Monochromacy & The Island of the Colorblind

Nystagmus

The human retina contains 2 types of light detecting photo-receptor cells:[1][2]

• ROD CELLS (active in low light – night vision) 

• CONE CELLS (active in normal daylight) 

The rods outnumber cones by ~ 20 to 1 in the human retina, but cones are essential for color vision.[20][25] 

There are 3 types of cone cells, each containing a photo-pigment with different color sensitivities:[20][25]  

One is most sensitive to short wavelengths (blue cones), one to medium wavelengths (green cones), and the third to medium-to-long wavelengths (red cones).[1][2][3]

Visual Spectrum (nm)

The mixture of these three base colors makes us perceive the vast palette of colors we see.[1]

The most common forms of color vision deficiency are inherited as recessive traits via alterations in genes located on the X chromosome. 

These genes encode the green and red sensitive photo-pigment molecules. 

Therefore, individuals affected will have difficulty distinguishing between green and red.[2][4][5][22]

UK stats show 8% incidence in males versus 0.5% in females.[2][4][5][22] 

MONOCHROMACY is a rare sub-type of color vision deficiency.[1][2][6] 

Monochromacy, as a color vision deficiency disease in humans, is referred to as ACHROMATOPSIA. It occurs in 4 forms:[1][2][7] 

• ROD MONOCHROMACY

• BLUE-CONE MONOCHROMACY

• CONE MONOCHROMACY, 

• CEREBRAL ACHROMATOPSIA

ROD MONOCHROMACY (RM) is 'typical' or 'complete' achromatopsia where the retina contains no cone cells. RM is characterized by the following symptoms:[1][20][21][22] 

• Complete inability to differentiate colors
• Poor visual acuity
• Severe light sensitivity 
• Hyperopia: difficulty seeing objects up close
• Nystagmus: involuntary eye movements (Photo at the top)
• Scotoma: central depressed vision (blind spot) 

Rod monochromacy is a rare and extremely severe form of an autosomal recessive retinal disorder. (Men and women have equal inheritance probabilities.)

It is the most commonly diagnosed form of total color blindness.[2]

The prevalence of the disease is estimated to be 1/30,000 worldwide.[7][11][20][21]







BLUE-CONE MONOCHROMACY (BCM) is the most common form of partial achromatopsia.[20][22][24] 

However, it is still a rare congenital cone dysfunction syndrome, affecting less than 1 in 100,000 individuals. 

BCM is characterized by the absence of red and green cone pigments,[20][23] but preserved function of the blue pigment.[20][21] 

Blue-cone monochromacy is inherited as an X-linked recessive trait,[20][21][23] with inheritance patterns similar to red-green color blindness discussed above.[1] 



CONE MONOCHROMACY is a very rare form of complete achromatopsia when only a single kind of cone is functional. Unlike blue-cone or rod monochromacy, cases of cone monochromacy have normal visual acuity and do not have light sensitivity.[1][2]  

In CEREBRAL ACHROMATOPSIA, or atypical achromatopsia, a person cannot perceive colors even though their eyes are capable of distinguishing them.[1]

It is thought the color receptors are working properly, but that the information is not reaching the brain or is not being processed.[1] 

Unlike the other types of monochromacy, cerebral achromatopsia is not inherited but an acquired color vision defect. It may be caused by trauma or illness.[1]

THE ISLAND OF THE COLORBLIND

A satellite image of the Pingelap atoll
at low-tide. [26]

In 1775, Typhoon Lengkieki struck and devastated the small Micronesian 
atoll of Pingelap.[12] 

The typhoon and ensuing famine, left only around 20 survivors; one of whom, a man named Nanmwarki Mwanenihsed, is now believed to have carried the gene for achromatopsia.[12][13][14]

Since achromatopsia is an autosomal recessive disorder, inbreeding between the descendants of Nahnmwarki Mwanenised would result in an increased recessive allele frequency.[12][14]

A population bottleneck is a sharp reduction 

in the size of a population due to environmental 

events or human activities.[19]

The prevalence of achromatopsia on the island has been traced back to that 'population bottleneck'.









The achromatopsia disorder first appeared in the second generation after the typhoon.[12] 

By the sixth generation, the incidence had risen to approximately 4.92%,[12][15] due to the *founder effect and inbreeding.

*In population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population.[18]

Presently, due to the small gene pool and rapid population growth, the disorder is now prevalent in almost 10% of the population, with a further 30% being unaffected carriers.[12][16]

In 1996, the BBC aired the documentary, The Mind Traveller | The Island of the Colorblind.[1]

The film quality appears a bit dated but it is worth watching as the documentary provides a glimpse into what life is like for individuals living with achromatopsia.

