Colour blindness is actually an inaccurate term for colour perception deficiency. It is a lack of perceptual sensitivity to certain colours, and is most commonly due to an inherited condition. The most common form by far is congenital anomalous trichromacy, which, simply put, is an inherited condition in which there is partial or complete inability to perceive a wavelength of colour. This form of colour perception challenge affects more men than women since it is encoded on the X chromosome and is often passed from mother to son.
The human eye has been always a subject of wonder, inspiring scientists and artists alike. Eyes are both beautiful and unique, with the retina in each person’s eye being just as unique as a fingerprint. The most striking feature of eyes though is their colour, occurring in multiple colours and shades.
Eye colours around the world
The most common eye colour is brown, occurring in most of the world’s population. Notably, in Africa and Asia, eye colour that isn’t brown is very rare. Blue eyes are the next most popular and are more typical to northern Europe. Estonia, Finland and Ireland have the largest percentage of blue-eyed people in their respective populations.
Green eyes are generally regarded to be the most attractive but they also happen to be the rarest. They are more common to countries in north and central Europe but can also be found in western Asia. Hazel eyes are similar to brown eyes but are more striking because of how their colour varies from copper to green depending on the light and angle.
Other eye colours such as silver, grey, amber and gold are generally extremely rare and are mostly variations of brown, blue, hazel and green. There are also anomalous eye colours such as red eyes and pink eyes, which are almost entirely associated with albinism.
Why are eye colours different?
Traditionally it was thought that brown eyes are the expression of a dominant gene while blue eyes are the expression of a recessive gene, but it is now established that the genetics of eye colouration is far more complex than that because multiple genes are involved.
Eye colour is a result of how the eye interacts with light and there is no pigmentation other than brown. The iris has two layers, the first being the stroma, which is clear and colourless, and beneath it, pigmented epithelial cells. It is how these two layers interact with light that causes the difference in eye colour.
The epithelium of the eye contains cells which are pigmented by melanin, the same pigment that affects hair and skin colour. The stroma is generally clear but can have deposits of pigmentation. The quantity of pigmentation and the structure of the stroma affect how light is scattered as it is reflected from the iris and subsequently, it affects the perceived colour of the eye.
Blue eyes result from a stroma that has very little melanin in it. The longer wavelengths of light are absorbed by the epithelium while the shorter wavelengths are scattered in the stroma. Blue light is scattered the most and as a result, the eye looks blue.
Brown eyes, in contrast, are the result of the stroma being rich in melanin and both short and long wavelengths of light are absorbed. Newborns of caucasian descent often have no melanin in their eyes so they appear blue but may shift to brown as they grow older and the melanin in their eyes increases.
Anomalous Eye Colours
There are conditions that result in eye colour that is different from the normal spectrum. The most common of these is ocular albinism, which is the absence of melanin. In its most extreme forms, there is absolutely no melanin at all in both the stroma and the epithelium of the iris. This gives the eye a pink or red appearance from haemoglobin in the capillaries of the iris. Another condition that affects eye colour is heterochromia, in which either one part of the iris is different from the rest or one iris is a completely different colour from the other.
Regardless of their colour, all eyes are beautiful. No matter your eye colour, it’s important to always protect your eyes when outdoors whether you need a prescription or not. Visit Vision Works for affordable eye tests and comprehensive eye care to keep your eyes healthy.
Our eyes are incredibly fascinating and complex. There is always something new to learn about them and in this article, we bring you some amazing facts about them.
- Ultra Speed: The average blink lasts for about 1/10th of a second, making your eye muscles the fastest in your body.
- Ever Vigilant: Before any kind of activity or even just after waking up, it takes some time for most parts of your body to warm up to their full potential, but your eyes are on their “A game” 24/7.
- Super Healing Ability: Eyes heal incredibly fast. With proper care, it only takes about 48 hours for the eye to repair a corneal scratch.
- High Brain Power: Seeing is so crucial to how we function that it engages and involves up to 50% of the brain.
- I cry the Dry Cry: Newborns don’t produce tears. They make crying sounds, but the tears don’t start flowing until they are about 4-13 weeks old.
- Sightless: Around the world, about 39 million people are blind and roughly 6 times that many have some kind of vision impairment.
