Whether caused by a tumor, a stroke, a blow to the head, or they were just born that way, people with achromatopsia have a limited or no ability to see color.

Each of the possible physical causes of the condition, such as a growing tumor or a brain hemorrhage, damage either the thalamus (a relay station in the brain for sensory signals) or the cerebral cortex, such that the pathways for discerning color are permanently disabled.

On the other hand, hereditary achromatopsia results from of one of four related genetic mutations, each of which prevents the eye from properly responding to light and color.

In the retina at the back of the eye, rods and cones live in greatly unequal proportions (120 million rods to 6 million cones). The rods are essentially all the same as they are merely sensitive to the absence or presence of light. Together with their sensitivity and great numbers, rods enable people to see even in low light conditions; however, they are no help when it comes to seeing colors.

That’s where cones come in. Divided into three groups, one each for perceiving green, blue and red, the cones discern color and are better at seeing detail.

Whether it is a rod or a cone, photoreceptor cells have a charge. The area surrounding the cell has higher levels of positively charged sodium ions (Na+) than the inside of the cells. When it’s dark, the cell’s membrane is permeable, and, seeking equilibrium, the Na+ ions move into the photoreceptors, causing the cell to become more positive than it otherwise would. When light strikes the cell, the cell membrane’s permeability decreases, preventing Na+ from entering it, and allowing the cell to become more negative, or hyperpolarize, and thereby activate.

The cones in people with congenital achromatopsia are unable to properly hyperpolarize, and therefore, signals that would otherwise convey color aren’t transmitted. There are four chromosomes that have been identified as possible culprits for the condition: chromosomes 14, 8q21-q22, 2q11 and 10q24.

In addition to being color blind, those with hereditary achromatopsia also have decreased vision overall. The only photoreceptor cells in the fovea, the central region of the retina from where the clearest vision comes, when cones aren’t functioning properly, achromats also suffer from reduced visual acuity, particularly in daylight (with vision typically no better than 20/200).

Likewise, achromats are also often sensitive to light.

Hereditary achromatopsia affects about one in 40,000 people, although it is found in greater proportions in societies that encourage consanguineous marriages. For example, on the island of Pingelap in Micronesia, after a typhoon and famine reduced the population to only 20 survivors in 1775, one of whom had achromatopsia, as the population re-built itself, many of the community inherited the condition from the survivor, and continue to pass it on several generations later.

While there is no cure for the condition, effective treatment is available to mitigate symptoms and includes glasses to correct far- and near- sightedness as well as astigmatism, glasses with red lenses to reduce light sensitivity and glasses with wrap-around shields to reduce light interference.

If you liked this article, you might also enjoy our new popular podcast, The BrainFood Show (iTunes, Spotify, Google Play Music, Feed), as well as:

• Why is Snow White Given That Snowflakes are Clear?
• The Color Pink Doesn’t Exist? So Why Can We See It?
• Both Cats and Dogs can See Color
• What Causes Red Eyes In Photographs
• Why Squinting Helps You See Better

Bonus Facts:

• Red-green color blindness is different from achromatopsia, in that with the former, usually only one type of cone cell is inactive, while the other two work fine. In addition to being inherited, certain conditions can also cause color blindness including macular degeneration, glaucoma, cataracts and diabetic retinopathy.
• Color blindness predominantly affects men, and it is estimated that up to 8% of Caucasian men, 5% of Asian and 4% of African men have red-green blindness.
• Blue color blindness is the rarest type, and only 5% of those who are color blind suffer from it; however, it equally affects men and women as it is carried in a non-sex chromosome.
• The percentage of women who suffer from color blindness is below 1% of the population; for a woman to be color blind, she must have both a color blind father, and a mother who was herself a carrier of the gene.
• Most people are capable of discerning nearly 1,000,000 different colors. In addition, there’s a rare few who have tetrachromatic vision, and can even register colors between red and green, rendering them able to distinguish up to 100,000,000 colors.  For reference, HDMI 1.3 supports 30 (1 billion), 36 (68 billion), and 48 bits (281 trillion) of color.
• Crayola makes crayons in over 120 colors. When it first began in 1903, a Crayola box had only black, brown, blue, green, violet, yellow, red and orange; this was expanded to 16 colors in 1935. At the rate of growth since then (2.56% each year), it is estimated that by 2050, there will be 330 distinct shades.  (And, if you’re interested: Where the Words “Crayola” and “Crayons” Came From)
• One irony of crayons is just when you get really good at coloring, you grow out of the pastime. Fighting this trend, over recent years increasing numbers of adults have returned to the coloring hobby, and today special coloring books for adults are even being published. A growing hobby, just between the Christmas shopping seasons in 2013 and 2014, one store saw an increase in coloring book sales of 300%.