How Do Our Eyes See Color?Laura Mercurio
Part 3: How do our eyes see color?
The physiological reaction to color is the first step to understanding our reaction to color. Light enters the eye through the pupil. Hazel Rossotti describes the process in Color, Why the World Isn’t Grey as “When it reaches the retina on the inner surface eye, it passes through layers of transparent nerve fibers between capillary blood vessels on to the photosensitive cells at the ends of the nerve fibers.” The photosensitive cells are rods and cones. Rods are effective in dim light only and enable us to sense differences in brightness. Cones are effective in daylight and are for color vision. Both rods and cones contain pigments, which absorb light. In the retina there are four million cones, 100,000 in the fovea (yellow spot) opposite the center of the lens and 120 million rods, the greatest amount 20 degrees from the yellow spot. (Figure 3)
The rods pigment is called rhodopsin, “visual purple”. Jeremy Nathans who was one of the biologists responsible for isolating the cone pigments wrote in “Genes for Color Vision” 1989. “The light absorbing cone pigments are sensitive to wave lengths in either the long wave (red), intermediate wave (green) or short wave (blue) region of the visible spectrum. The relative amounts of light absorbed by each class of cones are translated into electrical signals by retinal nerves, where the overall pattern evokes the sensation of a specific hue. The three classes of sensors, the cones are now known to have overlapping but distinct sensitivities to light. For instance the red and green receptors both absorb orange but the red receptor does it more efficiently”. They have also found that there are several versions of the green receptor, which vary slightly in wavelength absorption. Most people tend to have one type of blue receptor, one or two red receptors, in 40:60 ratio and two or three copies of the green receptor. So it appears people do see color differently. The overlapping of the three color receptors has the greatest maximal total sensitivity to yellow light. Could this occurrence be why we see yellow to yellow-green light first and best? (Figure 4)
The absorption maxima of the pigments are blue 426 nanometers, green 530 nanometers and red 552 nanometers and 557 nanometers. At rest the cone pigment, a molecule that consists of a carotine like backbone is partially folded into a cavity in the protein molecule. When light strikes the photosensitive cells, a photon is absorbed if it is the wavelength energy to which the photocell is sensitive. The backbone of the cell straightens and an impulse passes to the nerve.
The optic nerve impulse takes two major paths via the optic chasm- 1) to the hypothalamus via the retino-hypothalamic track to the supracheasmatic nucleus, 2) largest amount goes to the lateral geniculate (posterior of the thalamus) then to the primary visual cortex. (Figure 5)
The hypothalamus eye-brain connection is responsible for the physiological effects of light and color. The hypothalamus sends signals for physiological changes through the autonomic nervous system and through the release of hormones via the pituitary gland. It controls the body’s homeostasis, which includes integration of internal activities over autonomic and endocrine functions. The hypothalamus is connected to the pituitary gland which is master endocrine gland that stimulates the thyroids, adrenals, antidiurectic (fluid flows), gonads, etc. The hypothalamus is also connected to the pineal gland, which Jacob Liberman describes as the “body’s light meter” in Light: Medicine of the Future. “It releases milatonin- the regulator of regulators documented effects on reproductive function, growth, body temperatures, blood pressure, motor activity, sleep, tumor growth, mood and immune system”. It possibly effects longevity and aging. The largest amounts of the optic neurons go to the lateral geniculate and on to the visual cortex. This pathway begins the cognitive or psychological processes of color vision.
This is the third post in our series on color. Don’t miss out on What is color? And How do we perceive color?
For more information check out iaccna.com.
Hazel Rossotti Colour, Why the World Isn’t Grey, Princeton, NJ: Princeton University Press, 1983
Jeremy Nathans, “ The Genes for Color Vision.” Scientific American, February, 1989, pp. 42-49
Jacob Liberman, O.D., Ph.D. Light Medicine of the Future, Santa Fe, NM: Bear & Co., 1991