Eye Movements Visual Thresholds Critical Flicker Frequency and Field and Spatial Vision

Smooth pursuit and saccadic movements

Smooth pursuit movement gains were decreased with aging, while saccadic reaction times increased (54). Saccadic reaction times were slower in people older than 60 years, compared to young adults (55). Visual sensitivity to motion decreases with age, and the change is more pronounced in the central visual field compared to other areas (8).

Sensitivity and Visual Threshold

As discussed above, the decrease in pupillary aperture with aging results in lesser light input. The decline in the number of photoreceptor cells (rods) and other aging changes in the retina result in reduced availability and regeneration capacity of the photoreceptor pigment (rhodopsin), leading to reduced light utilization in the aged eye. As a result, the visual threshold, i.e., the minimum amount of light necessary to see an object, increases with aging. This is tested by measuring the change in visual threshold as a function of time spent in darkness (dark adaptation). It is known that the threshold for light detection decreases with increased duration of dark adaptation, because rhodopsin regeneration is enhanced in the dark. With advancing age, this regeneration is presumably deficient, resulting in higher light thresholds (i.e., lower sensitivity).

As illustrated in Figure 5, the enhanced light sensitivity after dark adaptation is markedly reduced with aging; in fact, the visual thresholds in the completely dark-adapted eyes of the aged group (80 years) is 100 times higher than that of the young group (20 years). However, as evident from the data, the pattern of change in sensitivity during dark adaptation is basically similar in the different age groups. This indicates that retinal function is quantitatively, but not qualitatively, impaired. The decline in threshold may be due to reduced oxygenation of the retina and the rods in the aged (7,8).

Recent Studies on Scotopic Sensitivity

Scotopic sensitivity shows a 0.5 log unit decrease with age; the loss is enhanced in the perimacular region (57). The area of scotopic spatial summation (Rico's area) was measured in adults in the age range of 20 to 85 years and was found to increase with age (58).

Critical Flicker Frequency

The rate at which consecutive visual stimuli can be presented and still be perceived as separate is called the critical flicker

FIGURE 5 Decline in light sensitivity with age. Data show changes in visual threshold after dark adaptation in different age groups (20, 40, 60, 80 years). Source: From Ref. 28, based on the original data of Ref. 56.

frequency (CFF). Determination of CFF in different age groups provides one way by which changes in visual function with age can be measured. These tests reveal a decline in CFF with aging, from a value of 40 Hz (cycles/sec) during the fifth decade to about 30 Hz in the eighth decade (59). The persistent miosis in the elderly must contribute to this decline, because the decline is less marked with fully dilated pupils. Foveal flicker frequency sensitivity in healthy aging eyes showed a decrease in foveal temporal contrast sensitivity coupled with losses in amplitude but not temporal resolution. The mean rate of loss was 0.78 decilog per decade after the age of 45 years (60).

Aging Changes in Visual Field and Spatial Vision With aging, a loss in the size of visual field is observed, ranging from 3% to 3.5% in middle age to two and four times as much at 60 and >65 years, respectively. Disturbances in visuomotor performance are particularly observed when changes in "useful or effective visual field" are measured, in contrast to measures of visual field under standard clinical conditions (61). Visual acuity reflects the ability to detect details and contours of objects. It is defined as the minimum separable distance between objects (usually fine lines) and is one of the measures of the visual system's spatial discrimination ability. As shown in Figure 6, visual acuity declines, commencing at 50 years, particularly worsening during and after the eighth decade, when it becomes detrimental to vision (8,61).

Recent Studies on Spatial Vision and Visual Acuity

A recently devised measure of spatial visual ability is contrast sensitivity, where the test format and conditions more closely resemble real-world conditions (61). Aging studies in this category indicate that at low spatial frequencies, such as when grating of wide bars are presented, little aging changes are observed, while at high frequencies (fine bars), a marked decline in contrast sensitivity is observed with aging, beginning at 30 years. This deficit may underlie certain reading disorders in the elderly, such as reading of very small or very large repetitive characters, where the elderly show nearly 70% deficit (61). A recent analysis on visual acuity changes with aging did not reveal significant change in high visual acuity, but all aspects of spatial vision showed a decrease, particularly under conditions of decreased contrast, luminance, and glare. Also found was reduced stereopsis, poor color discrimination, and decreased peripheral vision, particularly when divided attention was required (63).

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