Cerebral achromatopsia

Cerebral achromatopsia is a type of color blindness 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 but underlying physiological deficits of the disorders are completely distinct. A similar, but distinct, deficit called color agnosia exists in which a person has intact color perception (as measured by a matching task) but has deficits in color recognition, such as knowing which color they are looking at.

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 damage is almost always the result of injury or illness. Even when presented with this information, the patient believed he had retained his ability to perceive color even though the world around him appeared grey. He attributed this achromatism to "poor lighting" and it took several weeks for the patient to fully appreciate the extent of his disability. In addition, the characteristic comorbidity of prosopagnosia was present. After two months and frequent sessions with doctors, tests indicated his color had fully returned. The ischemia caused by lesions on the posterior cerebral arteries had subsided and follow up MRI scans indicated that blood flow had once again returned to the ventral occipital cortex.

The most common disorder seen alongside cerebral achromatopsia is prosopagnosia, the inability to recognize or recall faces. In some studies, the comorbidity is seen as high as 72%. 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. A case of cerebral achromatopsia, acquired after a cortical lesion, was described by Dr. Verrey in 1888. but the evidence was dismissed by both Gordon Holmes and Salomon Henschen, two eminent neurologists who had identified the position and limits of the primary visual cortex. Verrey's description all but disappeared until a color center was identified in macaque monkey. and until a review of the literature showed the common location of the rare cases of cerebral achromatopsia

Based on the knowledge gained from cerebral achromatopsia case studies, current research is focusing on learning more about the cortical area involved in color processing. A recent study provided some of the first direct evidence of color specific processing in the human ventral occipital cortex. A subject with a history of seizure activity was examined using fMRI and electrode implantation. Using the fMRI, researchers examined the areas of blood oxygenation in the occipital lobe as the subject viewed various color specific stimuli. The result of the experiment was the identification of an area in the subject, slightly anterior to the lesioned area in cerebral achromatic patients, that responded to variance in color stimulation. The resolution of the MRI was a limiting factor in identifying areas corresponding to specific colors. The next portion of the study used an electrode implanted in the right hemisphere in the location identified by the fMRI scan as pertaining to color processing. It was found the electrical activity of the area increased when the subject was presented with blue stimuli. The next, and most significant finding of the study, was that when the electrode was used to present an electrical stimulus in the subject's brain, the subject reported the perception of the color blue. Such a result is consistent with other reports of electrical stimulation in visual field maps eliciting perception of phosphines in subjects’ visual field. The color stimulus presented is not the only factor in determining the involvement of the ventral occipital cortex in color processing. The amount of attention and the type of object also affect the activation of the ventral occipital cortex. It has been noted that this area of the occipital lobe may not be a processing center but rather a pathway that is a critical intersection of several cortical areas involved in color perception.

The disorder is often presented as evidence of our incomplete knowledge of color processing. Color vision research is a well studied field of modern neuroscience and the underlying anatomical processing in the retina have been well categorized. The presence of another factor in the perception of color by humans illustrates the need for more research.

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:

Mr. I. could hardly bear the changed appearances of people ("like animated gray statues") any more than he could bear his own changed appearance in the mirror: he shunned social intercourse and found sexual intercourse impossible. He saw people's flesh, his wife's flesh, his own flesh, as an abhorrent gray; "flesh colored" now appeared "rat colored" to him. This was so even when he closed his eyes, for his preternaturally vivid ("eidetic") visual imagery was preserved but now without color, and forced on him images, forced him to "see" but see internally with the wrongness of his achromatopsia. He found foods disgusting in their grayish, dead appearance and had to close his eyes to eat. But this did not help very much, for the mental image of a tomato was as black as its appearance.