Shedding Light on Colour
Hold a prism up to the light, and the colours of the rainbow will project out in a beautiful spectral array. People once believed that the prisms themselves coloured light, but Isaac Newton realised that this was false. He became the first to understand the rainbow by demonstrating that all the prism did was refract white light and split it into its component colours: red, orange, yellow, green, blue and violet.
Our colour vision influences very human activities from art to poetry to what we wear and wear we live—but we rarely question its mechanics. What exactly is colour, and how do we see it?
The human eye perceives colour by seeing how matter interferes with white light—it absorbs, reflects, refracts, scatters, or refracts light into different wavelengths, so an object’s colour depends on the physics of an object in its environment. It also depends on the way the observer perceives colour. In humans, this external light then moves through the eye until it’s projected onto the retina, which consists of a thin layer of light-sensitive nerve cells called photoreceptors. The photons are absorbed by pigments in these cells, then converted into electro-chemical signals and sent to the brain where they are processed into vision.
There are two types of photoreceptor cells: rods and cones. Our retina contains approximately 125 million rods, which are monochromatic and so don’t perceive colour and are only used in dim light, but there are 6 million cones, which function best in bright light and are sensitive to a wide range of brightness. There are three types of cones, sensitive to short, medium and long wavelengths of light respectively—blue cones, green cones and red cones.
Cones are therefore sensitive to colour and are responsible for sending colour information to the brain. This means that the 8% of males and 0.5% of females who are colour blind either don’t have a particular type of cone, or one type of cone might be weak.
Wavelengths of light are measured in nanometres, and the colours that humans can detect range from approximately 400 nanometres (violet) to 700 nanometres (red). This is our visual spectrum, but it only makes up a tiny part of the entire electromagnetic spectrum, which spans from high-energy gamma rays to lower energy radio waves, with X-rays, ultraviolet and infrared in between. Our whole world is oriented around the way we see the visible spectrum and its component colours—but not everyone shares this same experience.
The majority of primates (including humans) and insects have three types of cones, and thus have trichromatic colour vision. But while humans have blue, green and red cones, insects have blue, green and ultraviolet cones, allowing them to see UV light beyond our spectrum of colour. Most mammals only have two types of cones and so are dichromats, while birds, turtles, and fish are tetrachromats, with four different types of cones: red, green, blue, and ultraviolet. In fact, many birds have ultraviolet plumage markings that play a role in mating selection.
But the mantis shrimp outstrips all the rest, with the most complex known visual system. Below is a comparison of human eyesight and the mantis shrimp’s eyesight. While humans only have three cones, the stalked eyes of the mantis shrimp have sixteen—and five of these cones are sensitive to ultraviolet light.
However, despite having such a large number of cones, the shrimp can’t process the colours like humans can as it doesn’t have the advantage of a large brain. It’s actually pretty lousy at distinguishing similar colours—it can only distinguish colours about 15 nanometres apart in wavelength, and rather than using brainpower to do this, the shrimp seems to rely on telling which photoreceptor cell responds more strongly.
So while the mantis shrimp has more cones, humans are more equipped to process colour. The genes for photoreceptors are on the X chromosome, and because women have two X chromosomes while men only have one, men are more likely to be born with colour deficiencies.
Interestingly, studies suggest that two to three percent of women may be born tetrachromats, with an extra cone between red and green. Theoretically, this means that they should be able to see wavelengths beyond a typical human being’s eyesight (though not as far as ultraviolet) and should be able to distinguish between colours that others think are identical. Further study is needed to verify this, but it’s intriguing to think that even among humans, colour vision—and therefore our experience of the world—could vary greatly.
While a small percentage of the population might have “extra-human” vision, a significant percentage still have a decreased ability to see colour because they have missing or damaged cones. But ongoing research is focusing on artificially replacing these cones in order to restore colour vision—one study successfully bred dichromat mice into trichromats, improving their range of colour vision by giving them another photoreceptor cone.
But mice are still a far leap from human vision, and so further research conducted in 2009 by the Washington National Primate Research Center worked with dichromat squirrel monkeys, who were born red-green colourblind. The brains and eyes of these monkeys are far closer to humans’ than mice, at least in terms of the mechanics of colour vision. In order to test their vision, the monkeys were taught to tap coloured dots on a screen, and when they tapped the correct dots, they were rewarded with grape juice. Since these particular monkeys were born red-green colourblind, they were initially unable to identify any red dots.
But by using gene therapy by injecting the missing photopigment gene into the monkeys’ retinas, the researchers demonstrated that the cones of the treated monkeys began to express new photopigments. They were eventually able to see a range of colours that they hadn’t previously been able to, and soon were successfully able to discriminate blue-green from red-violet, as shown when they were finally able to pick out the red dots in their colour vision tests. Thus, they became trichromats.
All the circuitry required for using a third cone is already present in dichromatic humans. Since the therapy proved effective in primates, and human genes were actually used in the research, there is definite potential to treat human colour-blindness, even in adults.
Thanks to Newton and his countless successors, we understand the mechanics of how the human eye detects and processes the spectrum of visible light. We know how crucial colour is to the way humans perceive their environments—and now we are taking steps towards improving our own vision via a working cure for colour-blindness, and thereby improving the way we see the world.