Cerebral achromatopsia
Encyclopedia
Cerebral achromatopsia is a type of color-blindness that is caused by damage to the cerebral cortex
of the brain, rather than abnormalities in the cells of the eye's retina. It is often confused with congenital achromatopsia. Cerebral achromatopsia differs because it is caused only by trauma or brain damage. The symptoms of both cerebral and congenital achromatopsia are similar but not identical, and their causes are widely different.
Also, cerebral achromatopsics have poor spatial acuity. Several other deficits also coincide with cerebral achromatopsia, most notably prosopagnosia
. Lesion analyses have also shown that there is a positive correlation between the lesion locations of prosopagnosia
brains and cerebral achromatopsia brains.
The critical difference between cerebral achromatopsics and people with other forms of color blindness is that cerebral achromatopsics retain the ability to perceive chromatic borders. For example, they see a red square on a green background effortlessly even when the red and green are equally bright. There are various ways of ruling out a role for unintended luminance differences in accounting for this, e.g. random luminance masking. It even appears that cerebral achromatopsics can discriminate contrasts on the basis of color direction, but they can't use these contrasts to compare the color of surfaces that do not adjoin directly.
Cerebral achromatopsia illustrates the way in which chomatic information can be used to achieve many goals, only one of which is the perception of surface color, and that those different goals may be met by distinct pathways within the visual system.
Remarkably, almost 50% of tested patients diagnosed with cerebral achromatopsia are able to perform normally on the color-naming test. However, these results are somewhat in question because of the sources from which many of these reports come. Only 29% of cerebral achromatopsia patients successfully pass the Ishihara plate test, which is a more accepted and more standardized test for color blindness.
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
of the brain, rather than abnormalities in the cells of the eye's retina. It is often confused with congenital achromatopsia. Cerebral achromatopsia differs because it is caused only by trauma or brain damage. The symptoms of both cerebral and congenital achromatopsia are similar but not identical, and their causes are widely different.
Physical damage
Cerebral achromatopsia differs from other forms of color blindness in subtle but important ways. It is a consequence of cortical damage that arises through bilateral ischemic infarction in the inferior occipitotemporal region of the brain. This damage is almost always the result of injury or illness. Because of the location of the tissues whose damage is associated with cerebral achromatopsia, and the requirement that it be damaged in both hemispheres of the brain, complete cerebral achromatopsia is very rare.Visual effects
Patients with cerebral achromatopsia deny having any experience of color when asked and fail standard clinical assessments like the Farnsworth-Munsell 100-hue test (a test of color ordering with no naming requirements). Patients may often not notice their loss of color vision and merely describe the world they see as being "drab". Most describe seeing the world in "shades of gray". This observation notes a key difference between cerebral and congenital achromatopsia, as those born with achromatopsia have never had an experience of color or gray.Also, cerebral achromatopsics have poor spatial acuity. Several other deficits also coincide with cerebral achromatopsia, most notably prosopagnosia
Prosopagnosia
Prosopagnosia is a disorder of face perception where the ability to recognize faces is impaired, while the ability to recognize other objects may be relatively intact...
. Lesion analyses have also shown that there is a positive correlation between the lesion locations of prosopagnosia
Prosopagnosia
Prosopagnosia is a disorder of face perception where the ability to recognize faces is impaired, while the ability to recognize other objects may be relatively intact...
brains and cerebral achromatopsia brains.
The critical difference between cerebral achromatopsics and people with other forms of color blindness is that cerebral achromatopsics retain the ability to perceive chromatic borders. For example, they see a red square on a green background effortlessly even when the red and green are equally bright. There are various ways of ruling out a role for unintended luminance differences in accounting for this, e.g. random luminance masking. It even appears that cerebral achromatopsics can discriminate contrasts on the basis of color direction, but they can't use these contrasts to compare the color of surfaces that do not adjoin directly.
Cerebral achromatopsia illustrates the way in which chomatic information can be used to achieve many goals, only one of which is the perception of surface color, and that those different goals may be met by distinct pathways within the visual system.
Testing
Testing and diagnosis for cerebral achromatopsia is often incomplete and misdiagnosed in doctor’s offices. The most common tests perform to diagnose cerebral achromatopsia are the Farnsworth-Munsell 100-hue test, the Ishihara plate test, and the color-naming test.Remarkably, almost 50% of tested patients diagnosed with cerebral achromatopsia are able to perform normally on the color-naming test. However, these results are somewhat in question because of the sources from which many of these reports come. Only 29% of cerebral achromatopsia patients successfully pass the Ishihara plate test, which is a more accepted and more standardized test for color blindness.
The Case of the Colorblind Painter
The most famous instance of cerebral achromatopsia is that of "Jonathan I." immortalized in a case study by Oliver Sacks and Robert Wasserman, and published as The Case of the Colorblind Painter. The essay tracks Johnathan I.'s experience with cerebral achromatopsia from the point where an injury to his occipital lobe leaves him without the ability to perceive color, through his subsequent struggles to adapt to a black, white and gray world, and finally to his acceptance and even gratitude for his condition. Especially pertinent is the analysis of how cerebral achromatopsia affects his practice as a painter and artist. Descriptions of cerebral achromatopsia's effects on his psychological health and visual perception are especially striking. For instance, in recounting Mr. I.'s descriptions of flesh and foods, the authors write:See also
- AchromatopsiaAchromatopsiaAchromatopsia , is a medical syndrome that exhibits symptoms relating to at least five separate individual disorders. Although the term may refer to acquired disorders such as color agnosia and cerebral achromatopsia, it typically refers to an autosomal recessive congenital color vision disorder,...
- Color agnosiaColor agnosiaColor agnosia , is a medical or psychological condition that prevents a person from correctly associating hue names with common objects. The sufferer retains the ability of distinguishing hues...
- Cortical blindnessCortical blindnessCortical blindness is the total or partial loss of vision in a normal-appearing eye caused by damage to the visual area in the brain's occipital cortex. This damage is most often caused by loss of blood flow to the occipital cortex from either unilateral or bilateral posterior cerebral artery...
- Color blindnessColor blindnessColor blindness or color vision deficiency is the inability or decreased ability to see color, or perceive color differences, under lighting conditions when color vision is not normally impaired...
- Ishihara color testIshihara color testThe Ishihara Color Test is an example of a color perception test for red-green color deficiencies. It was named after its designer, Dr. Shinobu Ishihara, a professor at the University of Tokyo, who first published his tests in 1917...