Posted in science

What causes colour-blindness?

Colour-blindness is a relatively common human condition – 8% of men are born colour-blind, as are 4.5% of women. This discrepancy between the genders is due to the gene linked to colour-blindness being on the X chromosome (i.e. women inherit two copies so both of the inherited genes must be faulty for them to be impacted). That being said, although the majority of cases of colour-blindness are genetic, a percentage of people can develop colour-blindness later in life. Colour vision tends to decline past the age of 60, and ill-managed chronic conditions such as diabetes, strokes, chronic alcoholism and Parkinson’s disease can all lead to eventual colour-blindness.

So what exactly causes people to be unable to see colour? It’s all linked to the many complex structures within the eye. There are two main light-sensitive cells within the human eye, both found in the retina. These cells are rods and cones. Both are found in vast numbers – in each individual retina there is in excess of 90 million rods and between 6 to 7 million cones. Despite their apparent similarities, rods and cones have very different and distinct roles when it comes to detecting light and processing images.

Rods are able to function in less light and are more sensitive than cones, and therefore they are mainly used for night vision. They are capable of responding to a single photon of light, making them around 100 times more sensitive than cones. However, they only contain one type of colour pigment. Rods are concentrated around the outer edges of the retina, and are used in peripheral vision during the daylight. However, as there are so many rod cells in the retina, multiple cells tend to share a neuron in order to amplify their signals. This makes the resolution of images detected by rods much less clear.

Despite there being many, many fewer cones than rods, they are responsible for much of the clarity of colour we experience. The exact opposite to rods, they function best in bright light. Three different varieties of cones are found in the retina of typical humans, L-cones, M-cones and S-cones. This means that as a species we are said to have ‘trichromatic vision’. Each different type of cone detects a different wavelength of light, and these wavelengths combine to give the large spectrum of visible colour seen by the majority of people. This range varies from species to species – amazingly, it has been reported that some types of snake can observe some wavelengths in the infrared spectrum.

There are three main pigments in the eye that allow us to observe colour, and – bizarrely enough – they vary from person to person due to genetic mutation and other factors. This means that everyone sees colour differently.

This intricate network of different receptors and pigments within the eye is very fragile, making it susceptible to damage and mutation. If a combination of rods and/or cones are faulty, then not all wavelengths of light can be absorbed. This leads to some colours appearing differently to the affected person, or perhaps not appearing at all. The most common form of colour-blindness is ‘red-green colour-blindness’. This is an umbrella term for people with either deuteranomaly or protanomaly. This is caused by one type of cone being slightly faulty, which causes the spectrum of visible colour to shift, rendering some colours slightly out-of-whack.

However, my parting thought is this: as no-one sees colour in exactly the same way, who’s to say who’s version of colour is correct?

Tabitha Watson

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