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Candice Davidoff, Maureen Neitz, Jay Neitz; The Relationship Between Photopigment Spectral Separation and Color Vision Acuity. Invest. Ophthalmol. Vis. Sci. 2014;55(13):6246.
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Within trichromats, variability exists in the separation of L and M photopigments depending on the amino acid sequence of the opsins. For L pigments, the presence of a tyrosine at position 116, an alanine at position 180, or a threonine at position 230 produces shifts toward shorter wavelengths, creating a range of spectral peaks of 549-559 nm. For M pigments, the presence of a serine at position 180 or an isoleucine at position 230 shifts the peak toward longer wavelengths, creating a range of 530-536 nm. Therefore, at the extreme, trichromats could have a separation of as little as 13 nm. Meanwhile, deutans could have two L photopigments as far apart as 10 nm. This predicts a continuum of red-green color vision acuity rather than discrete, well-separated categories defined by the classes of cones present in the retina.
We examined the color vision acuity of trichromats and deutans with a range of spectral separations using a forced-choice version of the Rayleigh match. Additionally, we characterized the distribution of L and M peak separations in the population using a sample of 896 men with normal color vision as screened by the HRR pseudoisochromatic plate test and the desaturated D-15. PCR was performed to selectively amplify L or M opsin genes and exons 2, 3, and 4 were sequenced to determine photopigment peak.
Over 98% of the trichromats had separations between 25 nm and 29 nm. However, two men had L and M peaks only 19 nm apart and one only 16 nm; these people tested as having normal color vision. Seven other men who also tested as having normal color vision had only L opsin genes 6-10 nm apart in expressed positions, making them deutans.
We conclude that normal color discrimination can be served by pigments much less well separated than the 30 nm found in most people.
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