Incomplete and Complete Achromatopsia

Biology 202
1998 Second Web Reports
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Incomplete and Complete Achromatopsia

Elaine de Castro

The human eye requires both rods and cones for normal vision. Over 100 million rods are located in the periphery of the human eye, and about 6 million cones compose the fovea. Rods, the more sensitive of the two to light, are not able to differentiate wavelengths, thus cannot detect color, and perceive shades of grey, black, and white. Cones, on the other hand, are of three types, containing particular pigments. They are categorized as red, blue, and green depending on to which wavelength they are most sensitive. These cones are what render color vision to humans.

Red-green color blindness is not uncommon in the general population. The unequal crossover in the X-chromosome which causes this disorder is much more easily achieved than the mechanisms which cause other types of color blindness, due to the proximity of the two pigment genes. Five to eight percent of men are affected with this genetic condition, and due to a lacking pigment, have trouble distinguishing between red, green and brown. (1)

Blue color blindness, also known as incomplete achromatopsia or blue-cone monochromatism, is an X-linked recessive disorder in which only the blue cones and the rods are functioning properly. A previously proposed theory states that signals from rods travel in the same pathways which carry signals from the blue-cones, making color vision in a blue-cone monochromat impossible. However, current research on blue-cone monochromats shows that signals from some rods and cones may be traveling by separate pathways to where wavelength discrimination takes place, making color vision possible in this type of monochromat, when both rods and blue cones are working simultaneously under twilight conditions. (6,7)

Total color blindness, or complete achromatopsia, is an autosomal recessive disorder. (2) This is defined as little or no function of the cone cells, (3) and is somewhat frequent among Moroccan, Iraqi, and Iranian Jews. (7) There is no treatment for this color deficiency, only ways to alleviate its expression. (4)

The consequences on the lives of complete achromats are many, and those of children should especially be considered. Many educational materials use different colors to teach. If vowels are red and consonants are black, the two are virtually indiscernible to an achromat (They may be discerned under certain light conditions, a phenomenon known as spectral differentiation). They may also not have yet learned to discern the different shades of grey, and may need help deciphering traffic signals. (4) Knut Nordby, a vision scientist in Norway, was diagnosed with complete achromatopsia and was wearing strong glasses at the age of nine months. His deficiency made him group colors by their shades of gray or black. He would place red and black blocks together, green and blue blocks together, and have separate piles of white and yellow. He stopped coloring pictures because he was often ostracized in his color choice (ex: light green or pink for the sky, white for the sun). (5)

Because the blue, or S-cone, detects wavelengths of lower magnitude than the red and green (M and L) cones, the vision of an incomplete achromat is restricted to the scotopic region and the lowest limits of the photopic region. A complete achromat has no functional cones at all and sees completely within the scotopic region. A person with normal color vision shows a maximum sensitivity at 530 nm, but for incomplete achromats, there is no wavelength discrimination beyond 520 nm, and near normal discrimination is observed between 440 and 500 nm. (2) This shows that not only are incomplete and complete achromats deficient in color perception, but they are hypersensitive to light. This sensitivity is due to the rods saturating at lower light intensities, so that bright light would blind the viewer.

This hypersensitivity, known as photophobia, can be moderated with certain appliances developed to reduce the intensity of the light entering the eyes. Visors, photochromatic sunglasses, polarizing filters, and colored lenses made for retinitis pigmentosa patients (which have a spectral cutoff to allow only lower wavelengths of light to pass through) all somewhat relieve the person from extreme pain and blindness from the brilliant light and its harmful glare. Since this affects the peripheral vision most, partial tunnel vision is also a consequence. Often, squinting and frequent blinking is adapted as a compensatory behavior so as not to saturate the rods. Some achromats believe that the aversion to light and their consequent restricted mobility and seemingly strange behaviors to compensate are unattractive socially and are more cumbersome than the lack of color vision. (5)

Nordby's brother and sister were also diagnosed with achromatopsia, and their childhood pictures show them playing outside at night, or in the cellar with the shades drawn. By not playing in the sun or keeping the curtains open, he felt the differences between his family and the social view of how children should play. (5) However, on the island of Pingelap where 12% of the population have achromatopsia, the achromats are excellent night fishermen. (1)

Another effect of achromatopsia is reduced visual acuity. The acuity of the human eye is mainly due to the foveal cones, which are dysfunctional in an achromat. This acuity also depends on the illumination, because of the mode in which rods function. Nordby's parents were told that their children would never be able to read or write due to their reduced visual acuity, and they sent them to schools for the blind, even though they were not completely blind. Knut could never hit a ball with a bat, and could not recognize his friends faces from afar. He and his siblings often use strong magnifiers to read, sew, or do other chores which require acuity. Nordby states that his precision is best when he does not need to squint or blink repeatedly, and that when details are brought close and are larger, he can distinguish clear edges. (5)

