Why do we see color?
This paper will investigate the question: Why do we see color? I will first give an account of color vision in humans and describe the two contending theories and their reconciliation. In the second part, I will give a brief evolutionary account of how humans have acquired color vision.
What is color vision? Simply, it is the ability to discriminate visible light on the basis of wavelength composition. Visible light for humans and most other mammals spans wavelengths between ~400 to 700 nm. This capacity allows us to see two million different surface colors. Our experience of these colors is through the individual surfaces of objects. These present colors when a bright white light falls on them. The object absorbs all but one of the wavelengths of this light, and the wavelength it reflects is its color.
The color solid is a reference tool for describing colors. The achromatic spectrum is “the gray scale” and it lies along the central axis (see Figure 1). These colors have no hue and are thus considered neutral. In the color solid, hue is represented by circumference, the distance from the central axis denotes saturation, and the distance towards or away from the whiteness represents brightness. There are three primary colors, and opposites are called complements as they can neutralize each other.
Like other animals, including primates and certain species of birds, fishes, reptiles, and insects, humans have visual photoreceptors that respond differently to various wavelengths of visible light. There are three cone photoreceptors in humans, and the cones of each type possess a photo pigment that is distinguished by a particular protein molecule that is light sensitive. The photopigment selectively absorbs from a particular region of the visible spectrum. The three kinds of receptors are known to absorb light over characteristic ranges of wavelength and each of the photoreceptors has a maximum absorption at particular regions within the spectrum. The cone photoreceptors are known as short-wavelength (S), medium-wavelength (M), and long-wavelength (L). Their respective sensitivities to maximal wavelengths are 420, 530, and 560 nm. (See Figure 2) The spectral sensitivity of a photoreceptor is best understood as a measure of the probability that the receptor will absorb a photon of a particular wavelength. The particular spectral sensitivity of a photoreceptor depends on the opsin it expresses. An opsin is a protein membrane-bound receptor found in both rod and cone photoreceptors. It’s sensitivity depends on the sequence of amino acids that make up the opsin protein. For the analysis of color, there need be a comparison of the signals from the different types of cones. The comparison occurs across space, and the arrangement of the three types of cones, according to modern measurements refute he idea that they are arranged along a lattice and instead seem to be randomly distributed in a mosaic.
There are two contending theories of color vision: the Young-Helmholtz-Maxwell theory and the Hering theory. The Young-Helmholtz-Maxwell is a trichromatic theory of vision that elaborates on Young’s empirical evidence concluding that the most sensitive points of the retina detect and are stimulated by only blue, red, and green, and that the optic nerve is composed of filaments corresponding to each primary color.
As a challenge to this Hering presented his “opponent-colors theory,” claiming that there are three qualitatively different processes present in the visual system, and each of the three is capable of responding in two opposite ways. These processes are red/green, blue/yellow, and black/white. Based on this, Hering declares the existence of four primary colors as the psychological basis for all color sensation—red, blue, green, and yellow.
Modern research claims these two theories are complementary: affirming that Young-Helmholtz three-color approach explains how the eye detects and perceives color. Hering’s four-color theory explains how color is encoded and sent to the brain via nerve pathways. In other words, the Young-Helmholtz account is concerned with the reception phase of color vision, while Hering is concerned with the processing phase—his focus is the phenomenal and the neural-processing aspects of color rather than the action of light on photo pigments.
The evolution of trichromatic color vision has lead to the color vision of humans, Old World monkeys and apes to be distinctly different from most other mammals. Other mammals typically are dichromatic, that is, have two photo-pigments S and an M/L intermediary. In humans, there are two distinct cone pigments for M and L. The genes encoding these are side by side on the X-chromosome and are identically in structure, while the S photo-pigment can be found on chromosome 7. Gene-sequence comparisons suggest that M and L opsin genes arose from a gene duplication that occurred 30-40 million years ago, giving early primates trichromatic color and successfully passing it down. New World monkeys are in nearly every species both dichromatic and trichromatic. Here, there is a polymorphism that causes individual variation in M and L cone pigments. While all monkeys share the common-S pigment some get only one from M/L range, and some get both. Evidence from universally trichromatic howler monkeys suggests that the polymorphic arrangement in New World monkeys was the state before universal trichromatic Old World monkeys before they underwent a gene duplication. Variation in color vision calls into doubt the capacity’s purpose. On one hand, trichromacy allows for discrimination in green, yellow, orange, and red targets, which would be useful in detecting fruits. This is a considerable advantage over dichromacy, and yet, on the other hand, raises the question why New World monkeys are not uniformly trichromatic. A reason for this could be that opsin gene duplications—like what happened to Old World monkeys and humans, and more recently Howler monkeys—are rare. Moreover, color vision requires an elaborate nervous system, so in circumstances where color vision would be of low utility, for example, in dim environments the selective pressure to maintain adaptations may lower.
Figure 1. Color Solid
Figure 2. Relative absorbance of particular wavelengths for the the 3 photoreceptors
Sources:
Guy Cowlishaw, Tim Clutton-Brock, Charles L. Nunn, Andrew Whiten, Richard Dawkins, Gerald H. Jacobs "Primates" The Encyclopedia of Mammals. Ed. David W. Macdonald. Oxford University Press, 2007. Oxford Reference Online. Oxford University Press. University of Pennsylvania. 5 February 2008 http://www.oxfordreference.com/views/ENTRY.html?subview=Main&entry=t227.e116-ss5
Lorrin A. Riggs, "Color vision", in AccessScience@McGraw-Hill, http://proxy.library.upenn.edu:4117, DOI 10.1036/1097-8542.149800
Myles Jackson "optics and vision" The Oxford Companion to the History of Modern Science. J. L. Heilbron, ed., Oxford University Press 2003. Oxford Reference Online. Oxford University Press. University of Pennsylvania. 7 February 2008 http://www.oxfordreference.com/views/ENTRY.html?subview=Main&entry=t124.e0538
Jacobs, G. H. (1996) Proc. Natl. Acad. Sci. USA 93:, 577–581.
Color Vision. By: Gegenfurtner, Karl R.; Kiper, Daniel C.. Annual Review of Neuroscience, 2003, Vol. 26 Issue 1, p181-206, 30p; (AN 10735015)
The machinery of colour vision. By: Solomon, Samuel G.; Lennie, Peter. Nature Reviews Neuroscience, Apr2007, Vol. 8 Issue 4, p276-286, 11p; DOI: 10.1038/nrn2094; (AN 24411246)
