Tritanopia (from the Greek tritos: third + an: not + opia: a visual condition) is a congenital form of severe color deficiency (dichromacy) affecting the blue-yellow opponent color system. Tritanopes do not distinguish between colors along a specific direction in color space (tritanopic confusion lines) and are able to match all colors using two primaries (unlike trichromats who require three primaries). Tritanopia follows a dominant pedigree pattern and is linked to chromosome 7, where damage to the gene encoding the photopigment opsin results in the absence of functioning S-cones, cone cells containing the short-wavelength (S-) photopigment. Tritanopia is rare, not sex-linked, with great uncertainty as to its incidence, although some estimates place this at 1 in 13,000 or fewer in the Caucasian population. The condition can be partial rather than complete, i.e., tritanopia lies at the extreme of a continuum of tritanomaly in which some capacity remains to discriminate colors along confusion lines.
At the retinal level, tritanopia is an absence of functional S-cones ( CIE Physiologically Based Colour Matching Functions and Chromaticity Diagrams), i.e., cone cells expressing a photopigment containing an S-opsin component, tuned for selective sensitivity to the short wavelengths of blue light [1, 2]. These cones comprise only about 8 % of the retinal mosaic. Even in normal trichromats, they are absent from the central region of the fovea, resulting in “foveal tritanopia” affecting the perception of sufficiently small stimuli when they are fixated [3, 4].
Congenital Tritanopia: Genetic Background
Functional loss of the S-cone photoreceptor cells is caused by point mutation in the gene on chromosome 7 that codes for the S-opsin component of the cone photopigment (sensitive to shorter wavelengths) ( Genetics of photoreceptors, genetics and color vision deficiencies, genes of cone photopigments, genes and cones). The resulting amino acid substitution may prevent the S-opsin from functioning, or it may fold into an aberrant conformation, blocking it from being transported to the ciliary segment of S-cone cells [1, 5].
The condition follows a dominant pattern of inheritance: S-cones must contain two undamaged copies of the gene to function properly [6, 7]. However, pedigrees often display variations in penetrance with incomplete tritanopia in some family members who retain some blue-yellow discrimination.
Variation in phenotype may also indicate that loss of S-cones is progressive, and incomplete, in some family members. This is supported by retinal imaging showing S-cone degeneration subsequent to formation of the retina .
Disease processes and exposure to environmental toxins can lead to the same end point, i.e., acquired tritanopia ( Color Perception and Environmentally Based Impairments). In particular, the phototoxicity of prolonged exposure to intense short-wavelength light can cause S-cones to degenerate. In other forms of acquired tritanopia, the damage is localized elsewhere in the processing of the S0 signal, such as the ganglion cells or the optic nerve.
A related phenomenon is observed in edge perception, where the border between two regions of equiluminant color dissolves if the colors lie on a tritanopic confusion line (e.g., yellow and gray or green and blue). The perception of border distinctness is tritanopic, with no contribution from the S0 signal, as a consequence of the sparseness of S-cones and chromatic aberration of the lens .
Because S-cones are insensitive to mid- and long-wavelength lights, tritanopic wavelength discrimination, as a function of wavelength λ, is similar to the normal trichromat function for λ > ca. 530 nm, with the discrimination threshold Δλ reaching a minimum at about 570–580 nm near the tritanopic “neutral point” (see below). Tritanopic discrimination deteriorates for blue-green hues where the L- and M-cones are equally sensitive, with Δλ peaking at ca. 470 nm, then improving at shorter wavelengths .
S-cones normally contribute little or nothing to perceived luminance, but S-cone signals can make a contribution under certain conditions of adaptation and temporal flicker. Specifically, with an intense long-wavelength background, S-cone stimulation exerts a negative but phase-delayed influence on luminance, as if inhibiting the combined L- and M-cone signal [11, 12].
A tritanope recognizes far fewer distinct colors than a trichromat ( Color categorization and naming in Inherited Color Vision Deficiencies). When naming spectral colors, tritanopes experience one “neutral zone” of subjective gray for yellow light, with midpoint at λ ca. 570 nm, and another for short-wavelength violets, at about λ = 410–420 nm . For a tritanope, all equiluminant colors, instead of being arranged in a two-dimensional plane ( Psychological Color Space and Color Terms), form a single-dimensional line, running from “cool” to “warm” colors between the two extremes of saturated green and saturated red. This single dimension combines the usually separate chromatic qualities of “saturation” and “hue.” Tritanopes distinguish approximately 44 steps along this line, each corresponding to a separate band of interchangeable colors . This tritanopic linear representation of equiluminant colors is part of a broader reduction of a three-dimensional into a two-dimensional color space.
The subjective qualities of tritanopic experience remain unclear. Tritanopes do use the words “blue” and “yellow” when describing lights and surface colors [4, 10]. Dichromacy simulation software, which converts color scenes to show how a tritanope would see these, uses the convention of a red-green scale .
Diagnosis of Tritanopia
Acquired blue-yellow deficits can be symptomatic of illness or chemical exposure ( Color Perception and Environmentally Based Impairments). Thus, standard tests of color abnormality, although targeted primarily at congenital red-green deficiency, do not ignore tritanopia completely. ( Pseudoisochromatic plates) and arrangement/panel tests both address a tritanope’s perceived similarity of colors along the confusion axis.
The Hardy-Rand-Rittler test ( Color Vision Testing) contains pseudoisochromatic diagnostic plates to detect tritanopia and gauge its severity, while the Farnsworth F2 plate is designed specifically for the purpose. (The widely used Ishihara series contain no plates targeting tritanopia.) Each pattern presented in these plates is defined by a color difference that disappears for tritanopes, so that the design vanishes or is supplanted by an alternative design, demarcated by a different chromatic distinction.
In the arrangement tests (Farnsworth-Munsell 100 Hue test, Farnsworth D-15 test), the one dimensionality of a tritanope’s color “plane” collapses a color circle ( Color Circle) into a line, causing drastic departures from a normal observer’s sequence. Errors, or cap transposition “distances,” are summed to quantify the severity of tritanopia. Plotted graphically, the transpositions in the D-15 manifest as diametrical circle crossings, indicating the tritanopic angle of the confusion axis ( Color Perception and Environmentally Based Impairments).
The “gold standard” of diagnosis of tritanopia is the Moreland equation, presented in a small viewing field (2°) in the Moreland anomaloscope , in which a cyan standard (480 nm with a small admixture of 580 nm) is matched by mixing indigo (436 nm) and green lights (490 nm). Decreasing discrimination along the tritanopic confusion lines increases the range of acceptable mixtures. Notably, when field size is increased beyond 1°, residual blue-yellow discrimination can be detected in some cases; at 8° only complete tritanopes accept the full range of color mixtures as a match to the cyan standard .
More recently developed computerized tests for color vision diagnosis, the Cambridge Colour Test and Colour Assessment and Diagnosis test ( Color Vision Testing), interactively quantify any loss of chromatic sensitivity along the tritanopic confusion line. For a tritanope, in the frame of the CIE (u′v′) 1976 chromaticity diagram ( CIE u′,v′ Uniform Chromaticity Scale Diagram and CIELUV Colour Space), a MacAdam ellipse of indistinguishable colors displays a major axis lengthened to infinity and orientation pointing to the tritanope copunctal point.
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