Farnsworth–Munsell 100-Hue test or the Farnsworth Panel D-15 (both have a tritan axis). However, none of these tests, alone or in combination, identifies a tritan disturbance unequivocally. Anomaloscope equations similar in concept to the Rayleigh red-green equation, including the Engelking–Trendelenburg equation [106, 107] and the Moreland blue-green equation [108–110], have been designed to detect tritan defects. The Moreland equation, which is now the most frequently employed, involves matching indigo (436 nm) and green (490 nm) mixture primaries to a cyan standard (480 nm plus 580 nm mixed in a fixed ratio).

Monochromacies

Monochromats can match test lights to just one primary light. In principle, they can lack two of the three cone types (S-, M-, and L-cone monochromats) or all three of them (rod monochromats or complete achromats).

Cone Monochromacies

Blue cone monochromacy is a rare form of monochromacy or total color blindness caused by loss or rearrangement of the X-linked (red-green) opsin gene array or by loss of the critical (locus control) region that regulates the expression of the gene array [111, 112]. It is also known as blue monocone monochromacy [47, 48], S-cone monochromacy, and X-chromosome-linked incomplete achromatopsia. The S cones are the only photoreceptors, other than the rods, believed to be functioning [47, 48, 57, 113, 114]. The disorder is characterized by severely reduced visual acuity [57, 114–116]; a small, central scotoma (corresponding to the blue-blind central foveola); eccentric fixation; infantile nystagmus (which diminishes or disappears with age); and nearly normal appearing retinal fundi. Color discrimination is typically very poor or nonexistent, although it may improve at mesopic levels when both the rods and S cones are functioning (for reviews, see [117, 118]). There also may be associated myopia [119, 120]. The photopic luminosity function peaks near 440 nm (the peak sensitivity of the S cones) rather than near 555 nm (see Chapter 15 on luminosity efficiency functions), with a resultant greatly reduced sensitivity to long-wavelength lights. A cone-rod break (the Kohlrausch kink) in dark adaptation curves occurs, denoting the transition from S-cone to rod function. Clinically, to distinguish blue cone monochromats from achromats (rod monochromats), a special four-color plate test [121] and a two-color filter test [116] are available.

Besides blue-cone monochromacy, there are two other forms of cone monochromacy, known as complete achromatopsia with normal visual acuity [122, 123]. Few cases have ever been described [122–130], and none is fully accepted as authentic (for a review, see [85]). In addition to rods, cone monochromats are conventionally assumed to have either L cones (in which case they are known as L- or red cone monochromats) or M cones (in which case they are known as M- or green cone monochromats), but not both. Although the S cones are assumed to be totally absent or inactive, they may be partially functioning, contributing to luminance but not color discrimination [129]. Evidence of remnant cone function has led to speculation that the defect may be wholly or partially postreceptoral in origin [123–125, 127]. Unlike blue cone or rod monochromacy, there is no reduced visual acuity, nystagmus, or light aversion, and the cone electroretinogram (ERG) appears to be normal.

Rod Monochromacy

In its complete form, the rare congenital disorder of rod monochromacy is also referred to as typical, complete achromatopsia; complete achromatopsia with reduced visual acuity; complete or total colorblindness Online Mendelian Inheritance in Man #216900 (OMIM 216900); or day blindness (hemeralopia). In its incomplete form, it is referred to as atypical, incomplete achromatopsia or incomplete achromatopsia with reduced visual acuity. Its autosomal recessive inheritance distinguishes it from the disorders caused by mutations or structural alterations of the cone opsin genes. However, its manifestation is functionally equivalent to the total loss of all three cone opsin genes. Even if the opsin genes are expressed, they are nevertheless never engaged for vision. To date, mutations in the following three genes are known to be associated with autosomal recessive achromatopsia: CNGB3, accounting for about 50% of affected individuals [131]; CNGA3, accounting for about 25% of affected individuals [132]; and GNAT2, accounting for fewer than 2% of affected individuals [133, 134]. An additional locus (ACHM1) has been assigned to chromosome 14 as a result of a single case of maternal uniparental isodisomy of chromosome 14 [135]. Neither the gene nor the frequency of the locus is known. In the majority of individuals affected by autosomal recessive achromatopsia, mutations in one of the three reported genes result in the complete form of the disorder. In a few cases, mutations in the CNGA3 gene [132, 136–138], and also in GNAT2, are associated with the milder phenotype of incomplete achromatopsia [139]. The estimated prevalence is less than 1 in 30,000 [85, 140, 141].

Most individuals have complete achromatopsia, in which the symptoms can be explained by a total lack of function of all three types of cones, with all visual functions mediated by the rods. The condition is then characterized by photophobia (increased sensitivity to light); severely reduced visual acuity (about 20/200), hyperopia (commonly, but not invariably); pendular nystagmus (which develops during the first few weeks after birth); a central scotoma that is associated with the central rod-free foveola (which is often difficult to demonstrate because of the nystagmus); eccentric fixation; and the complete inability to discriminate between colors (see [141]). On the Rayleigh anomaloscope equation, a complete achromat can always fully color match the spectral yellow primary to any mixture of the spectral red and green primaries, but a brightness match is only possible to red primary-dominated mixtures (because of the reduced long-wavelength sensitivity of the rods). Although visual acuity is usually stable over time, both nystagmus and sensitivity to bright light may improve slightly. The spectral luminous efficiency function, at all intensity levels, peaks at 507 nm, the peak wavelength of the rod or scotopic visual system (see Chapter 15 on spectral luminous efficiency). There is an absence of the Kohlrausch kink (cone-rod break) in the dark-adaptation curve. In the single-flash ERG, the photopic response, including the 30-Hz flicker response, is absent or markedly diminished, while the scotopic response is normal or mildly abnormal.

Rarely, individuals have incomplete achromatopsia, in which one or more cone types may be partially functioning along with the rods. The symptoms are similar to those of individuals with complete achromatopsia but generally less severe [85]. Color discrimination ranges from well preserved to severely impaired; photophobia is usually absent; and visual acuity is better preserved than in complete achromatopsia (it may be as high as 20/80).