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]
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]
atoll of Pingelap.[12]
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 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
Scientific American March 18, 2015 The Purpose of Our Eyes' Strange Wiring is Unveiled
References:
[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.
[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.
[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.
[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
"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
"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
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