Congenital Color Vision Defects

There are many distinct types of congenital color defect and each yields a characteristic pattern of results on behavioral tests of color vision (these include both laboratory tests of the kinds described previously and the familiar plate tests used for the clinical screening of color vision). Those most severely impacted have dichromatic color vision, like that of John Dalton. When tested in color matching experiments of the sort described previously, such individuals require only two primary lights to complete all the color matches. The number of discrete color discriminations that can be made is also severely reduced. For instance, in the wavelength discrimination test (Fig. 3B), dichromats basically fail to discriminate among wavelengths in the middle to long wavelengths while retaining an ability to discriminate wavelengths shorter than approximately 490-500 nm from wavelengths longer than that value. As a result, they are commonly characterized as "red/green color blind.'' This is a somewhat misleading description since dichromats are not blind to color in this part of the spectrum: Rather, they simply fail to discriminate among these colors. The full gamut of saturations that dichromats see is also significantly smaller than that seen by people with normal trichro matic color vision. There are two major types of congenital dichromacy, protanopia and deuteranopia. Individuals ofthe two types differ both in the details of color discriminations they can make and in their spectral sensitivity.

In addition to the dichromacies, there are also trichromatic individuals whose color vision nevertheless differs significantly from normal trichromacy. These people are said to have anomalous trichromatic color vision and, like the dichromats, their color vision defects can be detected by color discrimination tests. Anomalous trichromats are of major two types: protanomalous and deuteranomalous. An essential difference between the two types emerges from a simple color matching task in which an individual is asked to complete a color match by adding green and red lights in proportion to match a yellow test light. Dichromats so tested fail to set a reliable match since they cannot discriminate among colors in this part of the spectrum. All trichromatic individuals can make such matches. Relative to the normal, the protanoma-lous observer will require additional red light to complete the match, whereas the deuteranomalous individual needs additional green light. A wide range of other discrimination tests can be similarly used to distinguish among the trichromatic subtypes. Although the trichromatic anomalies are usually classified as color vision defects, it is important to note that in fact many anomalous trichromats have quite acute color vision.

Individuals with these congenital defects thus differ significantly from those with normal color vision in their discrimination abilities. What do they actually see? This intriguing question has not been easy to answer. Growing up as a minority in a color-coded world, most color defectives acquire a rich vocabulary of color names that they learn to use in a discerning fashion. They can do this because there are many secondary cues to allow one to apply color names in accord with the majority view (e.g., familiar objects are often characteristically colored and objects of different color often have systematic brightness differences). It is only when these secondary cues become scarce or disappear entirely (as they do in formal tests of color vision) that color defect become readily apparent. Some insights into the color world of the color defective come from examination of the occasional individual who has defective color vision in one eye and normal color vision in the other. Similarly, inferences can be drawn from comparison of the discrimination abilities of the normal and the color defective. These pieces of evidence suggest that the two major types of dichromat view the world in what to the normal would be varying shades of only two hues— blues and yellows. Furthermore, there are significant losses in the saturation of lights so that the perceptual world of the dichromat is distinctly pallid relative to that of the normal. Recently, computer algorithms have been developed that allow one to transform a digitized colored image to obtain a perceptual prediction of the dichromatic world. Although defective color vision is often treated as a benign condition, there is documented evidence to indicate that those who have defective color vision find that it provides a real barrier in many aspects of normal life—from things as mundane as making judgments about the ripeness of a piece of fruit to issues as far reaching as a choice of a profession.

A vast majority of those with defective color vision are male. The incidence of defective color vision among males is given in Table I. In total, approximately 8% of the in Western Europe and in the United States can be classified into one or another of these diagnostic categories with 5% of all males having deuteranomalous color vision. The incidence of red/ green defective color vision is low in females, being approximately the square of the frequency of the corresponding male color vision defects.

There are also rare congenital color vision defects that arise from change or alteration in the ability to discriminate among short-wavelength lights (tritanopia and tritanomaly). The incidences of these defects do not differ for male and females. Finally, in addition to dichromats and anomalous trichromats, a relatively few individuals lack color vision completely (are monochromatic) or only show an ability to discriminate color in very restricted test circumstances.

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