Colour Blindness
Image Source: Unsplash
INTRODUCTION
Colour Blindness, also known as Colour vision deficiency (CVD), decreases one’s ability to distinguish between specific colours. It affects around 300 million people in the world - approximately 1 in 12 men (8%) and 1 in 200 women [1]. There are several different types of colour blindness: Red-green colour blindness (Deuteranomaly, Protanomaly, Protanopia and Deuteranopia), Blue-yellow colour blindness (Tritanomaly, and Tritanopia) and Complete colour blindness, also known as Monochromacy (achromatopsia) [2] [3]. In this article, we will delve into what causes colour blindness, how to detect it, and how colour blind glasses work.
WHAT CAUSES COLOUR BLINDNESS
Colour vision deficiency is typically an inherited genetic disorder, making it a congenital medical condition; however, there are also other causes to CVD that can occur later in life, which includes eye diseases such as glaucoma or macular degeneration, brain and nervous system diseases like Alzheimer’s and multiple sclerosis, as well as the effect of certain medications like Plaquenil, used to treat rheumatoid arthritis [4].
Colour blindness is usually caused by genetic mutations. The ability to see colours (trichromatism) depends on three genes: OPN1LW, OPN1MW and OPN1SW. These genes code for proteins that are essential in the functioning of the receptor cells in your retina, known as cones and rods. Cones can detect colours under bright light, and the rods are useful for night vision, necessary for vision under dim light conditions [5]. The protein coded for by these respective genes are called opsins, and each cone has a specific opsin that is sensitive to particular wavelengths of light. The opsin made from the OPN1LW gene is sensitive to yellow-orange light, and cones that contain this pigment are called L cones as they are sensitive to long wavelengths of light. The opsin made from the OPN1MW gene is sensitive to yellow-green light, which is near the middle region of the visible spectrum, hence cones that contain this pigment are called M cones. As for the opsin made from the OPN1SW gene, it is sensitive to blue-violet light; therefore, cones that contain this opsin are called short-wavelength-sensitive or S cones (See Fig. 1) [6].
Fig. 1 A graph showing the stimulation level of the L, M and S cones when they are exposed to different wavelengths of light.
Image taken from https://ixora.io/projects/colorblindness/color-blindness-simulation-research/
All three of these genes are located in the X chromosome - which is the reason behind why red-green colour blindness is more common in men; it is a sex-linked condition. Males only need to inherit one X chromosome that contains the mutations in these genes which characterises colour-blindness (always passed down from their mother) to be affected by the condition; whereas, females have two X chromosomes, so they must inherit two X chromosomes that have the mutated genes in order to be affected [4]. Note that tritanopia and tritanomaly (Blue-yellow colour blindness) are not sex-linked and is related to chromosome 7; therefore, it can be acquired rather than inherited (More on this in Types of Colour Blindness) [8].
If mutations are present in the OPN1LW gene (codes for the red pigment cone) or the OPN1MW gene (green pigment cone), it results in red-green colour vision defects. If there are mutations present in the OPN1SW gene, defective S cones are made and some S cones may prematurely degrade. Without functional S cones, a person will find it difficult to detect differences between shades of blue and green, as well as finding it impossible to distinguish between blue and black [6].
People who are not affected by colour blindness have trichromatic vision. There are individuals who have anomalous trichromacy, meaning that they too also have trichromatic vision but one of their three cone pigments may have altered sensitivity to specific wavelengths of light. Dichromacy is when an individual only has two types of functioning photoreceptors. In very rare cases, there are people affected by monochromacy. In the world, about 1 in every 30,000 people have monochromatic vision. As the prefix suggests, it refers to the ability of only being able to see things in shades of gray - as a result of two or none of the cones working properly. This inability to see colour is one of the symptoms for diseases such as achromatopsia or inherited blue cone monochromacy [7].
TYPES OF COLOUR BLINDNESS
We will now explore the different forms of colour blindness: Red-green colour blindness, blue-yellow colour blindness and complete colour blindness. For each form, vision may be affected due to dichromacy, where one of the cones is completely absent or non-functional; anomalous trichromacy, the most common type of inherited CVD, where one of the the three cone pigments is altered in its spectral sensitivity [8].