"The Island of the Colorblind" written by Dr. Oliver Sacks (who presented the above documentary), was published in 1997.[17] 

***

Fin

UPDATE / RELATED

Business Insider Feb. 27, 2015  No one could see the color blue until modern times           

Scientific American March 18, 2015  The Purpose of Our Eyes' Strange Wiring is Unveiled

Valspar : Color For The Colorblind (People Experiencing Color for the First Time - Video)

References:

[1] http://www.color-blindness.com/2007/07/20/monochromacy-complete-color-blindness/

[2] http://en.wikipedia.org/wiki/Color_blindness

[3] "Colour vision deficiency - Causes - NHS Choices". Nhs.uk. 2012-12-14.

[4] Hoffman, Paul S. "Accommodating Color Blindness" (PDF).

[5] Neitz, Maureen E. "Severity of Colorblindness Varies". Medical College of Wisconsin.

[6] Spring, Kenneth R.; Parry-Hill, Matthew J.; Fellers, Thomas J.; Davidson, Michael W."Human Vision and Color Perception". Florida State University.

[7] http://en.wikipedia.org/wiki/Achromatopsia

[8]  Weiss, A. H.; Biersdorf, W. R. (1989). "Blue cone monochromatism". J Pediatr Ophthalmol Strabismus 26 (5): 218–23. PMID 2795409.

[9] Types of Colour Blindness". Colour Blind Awareness.

[10] Blom, Jan Dirk (2009). A Dictionary of Hallucinations. Springer. p. 4. ISBN 978-1-4419-1222-0.

[11] Francois, J (1961). Heredity in ophthalmology. St. Louis: Mosby.

[12] http://en.wikipedia.org/wiki/Pingelap

[13] "Gene Mutation for Color Blindness Found". Science News.

[14] Cabe, Paul R. (2004). "Inbreeding and Assortive Mating". Encyclopedia of genetics 2ed.

[15]  Hussels, I.E.; Mortons, N.E. (1972). "Pingelap and Mokil Atolls: achromatopsia". American Journal of Human Genetics 24 (3): 304–309. PMC 1762260. PMID 4555088.

[16] "The Achromatopsia Group".

[17]  Sacks, Oliver (1997). The Island of the Colour-blind. Picador. ISBN 0-330-35887-1.

[18] http://en.wikipedia.org/wiki/Founder_effect

[19] http://en.wikipedia.org/wiki/Population_bottleneck

[20] http://en.wikipedia.org/wiki/Monochromacy
[21] Eksandh L (2002). "Clinical features of achromatopsia in Swedish patients with defined genotypes". Ophthalmic Genetics 23 (2): 109–120. doi:10.1076/opge.23.2.109.2210.
[22] "Colour Blindness." Tiresias.org. Accessed September 29, 2006.
[23] Michaelides, M. "Blue cone monochromatism: a phenotype and genotype assessment with evidence of progressive loss of cone function in older individuals." Eye. 2005 Jan;19 (1) :2-10.
[24] Weleber, Richard G. "Infantile and childhood retinal blindness: A molecular perspective (TheFranceschetti Lecture)." Ophthalmic Genetics. 2002 Jun;23 (2) :71-97.
[25] Kalat, James (2013). Biological Psychology. Jon-David Hague. p. 158. ISBN 978-1-111-83100-4.
[26] http://en.wikipedia.org/wiki/Pingelap

Image Credits:

"Three Main Layers of the Eye" by Artwork by Holly Fischer - http://open.umich.edu/education/med/resources/second-look-series/materials - Eye Slide 3. Licensed under CC BY 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Three_Main_Layers_of_the_Eye.png#mediaviewer/File:Three_Main_Layers_of_the_Eye.png

"Optokinetic nystagmus" by Original uploader was Student BSMU at en.wikipedia - Originally from en.wikipedia; description page is/was here.. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Optokinetic_nystagmus.gif#mediaviewer/File:Optokinetic_nystagmus.gif

"X-linked recessive" by XlinkRecessive.jpg: National Institutes of Healthderivative work: Drsrisenthil - XlinkRecessive.jpg. Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:X-linked_recessive.svg#mediaviewer/File:X-linked_recessive.svg

"Autosomal recessive - en" by Kashmiri, based on earlier work by Domaina - Own work based on Autosomal dominant - en.svg and Autorecessive.jpg. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Autosomal_recessive_-_en.svg#mediaviewer/File:Autosomal_recessive_-_en.svg

"Rendered Spectrum" by Spigget - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Rendered_Spectrum.png#mediaviewer/File:Rendered_Spectrum.png

"Population bottleneck" by Original uploader was Mysid at en.wikipedia - Originally from en.wikipedia; description page is/was here.. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Population_bottleneck.svg#mediaviewer/File:Population_bottleneck.svg

"Pingelap" by Smurrayinchester - Own work. Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Pingelap.png#mediaviewer/File:Pingelap.png