Colorblindness: Through their Eyes

A palette of colors for colorblindness, or Color Vision Deficiency (CVD)

It’s estimated that 1 in 1,000 people in the United States suffer from some type of color blindness. Color blindness or color deficiency refers to the condition where some colors or all colors cannot be discerned. Let’s look at color blindness in more detail.

Background

There are photoreceptor cells in the human retina known as rod and cone cells that help people to detect different types of colors. Rod cells are active in low light, making it possible to see to some extent even in darkness. On the other hand, the three types of cone cells containing different pigments are active in normal lighting conditions to help people perceive different types of colors. These three different kinds of cone cells have different spectral sensitivities. They are classified as S cones, M cones, and L cones denoting their respective sensitivities towards short, medium, and long wavelengths of light. These cells are also known as blue cones, green cones, and red cones respectively based on their maximum sensitivity towards a particular region of the spectrum. However, the color based terminology is not considered to be accurate because the red cones exhibit maximum sensitivity in the yellow region of the spectrum.

The chromosome factor also contributes significantly in color blindness. As most of the color receptor cones are concentrated in the X chromosome, males suffer more from this visionary disorder than females due to the presence of a single X chromosome compared to two X chromosomes in females. As a result of this genetic trait, about 1 percent of females have a fourth type of cone known as tetrachromats but the function of this fourth cone in color discrimination is yet to be determined.

Classification

Based on Cause:

Color blindness can be genetic or acquired.

Congenital or genetic color blindness can be classified under three categories:

  • Monochromacy (Total Colorblindness): The absence of more than one type of cone cells or any defect in the cones leads to monochromacy and color vision is reduced to a single dimension. This may be categorized as rod monochromacy or cone monochromacy, which is a rare occurrence.
  • Dichromacy: It is a moderate type of color blindness where one of the cone cells is either absent or not functioning properly. This can be further classified under Protanopia, Deuteranopia, and Tritanopia.
  • Anomalous Trichromacy: It is the most commonly observed genetic color blindness in which one of the pigments of the cones exhibit different type of spectral sensitivity than the normal range. This leads to the inability in color discrimination deviating from the normal three dimensional visions. Protanomaly, Deuteranomaly, and Tritanomaly are the three sub-categories under Anomalous Trichromacy.

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Based on Clinical Appearance:

Color blindness based on clinical appearance can be of the following types:

  • Total Color blindness: This is a rare occurrence.
  • Partial Color blindness: This is a common occurrence.

Some people experience difficulty in discriminating between green and red or between yellow and blue.

Red-Green:

  • Anomalous Trichromacy (Protanomaly and Deuteranomaly)
  • Dichromacy (Protanopia and Deuteranopia)

Blue-Yellow:

  • Dichromacy (Tritanopia)
  • Anomalous Trichromacy (Tritanomaly)

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Causes of Colorblindness

Color blindness can be caused by a number of factors.

Evolution: A single recessive character of the gene which occurs at a very high level of about 5 percent may cause color blindness.

Genetics: Genetics is another important cause of colorblindness. Any mutation of the X chromosomes leads to this genetic disorder. A number of contributing mutations from 19 different chromosomes and 56 types of genes can give rise to color blindness.

Inherited color blindness can be due to the following factors:

  • Cone Dystrophy
  • Rod-Cone Dystrophy
  • Blue Cone Monochromatism
  • Achromatopsia
  • Retinitis Pigmentosa
  • Leber’s Congenital Amaurosis

Genetic color blindness can be congenital or it can be acquired gradually during childhood and adulthood. Depending on the mutation of the genes, this disorder can be static or it can progress in advanced stages of life causing retinal degeneration. Color blindness can also affect other parts of the eye causing total blindness. Researches reveal that color blindness affects 8 percent of males and only 0.5 percent of females. The reason is that males have XY chromosome combination while females have XX chromosome combination. The extra X chromosome in females suppresses the mutative variants of X chromosome while the mutant variants are exhibited in males through color blindness.

Other Causes:

Color blindness can also arise from damage in the brain and retina due to shaken baby syndrome, swelling or inflammation of the occipital lobe of the brain from any shock or trauma. Retinal degeneration during childhood caused by ultraviolet light is the prevalent cause of color blindness across the globe. Color blindness can also be caused by aging, leading to macular degeneration or prolonged suffering from diabetes.

Diagnosis of Colorblindness

Color blindness is mostly diagnosed through the Isihara Color Test. In this test, the patient has to identify a number of pictures and colored spots. The test is particularly useful in diagnosing red-green deficiency. A figure, in the form of Arabic digit is carved out in colored spots which remains embedded in a sheet of spots of different color. A person with normal color vision can easily make out the color difference of the sheet and the figure. A number of figures and color combinations are used to diagnose the specific type of color blindness. Children, not acquainted with numerals will not be able to take the test. In such cases, geometrical figures like circles or squares are used.

Apart from this color test, the Farnsworth Lantern Test is also carried in the US naval and armed forces. This test is more successful in differentiating the severity of color blindness among individuals. Clinical tests for diagnosing color blindness are quite simple and quick. These tests are also effective in distinguishing the major categories of this disorder. More flexible clinical tests are coming up to distinguish different types of color blindness with more accuracy.

Misconceptions and Compensations

  • Color blindness should not be misinterpreted as color swapping which is a condition whereby the sufferer can never see green as red or vice versa. A dichromat may often find it hard to distinguish between red and green colored things.
  • A dichromat views the color of objects in a faded manner.
  • Dichromats have the advantage of distinguishing the camouflaged colors where persons with normal vision fail.
  • For color blind people, traffic lights are modified according to their convenience to follow the traffic signals.
  • Color blindness does not always mean monochromatism, barring few cases.
  • Color blind people in most cases can discriminate blue-yellow colors.
  • Most color blind people are anomalous trichromats rather than dichromats.

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