Chapter 10: Retina
COLOR VISION DEFECTS
The perception of color is a cortical response to specific physical stimuli received by the retina. A narrow band of the electromagnetic spectrum, wavelengths between 400 and 700 nm, is capable of being absorbed by visual pigments contained in the outer segments of human cone photoreceptors. As described above, spectral sensitivity studies of cone photopigments have identified blue, green, and red cone photoreceptors. A minimal requirement for color discrimination is the presence of at least two kinds of cone photopigment, and normal color vision requires the presence of all three. Color vision testing is described in Chapter 2. In a broad sense, color vision defects are either congenital or acquired. While hereditary congenital color defects are almost always "red-green," affecting 8% of males and 0.5 % of females, acquired defects are more often of the "blue-yellow" variety and affect males and females equally. Congenital color vision defects affect both eyes equally, while acquired color defects frequently affect one eye more than the other. Most congenital color vision defects are X-linked recessive and are constant in type and severity throughout life. Acquired color vision defects generally vary in type and severity, depending upon the location and source of the usually ophthalmoscopically observable ocular pathology.
Dichromats are individuals whose cone photoreceptors contain only two of the three cone photopigments. Persons with a red-green color deficiency related to red-sensitive pigment loss were historically described first, and the condition is therefore referred to as protanopia. A second type of red-green deficiency involving green-sensitive pigment loss is known as deuteranopia. Blue-yellow color blindness is the third form and is referred to as tritanopia. While a color vision defect is present, there is no acuity loss in these patients.
Based on a color matching classification, the most common color vision deficit is that of anomalous trichromats. These individuals require three primaries for matching an unknown color but-unlike normal trichromats-use them in "anomalous" amounts. Each of the anomalous trichromats has a defect analogous to that of the dichromats described above.
There are two forms of monochromatism, and although both leave the affected individual completely without color discrimination, they are two quite separate entities. In rod monochromatism, the individual is born without functioning cones in the retina, and such a loss accounts for the associated symptoms of low visual acuity, absent color vision, photophobia, and nystagmus. The generalized loss of cones in this condition is shown unequivocally by the photopic electroretinogram. In cone monochromatism, affected individuals with this extremely rare condition have no hue discrimination but do have normal acuity and no photophobia or nystagmus. Cone monochromats do have cone photoreceptors, but all the cones contain the same visual pigment.
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10.1036/1535-8860.ch10