- Mission Impossible: Doctors have yet to find a way to transplant an eyeball. The optic nerve that connects the eye to the brain is too sensitive to reconstruct successfully.
- Team Efforts: The cells in your eye come in different shapes. Rod-shaped cells allow you to see shapes, and cone-shaped cells allow you to see colour.
- Bat Those Eyelids: You blink about 12 times every minute.
- Size Matters: Your eyes are about 1 inch across and weigh about 0.25 ounce.
- Pretty Colours: Some people are born with two different coloured eyes. This condition is heterochromia.
- Behind the scenes: Even if no one in the past few generations of your family had blue or green eyes, these recessive traits can still appear in later generations.
- Blind Spot: Each of your eyes has a small blind spot in the back of the retina where the optic nerve attaches. You don’t notice the hole in your vision because your eyes work together to fill in each other’s blind spot.
- Hyperactive: Out of all the muscles in your body, the muscles that control your eyes are the most active.
- Easy to fix: 80% of vision problems worldwide are avoidable or even curable.
Eyes are incredibly fascinating in how elegantly they function. Remember to take care of your eyes and keep them working at their absolute best. Book an eye test today and get a good start on making sure your eyes stay amazing.
How many people are affected by colour blindness?
Roughly 8% of all men and about 0.5% of all women are colour perception deficient. Colour blindness is commonly the impaired ability or complete inability to perceive one colour, and 99% of all colour blind people can see colour to some degree. Achromatopsia, the complete absence of colour vision, is extremely rare, occurring in only one out of every 33 000 people. About half the people with colour perception deficiency have mild perceptual challenges while the other 50% will have moderate to severe challenges.
What causes congenital anomalous trichromacy?
Normal colour vision uses all three types of light cones correctly and is known as trichromacy. People with normal colour vision are known as trichromats. The lack of perceptual sensitivity to certain colours is an anomaly. The different anomalous conditions are protanomaly, is a reduced sensitivity to red light; deuteranomaly, a reduced sensitivity to green light and is the most common form of colour perception deficiency; and tritanomaly, a reduced sensitivity to blue light and is extremely rare. Reduced or absent ability to perceive either red or green light results in very similar visual problems so they are often put together and known as red-green colour blindness.
Colour perception deficiency affects the retina, which is the light-sensitive tissue at the back of the eye. The retina contains two types of light receptor cells, called rods and cones. These cells transmit visual signals from the eye to the brain. Rods provide vision in low light (night vision) while cones provide vision in bright light (daylight vision), including colour vision. There are 3 types of cones, and each is sensitive to the red, green and blue wavelengths of light respectively. The brain determines what colour it is seeing by observing the ratio between the signals it receives from each of the three types of cones. For instance, when the blue cones and the red cones pick up light, the brain then interprets it as purple.
In anomalous colour perception, one or more types of cones in the retina are either partially or completely non-receptive to their particular wavelength of colour and this then affects how the brain interprets colour. For instance, red-green colour blindness (the most common) is caused by the partial or complete inability of either red cones or green cones to perceive their specific colour.
What is the daily life of a colour challenged person like?
Colour perception deficiencies bother affected children from their earliest years. At school, colouring can become a difficulty when one has to take the blue crayon – and not the pink one – to colour in the ocean. When cooking, red deficient individuals battle to tell whether their piece of meat is raw or well done. Neither can they tell the difference between green and ripe tomatoes or between ketchup and chocolate syrup. Some food can even appear very unappetising to them. For example, people with a green deficiency cannot possibly eat spinach as to them it may look like a cow pat! They can however, distinguish some citrus fruits. Oranges seem to be a brighter yellow than that of lemons.
To these individuals, purple looks like a dark blue, almost black and red is a very dark grey also appearing as almost black. Some of them cannot fully appreciate a rainbow because they can only see two light colours, blue and yellow. When driving, they can only distinguish traffic lights from street lights in terms of brightness. They cannot tell whether a woman is wearing lipstick or not. When at the beach with a redheaded woman, whose skin cannot be exposed to the sun for a long time, a colour deficient person may not be able to warn her if she is getting sun burnt. However, they distinguish faded and grey colours far more easily, which may even appear attractive to them.
It is a great idea for everyone to get tested for colour perception deficiency, get in touch with us today to book your test.