Above: Image © istockphoto.com/PashaIgnatov

Did you know? The genes that make cones, the eye cells that allow you to see colour, are on the X chromosome. As a result, men are more affected by colour-blindness than women.How many different colours can you name off the top of your head? Ten? Twenty? Fifty? I’m willing to bet that no matter how many colours you listed, it’s not even close to the number of colours your eyes are capable of seeing.

Scientists estimate that the average, healthy human can distinguish between well over a million different shades. However, not all men (or women) are created equal in this regard. Some people can only see only distinguish between a few hundred different shades, while others can see billions of colours.

A rainbow of colours as seen by a person with no coloour-blindness.

The same rainbow of colours as seen by a person with protanopia.

The same rainbow of colours as seen by a person with deuteranopia.

The same rainbow of colours as seen by a person with tritanopia.

The retinas at the back of your eyes are home to two types of photoreceptor cells that convert light into signals that are sent to the brain, allowing you to see. These specialized cells are called rods and cones. You have 20 times more rods than cones, and rods allow you to see in low light. However, cones are 100% responsible for colour vision.

Did you know? Compared to less than 1% of women, 8% of men are colour-blind!There are three types of cones that correspond to different wavelengths of light: long wavelengths stimulate red cones, short wavelengths stimulate blue cones, and medium wavelengths stimulate green cones. By activating different combinations of cones, you can see the world in colour. Colour-blindness occurs when one type of cone is completely missing or simply doesn’t work.

Naturally, there are three types of colour-blindness, depending on which type of cone is missing or out-of-order. The loss of red cones is called protanopia and the loss of green cones is called deuteranopia. Both of these conditions are commonly referred to as “red-green” colour-blindness. They make it very difficult to distinguish between shades of red, yellow, orange, and green.

Did you know? People with typical vision are called “trichromats”, because their eyes have three types of functional cone cells. Colour-blind people with one defective and two functioning types of cones are called “dichromats”.“Blue-yellow” colour-blindness, or tritanopia, is very rare condition that results from the loss of blue cones. People with this condition have difficulty distinguishing blue from green and yellow from purple.

Another rare form of colour-blindness is called achromatopsia. Incomplete achromatopsia involves the loss of two out of the three cone types. Since the brain needs to compare signals from at least two different cones to properly identify colours, people with this condition have severely limited colour vision. Complete achromatopsia is the loss of all three cone types. People with complete achromatopsia see the world entirely in shades of grey.

Most types of colour-blindness are the result of a genetic mutation that prevents the body from making certain cone cells, or causes it to make cone cells that simply don’t work. Some mutations may cause cone cells to partially work, leading to a milder form of colour-blindness. Colour-blindness can also be the result of brain damage, chronic illness, or taking certain medications.

Did you know? Most mammals, including dogs and most New World monkeys, are dichromatic, like colour-blind humans.On the other end of the spectrum, researchers recently discovered that a small number of people actually have four types of cones in their retinas! This probably makes them capable of distinguishing between up to a billion different shades, including colours that the average person can’t even imagine. However, the prize for superior colour vision has to go to an animal called the mantis shrimp, which has sixteen different types of photoreceptor!

References

General information

Acquired Colour Vision Defects (Colour Blind Awareness) All about Color Blindness (Colblindor) Color vision deficiency (Henetics Home Reference, US National Library of Medicine) Colorblindness and Achromatopsia (Achromatopsia.info, Low Vision Centres of Indiana) Humans with Super Human Vision (Veronique Greenwood, Discover Magazine) Mantis Shrimp Vision Preview (Michael Bok, Arthropoda) Photoreceptors (The brain from top to bottom, Bruno Dubuc) What is Color-Blindness? (J.L. Morton, Color Matters) What Is Colorblindness and the Different Types? (Terrace L. Waggoner, TestingColorVision.com) What Is Deuteranopia? (wiseGEEK)

Scholarly publications

Jacobs GH, et al. 1996. Trichromatic colour vision in New World monkeys. Nature. 382(6587):156-158. Neitz J, Neitz M. 2011. The genetics of normal and defective color vision. Vision Research. 51(7):633-651.

Kristen Chafe

I'm a neuroscience graduate student at Dal and I love talking about brains! 

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