Colour Blindness

coloured_irisContrary to its name, colour blindness does not refer to actual blindness. Rather it is a condition in which the ability to properly see colours, or differentiate between them, is either diminished or completely lacking. In rare cases it can be caused by injury or specific drugs, but in most cases its origins are purely genetic, and affects the development of one or more sets of retinal cones that perceive colour in light.

Colour blindness affects males much more than females, and is found in 8% and 1% of each, respectively. While it is typically considered to be a mild disability, there are some circumstances where some types of colour blindness can be advantageous. Due to the way that colours are processed differently, certain types of camouflages, which are intended to blend in with their surroundings, may appear to stand out. Some colour blind people can differentiate between two similar colours that most people perceive as two samples of the same colour.

Types of Colour Blindness

In some rare situations, colour blindness can be caused by an injury to the eye, optic nerve, or brain, affecting its ability to process colours properly. Some diseases, drugs, or chemicals can also trigger the condition, but are incredibly uncommon. A vast majority of colour blind people can attribute their condition to genetics. Depending on how the development of their eyes and brains were effected, the details and parameters of their colour blindness may be different. In fact, there are quite a few different variations of colour blindness, with details on them listed below.

  • Monochromacy – Often referred to as “total colour blindness", monochromacy is inability to distinguish colours at all. Anyone with this type of colour blindness views the world in black and white, like old television and movies. It is caused when two or all three of the cone pigments within the eye are missing or defective, and colour and lightness vision is reduced to a single dimension. Under monochromacy, there are two sub types.
    • Rod Monochromacy – Also called achromatopsia, this form is an exceptionally rare disorder. Those that have this form of colour blindness are totally unable to distinguish any colours as a result of either absent or non-functioning retinal cones. Issues commonly associated with this form are light sensitivity, involuntary eye oscillations, and poor vision.
    • Cone Monochromacy - A rare total colour blindness that usually doesn't affect visual acuity at all, unlike rod monochromacy. This form, however, can be the result of having multiple forms of dichromatic colour blindness simultaneously. For instance, someone with both protanopia and tritanopia can be considered to have cone monochromacy.
  • Dichromacy - A relatively severe form of colour blindness in which one of the three basic colour mechanisms is either absent or non-functional. Because it is hereditary in nature, and effects the X chromosome, is it found predominantly in males. There are three sub categories of dichromacy, protanopia, deuteranopia, and tritanopia, each occurring when one of the cone pigments is missing and colour is reduced to two dimensions.
    • Protanopia - In reference to the three primary colours, and stemming from the Greek “prot”, meaning “first”, this form of dichromacy is caused by the complete absence of red retinal photoreceptors. People with this form of colour blindness have difficulty distinguishing between blues and greens, as well as between reds and greens. Completely pure red cannot be seen at all, and appears black. Purples and blues cannot be distinguished from one another, and orange-reds appear like very pale or dim yellows. It is hereditary, and present in 1% of males only.
    • Deuteranopia - “Dueter”, Greek for “second”, refers to the next primary colour, green. This form of colour blindness is caused by the absence of the green photoreceptors within the eye. It affects differentiating between colours much in the same way as protanopia, but without the dimming effect. Like protanopia, it is only found in about 1% of the male population, not in females.
    • Tritanopia - “Trit, Greek for “third”, refers to blue, and the complete lack of colour receptors responsible for detecting it. This form of colour blindness is very rare, and involved the presence of only two cone pigments, red and green. For people with this form, blues appear greenish, yellows and oranges appear pinkish, and purple colours appear deep red. It is hereditary, but unlike protanopia and deuteranopia, tritanopia is not sex-linked, meaning it appears in both males and females.
  • Anomalous Trichromacy – A more common type of colour blindness, it is hereditary in nature, and involved one of the three cone pigments being less sensitive, rather than completely absent or damaged. The symptoms are very similar to their dichromatic counterparts, only less severe.
  • Protanomaly - A mild form of colour blindness in which the sensitivity of red retinal receptors is diminished, but not absent. The result is poor discrimination between reds and greens. It is hereditary, sex-linked, and present in 1% of males.
  • Deuteranomaly, - By far the most common type of colour vision deficiency, this form of colour blindness mildly affects discrimination of reds and greens. It is found in approximately 5% of European males, but not females.
  • Tritanomaly - A very rare form of colour blindness that mildly affects discrimination of blues and greens, as well as yellows and reds. Like all the other forms, it is hereditary, but can be found in both males and females alike.

What Causes Colour Blindness?

In a vast majority of instances of colour blindness, the cause is genetic, with the condition being passed down from parent to child. In fact, there are many know gene mutations that are capable of causing colour blindness, at least 19 different chromosomes and 56 individual genes. Because of the nature of these anomalies, which are typically linked to the X chromosome, they affect men far more than women. If a mutated X chromosome is inherited by a woman, a second, normal X chromosome can override it, resulting in normal vision. Men, however, only have one X chromosome, and will be affected should they inherit a mutated gene.

Hereditary forms of colour blindness can be present at birth, or may develop sometime during childhood, adolescence, or even adulthood. Different genetic mutations have different ways of affecting vision, not just in terms of colour identification, but in the progression of the condition. Some forms will stay consistent throughout life, while others may worsen over time.

In far fewer instances, colour blindness can be caused by injury, or another form of trauma. Brain or retinal damage can certainly affect vision, as well as any type of injury that causes swelling of the brain in the occipital lobe. Prolonged exposure to UV light can also lead to colour blindness. Not all of these injuries will result in colour blindness immediately, as the damage may not present itself until later on in life. It can even be caused by a serious deficiency in vitamin A.

How Is Colour Blindness Diagnosed?

ishihara_colour_vision_test

The post popular test to detect red-green colour blindness is the Ishihara colour test, which consists of a series of images comprised of coloured spots. A figure, such as a letter or number, is hidden in the image, composed of one colour of spots, while being completely surrounded with an alternate colour. With normal visual abilities, the image will appear to stand out, and can be identified easily. However, with a condition of colour blindness, the image may not be perceived at all. Different colours and contrasts are used to isolate different forms of colour blindness. One of the disadvantages of this test is that it relies on the subject’s ability to identify numbers or letters, which means it isn't useful for use with small children who have not yet learned those symbols.

Other variations of this test exist, as well. Several branches of the United States military use the Farnsworth Lantern test, which allows anyone with mild colour blindness, approximately 30% of all colour blind people who attempt, to pass.

The Farnsworth-Munsell 100 hue test involves arranging a set of coloured object in order, which is based on a transitional colour gradient. Two distinct colours are placed at either end, and the subject is left to arrange the other items in an ordered fashion between them. Those with difficulty distinguishing between some colours will unlikely be able to complete the test accurately.

How Is Colour Blindness Treated?

Unfortunately, there is currently no cure for colour blindness. There are lenses available, both contact and eyeglass, that will help differentiate between colours that might otherwise appear very similar or even identical, but they can not help the viewer perceive the true and accurate colour. In some cases, the coloured lenses might make it easier to distinguish between some colours, but harder for others.

Modern technology is providing some solutions, however. Often referred to as “colour blind mode” computers, tablets, and cell phones can adjust the colours they display to those more easily recognized by the user. While the image on the screen may not be an accurate representation of the original work, it can be tailored to individual types of colour blindness, making the device more usable and less confusing.

Research is currently being performed on squirrel monkeys, which naturally only have dichromatic vision. By using gene therapy, they have been able to alter the monkey's eyes, enabling trichromatic vision, like humans. There is still much more research and testing needed, but this shows promise for possible future treatments of colour blindness in humans.