deletions), or altered (due to intragenic recombination between genes of different types or possibly point mutations). Phenotypically, the results of the gene alterations are (1) dichromacy (when one of the cone pigments is missing, and color vision is reduced to two dimensions); (2) anomalous trichromacy (when one of the three cone pigments is altered in its spectral sensitivity, but trichromacy is not fully impaired); or (3) monochromacy (when two or all three of the cone pigments are missing, and color and lightness vision is reduced to a single dimension). Other inherited types of color blindness arise from mutations in genes not encoding the cone opsins but rather components of cone structure and function. Those associated with the loss of function of all three cone types are referred to as complete achromatopsia or rod monochromacy.

Protan and Deutan Defects

The most common inherited color vision deficiencies are the loss (protanopia and deuteranopia) and alteration (protanomaly and deuteranomaly) forms of protan (L-cone) and deutan (M-cone) defects. Also known as red-green color vision deficiencies, they are associated with disturbances in the X-linked opsin gene array. They manifest in early infancy, mostly in males; the condition is not accompanied by ophthalmologic or other associated clinical abnormalities. Among Caucasians, about 8% of males and 0.5% of females have red-green color vision defects; these defects are less frequent among males of African (3–4%) or Asian (3%) origin (for more details, see [85]). The two genes associated with red-green color vision defects are OPN1LW (opsin 1 long wave), encoding the L-cone pigment, and OPN1MW (opsin 1 middle wave), encoding the M-cone pigment.

Red-green deficiencies are diagnosed by a variety of special color confusion charts (e.g., the Dvorine, Ishihara, and Stilling pseudoisochromatic plates), hue discrimination or arrangement tasks (e.g., the Farnsworth–Munsell 100-Hue test, the Farnsworth Panel D-15, the Lanthony Desaturated D-15), and lantern detection tests (e.g., the Edridge– Green, Holmes–Wright), all of which exploit the color deficits of the color blind and have been designed to screen selectively for protan and deutan defects (for an overview of the available clinical tests, see [86–89]).

Traditionally, observers with protan and deutan defects are most efficiently and definitively characterized by the nature of their Rayleigh matches [90] on a small viewing field (≤2° diameter) anomaloscope. In the task, the observer is required to match a spectral yellow (ca. 589-nm) primary light to a juxtaposed mixture of spectral red (ca. 679-nm) and green (ca. 544-nm) primary lights by adjusting the intensity of the yellow and the relative proportions of the red and green lights. Most trichromats reproducibly choose a unique match between the red/green mixture ratio and the yellow intensity. In contrast, individuals with protan and deutan defects have displaced Rayleigh match midpoints (i.e., the mean value of the red/green ratio required to match the yellow primary falls outside the normal range) or extended or complete matching ranges (they accept more than one red/green ratio).

Protanopia and Deuteranopia

Protanopia and deuteranopia are the dichromatic or loss forms of protan and deutan defects, respectively. Although some protanopes and deuteranopes are true reduction

dichromats, having only one X-chromosome-linked cone photopigment, which is identical to the normal M- or L-cone pigment, others are not. Some have a hybrid X-chromosome-linked cone photopigment, which is intermediate in spectral position between M and L, while others have two cone photopigments with identical or nearly identical spectral sensitivities. By definition, dichromats require only two primaries to match all color stimuli. As a result, they confuse or fail to discriminate colors that are easily distinguished by normal trichromats. In the Rayleigh matching task, deuteranopes (lacking M cones) or protanopes (lacking L cones) are able to fully match the spectral yellow primary to any mixture of the spectral red and green primary lights by merely adjusting the intensity of the yellow regardless of the red-to-green ratio. Thus, instead of a unique match, they will have a fully extended matching range that encompasses both the red and green primaries.

As first pointed out by Maxwell [19] and demonstrated by von Helmholtz [91], when the colors confused by dichromats are plotted in a chromaticity diagram, the axes of which may be generated from transformations of standard CMFs, they lie on a series of straight lines called confusion loci that converge to either the protanopic or the deuteranopic copunctal points, which correspond to the chromaticity of the missing fundamental primary (see the section on cone spectral sensitivity measurements). In both protanopes and deuteranopes, the spectrum is dichromic, consisting of just two pure hues [92]. The midpoint of the zone—the neutral point—which, by definition, falls on the confusion line passing through the physiological white point, is relatively easy to specify. For protanopes and deuteranopes, representative neutral point values for a white standard light (of color temperature 6,774 K) are 492.3 nm [93–95] and 498.4 nm [93, 94, 96], respectively. The photopic spectral luminous efficiency function of deuteranopes is normal or relatively slightly more sensitive at long wavelengths than that for normals, whereas that of protanopes is much less sensitive than that for normals at long wavelengths (e.g., [97]). This imbalance arises because the normal luminous efficiency function is dominated by L cones (see Chapter 15 on luminous efficiency functions).

Figure 4 shows a simulation of a scene perceived by a normal trichromat (A), protanope (B), deuteranope (C), and tritanope (D). It gives an approximate impression of the sorts of color confusions they make.

Photopigment Variability and Protanomaly and Deuteranomaly

Protanomaly and deuteranomaly are the alteration forms of protan and deutan defects, respectively. The color deficits associated with these forms of anomalous trichromats are usually less severe than those of dichromats, but there is considerable variability among individuals. They can be categorized as simple or extreme, according to their matching behavior on the Rayleigh equation [98]. Many simple anomalous trichromats may be unaware of their color vision deficiency, whereas many extreme anomalous trichomats may have nearly as poor color discrimination as dichromats. Unlike dichromats, they do not have a neutral zone and see more than two hues in the spectrum.

Anomalous red-green trichromacy arises because the spectral sensitivity function of either the L- or M-cone photopigment is shifted from its normal location to an intermediary or anomalous position that lies closer to the location of the spectral sensitivity function of the remaining normal M- or L-cone photopigment (for a review, see [85]). These shifts are caused by the inheritance of hybrid LM or ML cone photopigment opsin