two polymorphic variants of the normal L cone pigment are known as protanomalous trichromats, whereas those with a hybrid ML pigment replacing one of the two polymorphic variants of the normal M cone pigment are known as deuteranomalous trichromats. The color vision deficits of anomalous trichromats are usually less severe than those of dichromats, but there is considerable variability among individuals. In general, the smaller the separation between the spectral sensitivities of the normal and anomalous hybrid pigments, the poorer the anomalous trichromat’s color discrimination (for more details, see [85]). Anomalous trichromacy can sometimes arise in individuals lacking either the normal L- or M-cone photopigment because of photopigment optical density differences between photoreceptors containing ostensibly the same remaining L- or M-cone photopigment [100].

Tritanopia

Tritan defects affect the S cones. They are often referred to as yellow-blue disorders, although the term blue-green disorder more accurately describes the typical types of color confusions. As for protan and deutan defects, congenital tritanopia arises from alterations in the gene encoding the opsin, but unlike protan and deutan defects, it is autosomal in nature and linked to chromosome 7. The gene associated with tritan vision defects is OPN1SW (opsin 1 short wave) encoding the S-cone pigment. Tritan defects affect the ability to discriminate colors in the shortand middle-wave regions of the spectrum. They often go undetected because of their incomplete manifestation (incomplete tritanopia) and because of the nature of the color vision loss involved. From a practical standpoint, even complete tritanopes are not as disadvantaged as many protanomalous and deuteranomalous trichromats because they can distinguish between the environmentally and culturally important red, yellow, and green colors. On the other hand, the most frequently acquired color vision defects, whether due to aging or to choroidal, pigment epithelial, retinal, or neural disorders, are the type III acquired blue-yellow defects (see [88]). These are similar, but not identical, to tritan defects. Unlike tritan defects (which are assumed to be stationary), acquired defects are usually progressive and have other related signs, such as associated visual acuity deficits (see [85]).

In the complete tritanope, confused colors should fall along lines radiating from a single tritanopic copunctal point, corresponding to the chromaticity of the missing S-cone fundamental primary [101, 102]. The spectrum is divided by a neutral zone, which occurs near yellow, at about 569 nm (6500 K white; [103, 104]). The violet end of the spectrum may also appear colorless. The resulting loss of hue discrimination is in the violet, blue, and blue-green portions of the spectrum. “Yellow” and “blue” do not occur in the color world of complete tritanopes. They confuse blue with green and yellow with violet and light gray, but not yellow with blue, as the descriptive classification “yellow-blue” defect would seem to imply. Figure 4D shows a simulation of a scene perceived by a tritanope. The photopic spectral luminous efficiency function in tritanopes is normal [102].

The diagnosis of tritan defects is difficult, and the condition often eludes detection. Special pseudoisochromatic plate tests have been designed, including the AO (HRR), Tokyo Medical College, Farnsworth or F2 (Plate I in [105]), Velhagen, Standard (2nd edition), Stilling, and the Lanthony tritan album. Additional information can be gained from examining chromatic discrimination on pigment arrangement tests, such as the