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Biology 103
2001 Third Web Report
On Serendip

Pingelap: Island of the Colorblind

Julie Wise

Pohnpei Landscape. Photo (c) FSM Visitors Board.

Envision a tropical paradise, not unlike the island scene pictured above, complete with breathtaking scenery that includes crystal blue waters and luscious plant-life. Now imagine that you cannot see any of these things in color. This is the situation that between 5% and 10% of the native population of Pingelap Atoll, part of the Micronesian State of Pohnpei, find themselves in (3).

Supposedly, a freak typhoon-like storm ravaged the island in the late eighteenth century and killed a number of the island's inhabitants. Approximately 20 people survived to replenish the isolated island's population. Roughly four generations after the typhoon, the citizens of Pingelap began exhibiting symptoms of a rare recessive disorder known as Achromatopsia. Achromatopsia is characterized by extreme light sensitivity, poor vision, and complete inability to distinguish colors (3). This anomaly is the focus of Oliver Sacks' new book The Island of the Colorblind and its publication has succeeded in raising public awareness about the rare hereditary disease of Achromatopsia. Of the roughly the 3000 people living in Pingelap today, 5% to 10% of them are affected by the disorder and about 30% are carriers (3). All of these people are able to trace their ancestry to a single male typhoon survivor who researchers believe was the carrier of the disease that emerged when some of his descendents intermarried (3).

The three primary colors - as far as light is concerned - are red, green, and blue. In order to "see" images, the human eye enables light to stimulate the retina (a neuro-membrane lining the inside of the back of the eye). The retina is made up of what are called rods and cones. The rods, located in the peripheral retina, give us our night vision, but can not distinguish color. Cones, located in the center of the retina (called the macula), let us perceive color during daylight conditions.

The cones of the eye each contain a light sensitive pigment which is sensitive over a range of wavelengths (each visible color is a different wavelength from approximately 400nm to 700nm). Genes contain the coding instructions for the pigments present in the cones, and if the coding instructions are wrong, then the wrong pigments will be produced, and the cones will be sensitive to different wavelengths of light (resulting in a color deficiency). The colors that we see are completely dependent on the sensitivity ranges of these pigments.

Many people tend to think people who are "colorblind" live their lives in black and white - like watching a black and white movie or television. This is a very common misconception. It is extremely rare to be totally colorblind (Monochromasy - complete absence of any color sensation - as is the case of the citizens of Pingelap). There are many different types and degrees of colorblindness and they are more correctly labeled 'color deficiencies' (2).

People with normal cones and light sensitive pigments (trichromasy) are able to see all the different colors and subtle mixtures of them by using cones sensitive to one of three wavelengths of light - red, green, and blue. A slight color deficiency is present when one or more of the three cones light sensitive pigments are coded incorrectly in the person's genes and their peak sensitivity is shifted (Anomalous Trichromasy - includes Protanomaly and Deuteranomaly). A more severe color deficiency exists when one or more of the cones light sensitive pigments is altered (Dichromasy - includes Protanopia and Deuteranopia).

The Color Wheel as it appears to a person with normal vision(2)

People with normal cones and light sensitive pigments (trichromasy) are able to see all the different colors and subtle mixtures of them by using cones sensitive to one of three wavelengths of light - red, green, and blue. A slight color deficiency is present when one or more of the three cones light sensitive pigments are coded incorrectly in the person's genes and their peak sensitivity is shifted (Anomalous Trichromasy - includes Protanomaly and Deuteranomaly). A more severe color deficiency exists when one or more of the cones light sensitive pigments is altered (Dichromasy - includes Protanopia and Deuteranopia).

"Five percent to eight percent (depending on the study you quote) of the men and 0.5% of the women of the world are born colorblind. That's as high as one out of twelve men and one out of two hundred women"(2). Protans (red-weak individuals) and Deutans (green-weak individuals) make up 99% of this group.

Protanomal (one out of 100 males):

Protanomaly is referred to as "red-weakness." "Any redness seen in a color by a normal observer is seen more weakly by the Protanomalous viewer, both in terms of its "coloring power" (saturation, or depth of color) and its brightness" (2). Red, orange, yellow, yellow-green, and green, appear somewhat displaced toward green, and all appear paler than they do to the normal observer. The redness component that a normal observer sees in a violet or lavender color is so weakened that the color that Normals call "violet" may look only like another shade of blue for the Protanomalous observer.

Under poor viewing conditions, such as when driving in glaring sunlight or in rainy or foggy weather, it is possible for a Protanomalous individual to mistake a blinking red traffic light for a blinking yellow one. He or she may similarly fail to distinguish a green traffic light from the various "white" lights in store fronts, signs, and street lights that line the streets (2).

Deuteranomaly (five out of 100 males):

The Deuteranomalous person is considered "green weak". Similar to the Protanomalous person, he/she is poor at discriminating small differences between the red, orange, yellow, green regions of the spectrum. For a Deuteranomalous individual, shades that appear to exhibit shades of green to a normal eye will appear redder. "One very important difference between Deuteranomalous individuals and Protanomalous individuals is Deuteranomalous individuals do "not" have the loss of "brightness" problem"(2). Some Protanomalous and Deuteranomalous people may not even be aware that their color perception is in any way different from normal. Many go through life with very little difficulty doing tasks that require normal color vision.

