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Vision and Perception - Visual Processing

age aging differences clinical light sensitivity contrast adults

While diseases of the eye offer clear limitations to the acquisition of information and the accurate perception of the world, they only affect a minority of older adults. There are other changes which occur in vision that are considered normative, that is, they occur to most people. These alterations in structure and function can be shown to have a marked effect on the visual experiences of older persons. An appreciation of these factors can help us to understand the perceptions of elderly adults and to create behavioral interventions to compensate for their effect.

Light sensitivity. Perhaps the most important limit on data acquisition is the reduction in light sensitivity that occurs in adulthood. It is a common experience for a child to be chided by a parent or grandparent to turn on more lights while they read. ‘‘You’ll ruin your eyes trying to read in that light!’’ the parent may exclaim. This event illustrates the difference in light sensitivity between the child and older person. While the child has sufficient sensitivity to light to be able to read easily, the parent would require more light to perform the same task.

Our maximum sensitivity to light starts to decline in the third decade of our lives. Indeed, starting with age twenty, the intensity of illumination must be doubled for every increase of thirteen years for a light to be just seen.

One reason for the reduction in sensitivity is that less light actually reaches the retina, the receptive surface of the eye, as we age. It has been estimated that the retinal illuminance of a sixty year old is only one-third that of a twenty year old. The reduction in retinal illuminance can be attributed to several factors including the marked reduction in pupil diameter known as senile miosis. That is, in older adults the pupil simply does not open as wide to capture light. The gradual opacification, or cloudiness, of the lens and the reduction of transparency in the vitreous body also contribute to the reduction in retinal illuminance. Finally, there is evidence of the loss of photoreceptor cells that would reduce light sensitivity.

A simple intervention to compensate for the reduction in light sensitivity is to increase the level of illumination for older adults. However, care must be taken to avoid glare effects, which are more common in elderly adults. Excessively bright light or light which is scattered by opacities in the lens can reduce visual performance by dazzling a person or reducing the contrast of an object. One can compensate for glare effects in reading by using high contrast or large size material. In driving, however, glare is an issue for nighttime drivers who are exposed to the headlights of oncoming cars. A concern is that it takes a substantially longer time for persons to recover from glare, as they grow older. The temporarily impaired driver is at greater risk for an accident.

Acuity is the capability to resolve fine detail. It is ordinarily assessed by asking the patient to read letters or symbols that are printed in high contrast. The smallest element, which can be resolved accurately, is the acuity limit of the observer. We have all noticed as we grew older that optical corrections became common among our peers. Acuity improves from childhood into adolescence and then starts to show a steady decline in early adulthood. Indeed, even when adults are fit with their best optical correction, a gradual decline with age in peak acuity is noted starting late in the third decade of life. Thus, older adults with corrective lenses can be expected to have more difficulty than their younger counterparts in resolving fine detail.

To focus light on the macula, which is the portion of the retina capable of resolving fine detail, the lens must accommodate or change shape. The flexibility or accommodative power of the lens diminishes with increasing age. At about the mid-forties this loss of accommodative power becomes serious enough to affect the ability to focus on near objects. This loss of accommodative power for near vision is known as presbyopia.

Contrast sensitivity. The measurement of acuity assesses the ability to resolve small details at a high level of contrast. It is also important to determine the ability of a person to resolve objects under lower levels of contrast. In the assessment of contrast sensitivity the minimum contrast required to detect difference between light and dark regions is determined. A common definition of contrast is (Lmax - Lmin) / (Lmax + Lmin) where Lmax is the maximum luminance in a stimulus display and Lmin is the minimum level of luminance present.

One method in the clinical assessment of contrast sensitivity is achieved by having the patient read letters of fixed size that vary in contrast. At the top of the chart the letters are very dark against a light background. The contrast or darkness of the letters is successively reduced in each line of the chart. The lowest contrast at which the person can read the letters accurately marks their contrast sensitivity. A person who can read very light letters has a better contrast sensitivity than one who is successful only with dark letters.

Stimuli composed of gratings or stripes in which the contrast is sinusoidally modulated are also used in assessment. At high contrast levels, the grating appears to be composed of fuzzy stripes against a light background. The lighter stripe or the lower level of the contrast at which a person can detect the grating, the better their contrast sensitivity. The width of the stripes is also varied to permit the determination of contrast sensitivity for different size stimuli. The variation of stimulus size is described in terms of spatial frequency where the number of stripes per unit area on the retina is a measure of the spatial frequency of the stimulus.

Contrast sensitivity peaks in adolescence and starts to decline in early adulthood. As would be expected from the acuity data, older adults require very high contrast to resolve small objects, or high spatial frequencies, at even higher levels of illumination. This difficulty also extends to low and intermediate spatial frequencies under lower levels of illumination. Measures of spatial contrast sensitivity have been shown to be superior to acuity measures in predicting performance on a wide variety of tasks. Since the accurate processing of lower spatial frequencies are important for reading, face and object recognition, and road sign identification, the reduction of contrast sensitivity to these spatial frequencies places the older perceiver at a disadvantage for quick and accurate responding.

