Edit pad for Colour Blindness re-write.
Created | Updated Mar 17, 2009
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This is an article about Colour Vision and how this visual system can malfunction and procude Colour Deficient Vision (or 'colourblindness'). Colour deficient vision is a disorder in which the individuals affected have a partial or total inability to detect certain wavelengths of the visible spectrum. Before going through a history of the condition and modern techniques for diagnosing the variety of colour vision deficiencies, it is necessary to go through some of the fundamentals of how colour works and how humans see.
Light
Light is not coloured, it is invisible. If light were visible we wouldn't be able to see anything at all! Light is what makes vision possible. Light is only visible when it interacts with objects such a surface of some kind. This surface might be large like a house or as small as motes of dust in the air. We never see the passage of light only it's terminus upon objects it strikes.
The perception of colour, indeed all our visual perceptions, are derived from the processing by the central nervous system of various wavelengths of reflected light, which is generated when white light falls on an object. White light is composed of a combination of all the visible spectral wavelengths, the majority of which are absorbed by objects with photons being released of specific wavelengths corresponding to the colour of the lit surface, as reflected light.
Trichomacy in nature.
The evolution of coloured vision in other species than our own is relatively well understood. Among the vertebrates it is perhaps surprising to learn that placental mammals have amongst some of the poorest colour vision but that birds and reptiles, meanwhile, have some of the most complex colour vision.
Light is processed by specialist cells on the back surface or retina of the eye. There are two kinds of light sensitive cell, these are called rods and cones. The rods and cones are specially adapted neurones which convert light energy, by absorbing photons, into electrical signals that can be carried and interpreted by the nervous system of most organisms.
Rods, named for their cylindrical shape, are sensitive to the luminosity of light and function best in low light conditions. They are congregated around the periphery of the retina and are associated with peripheral vision (things sensed out of the corner of your eye)
INSERT CONE HERE
This is because the retinas of most placental mammals have only two types of cone cell and are only able to process mixes of two spectral lights, This is a condition of vision called Dichromacy and they are natural dichromats.
Trichromacy is when an organism is independently able to process three separate spectral lights to make up their range of colour perception. Organisms with trichromatic vision have three-cone colour vision are called trichromats. Trichromats include most fish and reptiles and marsupial mammals, but trichromacy is the exception rather than the norm among placental mammals.
Among those exceptions are primates, New World (Central and South America) and Old World monkeys (South and East Asia, the Middle East, Africa and Gibraltar), and of course, humans.
Four-cone colour vision or tetrachromacy is also found in nature also amongst some fishes and reptiles. A few species of reptiles and birds can have vision that is even more sophisticated than this. In addition some mammals developed alternatives to vision such as echolocation found in dolphins and bats or the electromagnetic sensitivity of the duck-billed platypus.
With the exception of the primate families the majority of placental mammals see no better than a someone who is a dichromat. Why this should be was a puzzling question but research conducted in only the last few decades suggests the evolution of colour vision has shown some interesting and explicable trends.
Amongst birds and reptiles1 the ability to see with greater acuity in either low and high light levels evolved fairly early.
Amongst predators either diurnal2 cold-blooded reptiles or nocturnal3 warm-blooded birds the ability to see prey to hunt with conferred an obvious advantage.
Amongst nocturnal hunters, the evolution of the rod coverage on their retinas gives them exceptional vision in low light levels. Over their evolutionary history the general complexity of their visual systems has increased. The same cannot be said of mammals. Mammals it seems inherited the reptialian thrichromatic vision but lost it. This is evident if if one examines the dichromacy common to most placental mammals, marsupial mammals retained the reptilian trichromacy. Trichromacy evolved again, independently twice more
The reason it is postulated seems to have been the nocturnal behaviours of the early mammals. During the Cretaceous Period where the day belonged to the dinosaurs, The Mammals, who were nocturnal, needed every spare photon available. And the selective pressure acting on the genes for vision which would later be passed on to a variety of decedent species selected against trichromacy which was slowly eradicated. Probably becuase the eyes of mammals that contained more rods than cones let the nocturnal hunters see better at the time they needed to hunt. Early mamals with more cones than rods were selected against by the nocturnal environment of the early mammals. The diurnal pattern of behavious and the consequent switch back to trichromatic vision emerged only later after mammals started to diversify in the Cenozoic era4 and it is then, and only in a limited set of species, that trichromacy re-appears.
The fact that it did so, and then twice independently suggests it is of significant benefit to diurnal creates which can hunt using the white light of the sun. But why did trichromacy reappear in primates? The prevailing theory is that it relates to the ability to detect ripe and unripe fruit (which have evolved in a kind of positive feedback to be brightly coloured in the red-spectrum (including yellow and orange) to help diseminate their seeds, which selects for bright coloured ripe fruit.
Maxwell, Young and Helmholtz
Trichomacy, Rods and Cones: Short (S), Medium (M) and Long (L)
Human Vision: Light and the human eye.
Explain how cones react to light proteins and vitamin A
How individual cones don't just react to RGB spectral lights but that all cones react to differing extents
The three cone types in trichromats are sensitive to short, medium and long wavelengths of light
SML dewcribes the frequency range of light for which the cones show peak sensitivity thus the blue cone is most sensitive in the 420nm, the green cone's peak sensitivity is green-yellow 534nm and red cones spectral peak s in yellow 564nm. The colour sensations of red, green and blue depend on the activity of more than one type of cone.
Spectral cone sensitivity
The peak sensitivity of the spectral cone cells are not in their respective nomenclatures of blue, green and red, but rather respectively violet, yellow-green, and
Spectral cone sensitivity
Yellow and mixing red and green light.
Appearence of mixed colours is perceptual based on the colour matching that occurs at the retina
Colour
Hue, Luminosity, Saturation, Chromaticity (saturation and hue less luminosity) - terms
1Other orders such as invertebrates have a whole differ biochemical system, that aren't considered here.2Active during the day; sleeps at night3Active during the night; asleep during the day.465 Million Years Ago