Dichromasy- can be divided into Protanopia and Deuteranopia (two out of 100 males):

These individuals normally know they have a color vision problem and it can effect their lives on a daily basis. They see no perceptible difference between red, orange, yellow, and green. All these colors that seem so different to the normal viewer appear to be the same color for this two percent of the population (2).

I. Protanopia (one out of 100 males):

The Color Wheel from the prospective of a Protanope(2)

For the Protanope, the brightness of red, orange, and yellow is much reduced when compared to normal. "This dimming can be so pronounced that reds may be confused with black or dark gray, and red traffic lights may appear to be extinguished" (2). Protanopes may learn to distinguish reds from yellows and from greens "primarily on the basis of their apparent brightness or lightness, not on any perceptible hue difference" (2). Likewise, violet, lavender, and purple are indistinguishable from various shades of blue because their reddish components are so dimmed that they become to be invisible.

II. Deuteranopia (one out of 100 males):

The Color Wheel from the prospective of a Deuteranope(2)

The Deuteranope suffers the same hue discrimination problems as the Protanope, but without the dimming. The names red, orange, yellow, and green really mean very little to a Deuteranope aside from being different names that every one else around him seems to be in concurrence upon. Similarly, violet, lavender, purple, and blue all appear to be the same to a viewer with Deuteranopia.

Achromatopsia

Achromatopsia, as it appears on the island of Pingelap, differs from the above mentioned disorders in that persons born with congenital Achromatopsia have never seen color at all. Achromatopsia occurs when two copies of the mutated genes that code for the disease are present. Olof H. Sundin and his colleagues at The Johns Hopkins University reported in the July 2000 issue of Nature Genetics (25: 289-293) that "Pingelapese islanders with Achromatopsia have a single mutation in both copies of a gene dubbed CNGB3. The gene codes for one component of a type of ion channel in the plasma membrane of cone cells--specialized nerve cells in the eye's retina. The ion channels are essential for generating electrical responses to red, green, and blue light in cone cell receptors; cone cells in people with Achromatopsia do not respond to light" (3). Congenital Achromatopsia is an extremely rare hereditary vision disorder that affects 1 person out of 33,000 in the United States (5). Congenital Achromatopsia is sometimes referred to as "stationary cone dystrophy" because it is not progressive nor does it lead to blindness (5). Coping with light sensitivity is the most significant problem faced by Achromats. "In moderately bright indoor spaces or outdoors just after dawn or just before dusk, some Achromats adapt to their reduced level of visual functioning without resorting to tinted lenses, by using visual strategies such as blinking, squinting, shielding their eyes, or positioning themselves in relation to light sources. Others routinely wear medium tinted lenses in such settings. At higher levels of illumination, the vision of persons with Achromatopsia decreases. In full sunlight, outdoors, or in very bright indoor spaces, almost all Achromats use very dark tinted lenses, in order to function with a reasonable amount of vision, since their retinas do not possess the photoreceptors needed for seeing well in such settings" (5). "Congenital Achromatopsia should not be confused with Cerebral Achromatopsia, which is an acquired form of total colorblindness that can result from trauma, illness, or some other cause [as is the case in Sacks' account of The Case of the Colorblind Painter (Found as part of his book An Anthropologist on Mars) - a highly informative and enjoyable read]. Persons who develop Cerebral Achromatopsia report that they see a monochromatic world, all in shades of gray. They are able to see gray because of having previously experienced color vision, making it possible for them to perceive the absence of color as gray. This is in sharp contrast to the visual perception of congenital, complete Achromats (i.e., complete rod Monochromats), who report that the concept of "gray" is as mystifying to them as is the concept of any of the other colors. Persons with Cerebral Achromatopsia are diagnosed by neurologists, rather than eye specialists. Their loss of color perception is not accompanied by severely impaired vision, extreme light sensitivity, or any abnormality in the photoreceptors of the retina, as is the case with persons who have congenital Achromatopsia" (5).

Forums such as The Achromatopsia Network are concerned with and strive to promote the following issues concerning Achromats:

The case of the "Island of the Colorblind" is an extremely interesting juxtaposition of genetics, neurology, and the reality of the world as each person perceives it. It illustrates the necessity of diversity within populations to ensure that genetic mutations such as congenital Achromatopsia are suppressed through the natural selection process. Cases of similar oddities have been able to hold the attention of neurologists, physicians, and lay people for centuries. Advancements in the neurosciences continue to illuminate new understandings and possible explanations for an array of life's mysteries, and the case of 'the island of the colorblind' is no exception.



WWW Sources

(1) The Mind Traveler: The Island of the Colorblind .A website with information on Knut Nordby who was born completely colorblind.

(2) What is Colorblindness and the different types? What is Colorblindness and the different types? Information about colorblindness and color deficiencies. Color wheel images used in this paper were taken from this site.

(3) Not seeing Red (or Blue or Green).Article from BioScience Online. Issue: August, 2000.

(4) The Island of the Colorblind.Information on Oliver Sacks' book The Island of the Colorblind.

(5) What is Achromatopsia? From the Achromatopsia Network.

(6) About Oliver Sacks' book "The Island of the Colorblind" and the PBS Documentary film "Island of the Colorblind".

(7) Special needs of persons with Achromatopsia. Information from The Achromatopsia Network.




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