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Human Color Vision: Part 2 Lecture Notes
Key Words and Termstrichromatic color vision, refraction of light, color mixing, cone pigments, spectral sensitivity, visual pigment genes, gene duplication, color blindness, unequal recombination, polymorphic variation
Lecture NotesIsaac Newton’s prism experiments: sunlight is made up a continuous series of lights that differ in their angles of refraction when passed through a prism, the different lights appear as different colors, and various combinations of these lights can create the color sensation of a physically different light.
Thomas Young’s hypothesis that human trichromatic color vision was based on three types of light sensors within the retina, with distinctive absorbance curves that are broad and substantially overlapping.
The absorbance spectra of the human visual pigments and the resulting wavelength discrimination curve measured psychophysically.
James Clerk Maxwells’ conceptualization of color space as a three dimensional Cartesian coordinate system
The chemical basis for spectral tuning among visual pigments that use the same 11-cis retinal chromophore but differ in their absorbance spectra. The key determinant of the position of the absorbance curve is the degree of pi-electron delocalization: the more delocalization, the greater the red-shift.
The amino acid sequences of human visual pigments reveal a high degree of similarity between M and L pigment genes.
An analysis of the genes coding for the human visual pigments shows that the L and M pigment genes reside in a head-to-tail tandem array on the X-chromosome. This arrangement is the result of a relatively recent gene duplication. The S pigment gene and the rhodopsin gene are on autosomes.
The common variations in human color vision: dichromacy (two rather than three types of cones) and anomalous trichromacy (one of the three cone types has a shifted absorbance spectrum). The vast majority of individuals with a color vision variation are males and they have the variation within either the M or L pigments – a simple consequence of the X-chromosome location of these two genes.
Arrangement and rearrangement of L and M pigment genes as a consequence of frequent unequal homologous recombination, either between the genes (intergenic recombination) or within the genes (intragenic recombination). The result: variability in the number of genes in the array, and the common finding of hybrid genes (one part from an L pigment gene and one part from an M pigment gene).
A common single amino acid polymorphism in the human gene pool (alanine vs. serine at position 180 in the L pigment gene) gives rise to a several nanometer shift in spectral sensitivity, dividing the population of human trichromats into different perceptual groups.
John Dalton’s many contributions to the study of variant color vision: his astute description of his own dichromacy, his instructions that his eyes be examined post-mortem for signs of a pre-retinal discoloration (which was not found), and finally, 150 years later, DNA analysis showing that he was missing the M pigment gene. |
