Protanopia [from the Greek protos (first) + an (not) + opia (a visual condition)] is a congenital form of severe color deficiency (dichromacy) affecting the red-green opponent color system. Protanopes do not distinguish between colors along a specific direction (protanopic confusion lines) in color space and are able to match all colors using two primaries (unlike trichromats who require three primaries). Protanopia is linked to the X-chromosome and arises from loss or alteration of the gene encoding the opsin of the long-wavelength (L-) photopigment; this malstructure results in the absence of functioning L-cones in the retina. Protanopia follows a recessive pedigree pattern, with incidence of ca. 1 % in the Caucasian male population. It is distinct from protanomaly in which some degree of discrimination among such colors remains.
Functional loss of the L-cone photoreceptor cells is caused by a mutant cone photopigment gene on the X-chromosome (“ Genetics of photoreceptors, genetics and color vision deficiencies, genes of cone photopigments, genes and cones” by Rodriguez-Carmona). The gene coding for the L-opsin may be missing altogether from a protanope’s genome; it may be replaced by a second copy of the M-opsin (sensitive to slightly shorter wavelengths), or it may be present but damaged so that the protein is not expressed or cannot be transported to the ciliary segment of cone cells expressing that gene .
When present, the L-opsin gene is located on the X-chromosome, followed by one or more copies of the M-opsin gene (“ Genetics of photoreceptors, genetics and color vision deficiencies, genes of cone photopigments, genes and cones” by Rodríguez-Carmona). This architecture, and the high similarity of the two genes, facilitates mismatching of DNA during meiotic crossover, leading to deleterious mutations of the L-opsin encoding gene .
With twoX- chromosomes, females are far less prone to protanopia. Notably, mothers and daughters of protanopes can be heterozygous for protanopia (i.e., carriers of the condition) with one normal and one aberrant X-chromosome; the former is active in a portion of their cone cells, providing enough L-cones to sustain normal trichromacy. However, minor departures from normality are discernible , including reduced sensitivity to long wavelengths, known as Schmidt’s sign, and reduced subjective prominence of red-green dissimilarity .
Luminous Efficiency, Luminance Effect, and Wavelength Discrimination
In the absence of L-cones tuned to longer wavelengths, protanopes display a distinctive insensitivity to 550–750 nm lights (orange and red) . Their luminosity function has peak sensitivity around 540 nm compared to the maximum near 555 nm of normal trichromats, i.e., it is blue shifted toward the λmax of the M-cone pigment [1, 7]. Phenomenally, reds appear relatively dark for protanopes and are confused with grays and bluish turquoise.
Thus, protanopes attend more to luminance changes than normal trichromats as a color cue, compensating for their poor hue discrimination . Notably, at lower photopic and mesopic luminance levels, protanopes’ color discrimination slightly improves, perhaps reflecting input from rod cells .
Wavelength discrimination, as a function of wavelength, is considerably worse for protanopes than for normal trichromats. Their lowest discrimination thresholds (Δλ), approaching the trichromatic value, are around the protanopic “neutral point”, ca. 490 nm (see below). Δλ rises on both sides of this minimum, increasing rapidly on the short-wavelength side and on the long-wavelength side until wavelengths are not distinguished beyond approximately 550 nm .
A protanope recognizes far fewer distinct colors than a trichromat (“ Color Naming” by Bonnardel). Instead of all possible equiluminant colors being arranged in a two-dimensional plane (“ Psychological Color Space and Color Terms” by Bimler), for a protanope they form a single-dimensional line, running between the two extremes of saturated blue and saturated yellow, separated by a “neutral zone” of subjective gray, including blue-green spectral light, with midpoint at ca. 490 nm . Protanopes distinguish approximately 21 steps along this line, each corresponding to a separate band of interchangeable colors . The usually separate chromatic qualities of “saturation” and “hue” are thus combined along this line. This protanopic linear representation of equiluminant colors is part of a broader reduction of a three-dimensional into a two-dimensional color space.
Whether the subjective qualities of the two extremes are identical to the normal trichromatic “blue” and “yellow” remains controversial and rests on reports from a handful of individuals with unilateral dichromacy. The blue-yellow convention is used in dichromacy simulation software, which converts color scenes to show how a protanope would see them .
Despite this model of a two-hue palette for self-luminous colors, the color percepts of protanopes remain in question. Their object-color palette was proved to include all six Hering component colors (red, green, blue, yellow, white, and black) . However, in contrast to normal trichromats, protanopes perceive only a weak green component in object colors and do not experience “unique green,” while their red and green components are confounded with an achromatic element (“ Color categorization and naming in inherited color vision deficiencies” by Bonnardel).