Because part of normal vision consists of the rods used in achromatopsia, this disorder can be somewhat relayed to the general population. Nordby chooses to describe his achromat experience somewhat like living in a black and white movie, but factoring in light aversion (like that we experience momentarily when moving from a dark atmosphere to a bright light), which causes nystagmus (side to side shifting of the eyes). Because of these circumstances, qualities such as surface reflectiveness and texture help differentiate objects. However, shadows can greatly affect the perception of the viewer, and the glare from windows can make it nearly impossible to see through at all. We must then also add the reduced visual acuity, such as when we are trying to see in dim light, or using our peripheral vision. (5)

Practically all areas of an achromat's life are affected. On the island of Pingelap, achromats are not considered marriageable by non-achromats, and the inability to read due to the small print makes it difficult for them to acquire a livelihood.(1) Nordby was often told he could only engage in simple clerical tasks or traditional work for the blind, which required little education. Traveling to new places is troublesome because signs on streets and public transportation vehicles are difficult to see and being considered for a driver's license is infeasible. Typing on a computer requires magnification lenses and a large screen, not to mention large fonts for the screen and printer. Even simple tasks like finding matching socks or picking berries off a bush can be perplexing. (5)

Technology and science have yet to find a truly effective way to correct this vision deficiency. The histology of the blue cone monochromat eye has yet to be discovered, much less the true functioning of the achromat eye. (6) The condition of complete achromatopsia is more than simply complete color-blindness, but a combination of other symptoms including reduced visual acuity, photophobia, and nystagmus. These manifestations have immense effects on the person's lifestyle, and achromatopsia should be recognized as its own unique disorder. Achromats should not be grouped with the blind, nor assumed to only do low-skill jobs. Further research and education will hopefully lend more remedies to the many inconveniences incomplete and complete achromats incur.

 

References:

1) The Island of the Colorblind - A book by Oliver Sacks

The Island of the Colorblind concerns Dr. Sacks' visit to the island of Pingelap where 12% of the population have achromatopsia. The Pingelap variant may be genetically distinct from the standard form of achromatopsia.

2) LETTERS TO NATURE

This website was found as a a link from http://rzsunhome.rrze.uni-erlangen.de:81/~sz0722/, a personal homepage by a student (Sebastian Alexander Bonhag) who is a blue-monocone monochromat. The authors, Andreas Reitner, Lindsay T. Sharpe, and Eberhart Zrenner address the question, "Is colour vision possible with only rods and blue-sensitive cones?" by performing experiments on five males with blue-cone monochromacy. It was assessed that these people indeed have color vision when their rods and blue cones are working simultaneously.

3) The Achromatopsia Network

An information and support network for those concerned with achromatopsia. Contains definition, symptoms, and suggestions on how to cope with the disorder.

4) Special Education Exchange

Special Education Exchange, or SpEdEx, is a resource for people interested in special education. This page includes descriptions of various color deficiencies and how they affect children in the classroom.

5) VISION IN A COMPLETE ACHROMAT: A PERSONAL ACCOUNT By: Knut Nordby

This page is an autobiography written by a research scientist who is a complete achromat. I highly suggest reading this page - Nordby recounts how this disorder has affected his life from early childhood on, and how he compensates for this deficiency on a daily basis. This is another website which was found as a link from http://rzsunhome.rrze.uni-erlangen.de:81/~sz0722/.

6) Color & Vision Research Laboratories

A library of experimental procedures and data sets relevant to color and vision research. This contains a report on estimating S-cone spectral sensitivities, by observing how blue-cone monochromats and color-normals visions' differ.

7) National Center for Biotechnology Information -- Online Mendelian Inheritance in Man

This website, maintained by Dr. V. A. McKusick and colleagues, contains information on genetic disorders, including complete achromatopsia. The assignment for the disorder's locus was stated as 2p11.2-q12. This site was developed by the National Center for Biotechnology Information.

 

 

Comments made prior to 2007

I am the mother of a son with Achromatopsia who just graduated from Johns Hopkins with an engineering degree. But, he has developed a serious depression and I am wondering if this is linked in any way.

Also, my son was left under the lights at the hospital for too long and his mask fell off 2 days after his birth. Another baby at the same hospital was also jaundice and under the lights. He was born the same month and has achromatopsia. We happened to meet him by chance at a wrestling meet. We find this interesting and wonder if the cones could have been burned out by intense lights used to treat the billirubium problem.

Any comments on cone / rod disorders from these lights? ... Ann Nelson, 19 August 2006