Color Vision

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Color, like beauty, is in the eye of the beholder. It is no more measurable by physical means than is the sensation of pain or the feeling of joy. The sensation of color can, however, be closely linked to physical attributes of the visual stimulus and to anatomic and physiologic properties of the afferent visual system.8 The human process of color photoreception can be explained by the trichromatic theory. Early investigators hypothesized that there were three classes of cones in the human retina. Later experiments have confirmed the presence of exactly three types of color pigments in cone photoreceptors. These pigments have spectral sensitivity curves that peak at 450nm (blue cones), 540nm (green cones), and 580nm (red cones), and individuals with congenital color deficiencies have an absence of one or more pigments.3

The terms "protan" (red) and "deutan" (green) originate from the Greek words for "first" and "second" and denote the two most common kinds of red-green confusion, whereas blue-yellow confusions characterize the "tritan" (third) type of defect.8 Congenital dyschromatopsias are mostly of the red-green type, binocular and stable throughout life. Color defects in patients with acquired dyschromatopsias frequently mimic

TABLE 2-6. Associated Color Vision Defects.

Red-green

Blue-yellow

Optic nerve diseases

Optic neuritis

X

Toxic

X

Leber's atrophy

X

Compressive

X

Traumatic

X

Dominant atrophy

X

Chronic disc edema

X

Ischemic

X

Retinal diseases

Retinal detachment (macula on)

X

Toxic

X

Peripheral chorioretinal dystrophies

X

Diabetic retinopathy

X

Retinoschisis

X

Cystoid macular edema

X

Cone dystrophy

X

Juvenile macular degeneration

X

Modified from Harper W. Acquired dyschromatopsias. Surv Ophthalmol 1987;32:10-32, with permission.

Modified from Harper W. Acquired dyschromatopsias. Surv Ophthalmol 1987;32:10-32, with permission.

the abnormal patterns seen in those patients with congenital defects. It has been noted by Kollner and others that patients with diseases of the macula tended to have blue-yellow defects while those with diseases of the optic nerve tended to have red-green defects. Unfortunately, not all acquired defects behave in this manner, and significant exceptions exist. Table 2-6 lists the most common disease entities and their relative color defects.

The most accurate way to measure color vision is with the use of spectral light sources (Nagel anomaloscope). Because such testing is very difficult and time consuming, alternative methods based on pigmented surface colors were developed.2 Tests that use this method are the pseudoisochromatic plates, the Farnsworth-Munsell 100 hue test, and Farnsworth D-15 test. By far the easiest test to use in children is the pseudoisochromatic plates. The plates consist of dots arranged in a particular pattern that can be read quite easily by the normal individual. The plates were designed to detect congenital dyschromatopsias but are inadequate for classification. The patterns are designed to show large letters, figures, or numbers, which are easily recognized even by children. Although the child may not know the letter or number illustrated, they can be asked to trace the figure with their finger. More sophisticated testing and classification can be performed with the Farnsworth-

Munsell 100 hue test or the Farnsworth D-15 test2; these involve arranging colored caps between pairs of reference caps that are placed at each end of a long narrow rectangular box. Characteristic patterns of confusion are seen in individuals with congenital and acquired color vision defects. This test requires a fair amount of cognitive ability, and children usually must be 6 or 7 years of age before such testing is possible.

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