Diagnosis of Protanopia
A number of tests for protanopia are available, varying in sensitivity, specificity, and ease of use [13, 14] (“ Color Vision Testing” by Paramei, Bimler). Pseudoisochromatic plates and arrangement/panel tests, widely used in surveys/field studies, directly address a protanope’s confusions between color pairs along the confusion axis.
In the pseudoisochromatic tests (Ishihara test, Hardy-Rand-Rittler test), each diagnostic plate shows a normal-observer design (“ Marisa Rodriguez-Carmona Pseudoisochromatic Plates” http://link.springer.com/referenceworkentry/10.1007/978-3-642-27851-8_93-1 digit or curved lines) defined by a color difference which disappears for protanopes, so that the design vanishes or is supplanted by an alternative design demarcated by a different chromatic distinction.
In the arrangement tests “ Color Vision Testing” (Farnsworth-Munsell 100 Hue test, Farnsworth D-15 test), the one-dimensionality of a protanope’s color “plane” collapses a color circle into a line (“ Color Circle” by Green-Armytage), causing drastic departures from a normal observer’s sequence. Errors, or cap transposition “distances,” are summed to quantify the severity of protanopia. Plotted graphically, the transpositions in the D-15 manifest as diametrical circle crossings, indicating the protanopic angle of the confusion axis (“ Color Perception and Environmentally Based Impairments” by Paramei).
The “gold standard” of diagnosis of protanopia is the Rayleigh match on a small viewing field (2°) of the Nagel anomaloscope (“ Color Vision Testing”) . Unlike normal trichromats who reproducibly choose a unique ratio of red and green to match the yellow intensity, a protanope can match any red/green ratio to the yellow light by adjusting the luminance of the latter, dimming it if the mixed light is mainly red, while a green-dominated mixture requires a more luminant yellow. Notably, some protanopes, when setting Rayleigh matches tested with a large viewing field (10°), perform as anomalous trichromats. Various factors related to retinal eccentricity are presumed, e.g., rod intrusion, S-cone intrusion, spatial variation in the macular pigment, etc. .
More recently developed computerized tests for color vision diagnosis, the Cambridge Colour Test and Colour Assessment and Diagnosis test (“ Color Vision Testing” by Paramei, Bimler), interactively quantify any loss of chromatic sensitivity along the protanopic confusion line. For a protanope, in the frame of the CIE (u’v’) 1976 chromaticity diagram (“ CIE u’,v’ uniform chromaticity scale diagram and CIELUV color space” by Schanda), a MacAdam ellipse of indistinguishable colors displays a major axis lengthened to infinity and orientation pointing to the protanope copunctal point.
Incidences of Protanopia
The prevalence of inherited protanopia varies between human populations. For Caucasian populations, the reported incidence of protanopia is 1.01 % in males and 0.02 % in females . Recent surveys of red-green deficiency in populations of different racial origin found the incidence of protanopia in Caucasian males to vary between 0.96 % and 1.27 %. In a marked difference, the incidence is about half this rate in indigenous populations in Asia, Africa, Australia, and Central America but is observed to raise in geographic areas that have been settled by incoming migrants, in particular, in coastal areas visited by Europeans in historical times . The increasing prevalence of congenital red-green deficiencies, including protanopia, has tentatively been explained by a relaxation in selection pressures as hunting and gathering cultures evolve toward industrialized societies . More recently, however, it is rather attributed to genetic drift and founder events (variable sampling of the gene pool contingent on the population size) .
- 1.Sharpe, L.T., Stockman, A., Jägle, H., Nathans, J.: Opsin genes, cone photopigments, color vision and colorblindness. In: Gegenfurtner, K., Sharpe, L.T. (eds.) Color Vision: From Genes to Perception, pp. 3–52. Cambridge University Press, Cambridge (1999)Google Scholar
- 13.Birch, J.: Diagnosis of Defective Colour Vision. Oxford University Press, Oxford (1993)Google Scholar
- 14.Pokorny, J., Smith, V.C., Verriest, G.: Congenital color defects. In: Pokorny, J., Smith, V.C., Verriest, G., Pinckers, A.J.L.G. (eds.) Congenital and Acquired Color Vision Defects, pp. 183–241. Grune and Stratton, New York (1979)Google Scholar
- 16.Pitt, F.H.G.: Characteristics of Dichromatic Vision. Medical Research Council Special Report Series, No. 200. London: His Majesty's Stationery Office (1935)Google Scholar