Color perception. The ability to discriminate among colors peaks in the early twenties and declines steadily with advancing age. The discrimination of shorter wavelength colors, blues and greens, is particularly challenging for older observers. This reduction in color discrimination may be attributed to at least two sources. First of all, the lens yellows with adult aging causing a selective absorption of shorter wavelengths and consequently less light from that region to strike the retina. Secondly, there is evidence that there is a selective loss of sensitivity of the photoreceptors that are responsive to short wavelengths.

A consequence of the loss of sensitivity to shorter wavelengths is that white light, which is composed of all wavelengths, may appear faintly yellow. Also, blue objects may appear particularly dark and blues and dark greens may be indistinguishable. Such changes in color perception may affect the sartorial choices of older adults.

Depth perception. 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. Reductions in stereopsis sensitivity may affect the ability of a person to perform a number of important tasks such as hitting a curve ball, judging the distance from an object while parking a car, and walking. In the latter case, objects, such as sidewalk cracks and stair treads, whose depth is not appropriately discriminated, may become tripping hazards. More work is needed to fully appreciate the depth perception capabilities of older adults.

Motion perception. 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.

Beyond the detection of motion it is important to be able to judge the speed of an object. Accurate speed judgments permit drivers to merge onto highways and ballplayers to hit a baseball. In general, young adults are quite accurate in their speed judgments. One area where there is a critical failure in speed estimates that affects all ages is in the perception of large objects. A large plane appears to be floating very slowly on to the runway as it lands yet it is travelling at nearly two hundred miles per hour. A train approaching an intersection appears to be moving slowly enough for a driver to avoid a collision yet the seventy-mile-per-hour locomotive slams into the car. It is a strong illusion that large objects appear to move more slowly than their actual speed.

There has been limited work on the ability of older adults to judge the speed of automobiles. The evidence suggests that older people overestimate the speed of slowly moving cars while underestimating the speed of cars travelling at highway speeds. Such an effect may account for the hesitancy of older drivers to cross an active intersection or to merge on a highway. The importance of motion perception in general and speed judgments specifically demands that more work is required to understand the impact of aging on these abilities.

Stimulus persistence. The experience of a visual event does not end when the stimulus is removed. There is a phenomenal persistence of the event not to be confused with an afterimage. The latter occurs because of the fatigue and recovery of receptors while persistence is a continuation of information transmission. The duration of the persistence is inversely related to the luminance, contrast, and duration of the stimulus. That is, stronger visual events lead to shorter periods of visible persistence. It may be that weak stimuli persist longer to permit the perceiver to continue to extract needed information from the stimulus event. The cost of prolonged persistence is that separate stimulus events may blend together yielding indistinct perceptual events. An example of such blending is the fusion of light pulses in a fluorescent light. There are distinct pulses of light and dark intervals emitted by the fluorescent tube. Each light pulse results in a residual persistence of the light in our visual system. Because the rate of flicker is so fast, the persistence of the light is long enough to fill the dark interval leaving the viewer with the experience of continuous light. The pulse rate of light and dark intervals at which a person perceives the light as continuous is termed the critical flicker fusion (CFF) threshold.

Given the reduced light and contrast sensitivity of elderly adults and the inverse relationship between stimulus strength and persistence, it is to be expected that older perceivers will experience longer persistence. Indeed, the CFF threshold is lower for older observers. This means that an older adult presented with a relatively slowly flickering light will report that it is continuous while a young person will note the flicker. The fact that a stimulus event has weaker temporal integrity for elderly adults suggests that there may be significant misperceptions of sequentially occurring events. Indeed, it has been argued that prolonged stimulus persistence may be at the root of a number of perceptual deficits reported for elderly observers.

Perceptual span. We have noted a number of factors that limit the data available to older perceivers. A direct measure of the impact of these limitations on the construction of a percept may be made by noting the amount of information that a person can acquire in a brief glance. Such a measure is the perceptual span, which is also known as iconic memory. The visual information is available for only a brief period of time, such as a quarter of a second. The span is affected by the strength of the stimulus. Following the theme, which has been developed here, young adults are capable of capturing a large amount of data while older adults have a more limited capacity. The limit on the span of the older observers may be related to their reduced light and contrast sensitivities that result in relatively weak stimuli. This point was supported by a study that compensated for the reduced sensitivity of the elderly participants and found that under this special condition age differences in span were eliminated.

The limit on the perceptual span may be linked to what has been identified as the useful field of view (UFOV). The UFOV is the spatial extent within which highly accurate stimulus detection and identification can be performed. Measurement of the UFOV emphasizes the capability to acquire information in peripheral fields of vision where elderly adults have reduced light sensitivity. The UFOV of elderly participants is three times more restricted than young adults, meaning that they can examine only relatively small areas of the visual field. This restriction in the UFOV has been shown to be related to the incidence of automobile accidents at road intersections.

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