnson, p. 83). The results showed that fewer neurons respond to a given stimulus in older eyes than in younger eyes and that older neurons do not respond as quickly as younger neurons. This change has many effects on the elderly person's ability to perform daily tasks that will be discussed in further detail in the section Impact of Low Vision on Daily Life.
The brain also undergoes chemical changes that can affect the older visual system's overall functioning. Neurons throughout the body communicate through the use of chemicals called neurotransmitters. When the body produces too many or too few neurotransmitters, neurons do not communicate properly. As we age, the brain begins to produce neurotransmitters in abnormal proportions. This affects the visual system by changing the abilities of neurons in the visual pathways to communicate (Ordy, Brizzee, & Johnson, 1982, p. 85).
The loss of short-term memory is another neural change that affects vision in older persons (Ordy, Brizzee, & Johnson, 1982, p. 80). This change can impact vision in the elderly when they must divide their attention among multiple stimuli. Short-term memory loss can also impair vision when the elderly must organize incoming stimuli. These changes and impairments make several implications for the senior citizens' ability to drive as well as other tasks. These issues will be further discussed in Impact of Low Vision on Daily Life.
While many neural and anatomical changes are independent of each other, some changes are a combination of both types of change. As a result of both neural and anatomical changes, the older eye has limited motility as compared to younger, healthier eyes (Leigh, 1982, pp. 175-176; Lott, et al., 2001). Motility refers to the ability of the eye to move properly. Older individuals tend to have less control over their voluntary eye movements than younger people. Also, the older eye has a more limited range of movement. Aging eyes have a decreased ability to perform smooth pursuit eye movements, which are used when the eye tries to visually track a moving object. During saccadic eye movements, which are used to scan stationary objects, the eye jumps from one part of the scene to the next. The older eye performs the visual jumps used in saccadic eye movements more slowly than the younger eye. These changes in the ability to control eye movements can affect a number of tasks, such as reading.
The previous section, Changes in Vision, discussed the anatomical and neural changes that occur in older persons. While these changes in the visual system are interesting to study and understand, what makes the changes most important is their impact on the quality of the elderly person's vision. Each anatomical change, neural change, or combination of the two has a corresponding impact on 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 is decreased pupil diameter through which it must enter. The aging variable lens further filters light, which is more yellow and opaque than in younger eyes. Researchers estimate that the elderly retina receives approximately one third of the amount of light that a younger retina would receive (Ordy, Brizzee, & Johnson, 1982, p. 86). This means that older persons require much more lighting than younger persons. This need for more light interacts with a greater susceptibility to glare to cause 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 as discussed by Carter (1982, p. 122). 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 directly hit 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 (Sekuler, Kline, & Dismukes, 1982, p. 118). 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 (Haegerstrom-Portnoy, Schneck, & Brabyn, 1999). Decreased color discrimination results moslty from cell loss in the fovea (Ordy, Brizzee, & Johnson, 1982, p. 87). 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 (Carter, 1982, p. 122). 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.
Poor color discrimination interacts with poor contrast sensitivity to make boundary detection a challenge (Pastalan, 1982, p. 324). The term contrast refers to the amount of luminance difference between to 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. Figure 2 below shows examples of high and low-contrast print. Contrast sensitivity also interacts with size, meaning poor contrast sensitivity is less of a problem when a stimulus is large. The disabilities resulting from this lowered contrast sensitivity will be discussed in the next section.
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High Contrast Text |
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Low Contrast Text |
Figure 2: Examples of high contrast text (top box), where there is a large lightness difference between the text and the background, and of low contrast text (bottom box), where there is little lightness difference between the text and the background.
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 (Haegerstrom-Portnoy, Schneck, & Brabyn, 1999). 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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