Red-green colour blindness is the most common type, and individuals who are affected by this condition find it hard to tell the difference between red and green. This can be due to the anomalous trichromacy conditions of Deuteranomaly and Protanomaly. Deuteranomaly is the most prevalent form of red-green colour blindness and it makes green look more red. Its effect is mild and doesn’t interfere with a person’s daily life. Green looks more red because of an altered spectral sensitivity of the green retinal receptors, which results in an overlap between the sensitivities of the M and L cones; therefore, reducing the number of shades that can be detected (See Fig. 2). Protanomaly is similar to Deuteranomaly, but instead the L cone is shifted towards shorter wavelengths (to the left), making red look more green and less bright [8] [9].
Fig. 2 Showing the overlap in spectral sensitivities of the M and L cones. This specific diagram depicts the shift of the sensitivity of the M cone towards longer wavelengths - which causes Deuteranomaly.
Image taken from https://enchroma.com/pages/what-is-color-blindness [9]
Protanopia and Deuteranopia are dichromatic forms of red-green colour blindness. Protanopia affects 1% of males, and this condition is characterised by the absence of red retinal photoreceptors (L-cones) - making it hard to distinguish between blue and green, as well as between red and green. People who are affected by this condition can only perceive light wavelengths from 400nm to 650nm, instead of the normal 700nm; resulting in red to appear black. Moreover, purple can’t be detected amongst shades of blue; orange-tinted reds appear as dim yellows; orange-yellow-green shades appear as a similar yellow hue [8]. Deuteranopia is also found in 1% of males, and is similar to Protanopia, but instead of missing the red cones, the green cones (M-cones) are absent [8] [10].
Blue-yellow colour blindness is less common and people affected by it find it hard to distinguish between blue and green, and between yellow and red. The dichromatic form of blue-yellow colour blindness is Tritanopia. It is a very rare condition characterised by the absence of blue cone pigments (S-cones) [3]. This causes blue to appear greenish, yellows and oranges to look more pink, and shades of purple to appear deep red [8]. The colours also become less vivid [3]. Tritanomaly is the anomalous trichromatic form of blue-yellow colour blindness, where the S-cone is malfunctioning, making shades of blue and green, yellow-red and pink indistinguishable. What makes tritanopia and tritanomaly different to red-green colour blindness is that it is not sex-linked, it is caused by mutations in chromosome 7; making it acquirable rather than inheritable [8].
For a comparison between normal vision and dichromacy forms of colour blindness, see Fig. 3.
Fig. 3 Comparison of the apparent colours of red, yellow, green and blue by four observers. One unaffected by CVD (trichromatism), and the other three affected by CVD: Protanopia, Deuteranopia and tritanopia respectively (down the image)
Image Credit: Wikimedia Commons
In regards to monochromacy, there are two types: Rod monochromacy, also known as Achromatopsia, and cone monochromacy. Rod monochromacy is a very rare condition where there are no retinal cones or all of the retinal cones are non-functioning. This condition can cause photophobia (light sensitivity), involuntary eye oscillations (nystagmus) and poor vision [8]. On the other hand, cone monochromacy, is total colour blindness that is caused by having more than one type of dichromatic colour blindness; for instance, if one is affected by both protanopia and tritanopia, then they are affected by cone monochromacy. Those affected by cone monochromacy, unlike those with rod monochromacy, have relatively normal vision (See Fig. 4) [8].
Fig. 4 Comparison between normal vision (trichromatism) and cone monochromacy
Image taken from https://www.colour-blindness.com/variations/total/
DETECTION
How can we detect colour blindness? There are many tests available to measure colour vision defects but the most common one is the Ishihara test. It is used routinely and can detect those that are red-green colour blind but not blue-yellow colour blind. The test is made up of 38 circles that compose of dots in two or more colours. Some plates contain information (usually numbers) that people with normal vision (trichromatism) can see and others that contain information colour-blind people can see (See Fig. 5) [11]. To detect colour blindness in young children who haven’t learnt the numbers yet, shapes are used instead [12].
Fig. 5 Ishihara test
Image taken from https://www.webeyeclinic.com/color-blind/ishihara-test
COLOUR BLIND GLASSES
One of the most well-known colour blind glasses there are in the market are from a company called EnChroma.
EnChroma glasses were created for doctors to use as protection during laser surgery procedures. They were originally manufactured as sunglasses with lenses that had rare earth iron embedded in them which absorbed light, as well as other compounds that enhanced and strengthened the wavelengths of light, increasing the saturation and richness of the colours that one’s eyes are having trouble perceiving [13]. Take this scenario: 10 photons landed on the red cone and 100 landed on the green cone, for someone with normal vision, the object would look more green. If an equal number landed on both the red and green cones, yellow would be seen. However, for those who are red-green colour blind, there is a large overlap between the M and L cones; therefore, if an equal number of photons landed in both red and green cones, the person would perceive very little colour. What EnChroma’s glasses does is that its optical filters can block out certain wavelengths of light (using its band of absorption); correcting the ratios of the photons to the M and L cones and ‘push the cones away from each other’; in other words, the contrast between the red and green colour signals increases (See Fig. 6) [15] [16] [17].
However, the glasses don’t work for everyone who is colour blind. A study conducted in 2017 has shown that amongst 10 participants, there was significant colour improvement for only two of them. This is because the glasses won’t work for people with monochromatic vision as they don’t have any functional photoreceptors or are affected by cone monochromacy [14]. Therefore, they will only work for people who have mild or moderate colour blindness; most effective for those that have anomalous trichromatism [13].
Fig. 6 The coating of EnChroma sunglasses filters out specific wavelengths of light so that the ratio between red, green and blue is corrected, thus stimulating the S, M and L cones to the correct extents. This allows people with colour blindness to perceive colours they have never seen before.
Image Credit: Wikimedia Commons
Fig. 7 How EnChroma Glasses work
Image taken from https://abbeyeyecare.ca/enchroma-glasses/BIBLIOGRAPHY
[1] “Colour Blindness.” Colour Blind Awareness, www.colourblindawareness.org/colour-blindness/.
[2] “Types of Colour Blindness.” Colour Blind Awareness, www.colourblindawareness.org/colour-blindness/types-of-colour-blindness/.
[3] “Types of Color Blindness.” National Eye Institute, U.S. Department of Health and Human Services, www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/color-blindness/types-color-blindness.
[4] “Causes of Color Blindness.” National Eye Institute, U.S. Department of Health and Human Services, 26 June 2019, www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/color-blindness/causes-color-blindness.
[5] Person. “Deuteranopia: Red-Green Color Blindness.” Healthline, Healthline Media, 9 Nov. 2020, www.healthline.com/health/deuteranopia.
[6] “Color Vision Deficiency: MedlinePlus Genetics.” MedlinePlus, U.S. National Library of Medicine, 18 Aug. 2020, medlineplus.gov/genetics/condition/color-vision-deficiency/.
[7] “Monochromacy.” Wikipedia, Wikimedia Foundation, 3 Dec. 2020, en.wikipedia.org/wiki/Monochromacy.
[8] “Color Blindness.” Wikipedia, Wikimedia Foundation, 17 Dec. 2020, en.wikipedia.org/wiki/Color_blindness.
[9] “What Is Color Blindness.” EnChroma, enchroma.com/pages/what-is-color-blindness.
[10] “Deuteranopia – Red-Green Color Blindness.” Colblindor, www.color-blindness.com/deuteranopia-red-green-color-blindness/.
[11] “Diagnosis.” Colour Blind Awareness, www.colourblindawareness.org/colour-blindness/diagnosis/.
[12] “Color Blind Test for Kids.” EnChroma, enchroma.com/pages/kids-color-blind-test.
[13] Watson, Kathryn. “How Do EnChroma Glasses Work?” Healthline, Healthline Media, 13 Oct. 2018, www.healthline.com/health/how-do-enchroma-glasses-work.
[14] “How Do Color Blind Glasses Work?” Bard Optical, 19 May 2020, www.bardoptical.com/how-do-color-blind-glasses-work/.
[15] Zhou, Li. “A Scientist Accidentally Developed Sunglasses That Could Correct Color Blindness.” Smithsonian.com, Smithsonian Institution, 3 Mar. 2015, www.smithsonianmag.com/innovation/scientist-accidentally-developed-sunglasses-that-could-correct-color-blindness-180954456/.
[16] “How EnChroma Glasses Work.” EnChroma, enchroma.com/pages/how-enchroma-glasses-work.
[17] “This Is What Color Blind People See With These Viral Glasses.” YouTube, Tech Insider, 12 Mar. 2018, www.youtube.com/watch?v=1vWM2N3GRjE.
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