Cerebral achromatopsia
From Psy3241
Achromatopsia is the inability to see color, more commonly known as “colorblindness”. Normally inherited, achromatopsia is usually the result of deficient cone function in the eyes. A much rarer form of colorblindness is cerebral achromatopsia, which is the result of damage to the occipital cortex. People suffering from cerebral achromatopsia see in gray scale, not black and white as one would expect.
Patients with cerebral achromatopsia often cannot visualize or imagine colors, although studies have shown that some patients do retain this ability.
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Theoretical Background
Previous research has sought to determine where color perception takes place in the brain.
Louis Verrey
In 1890, Verrey examined the brain of a woman who, following damage to her occipital lobe in the left hemisphere, complained of seeing everything in her right visual field as gray scale. He found a small amount of damage in the visual cortex, which he determined was the location of color perception. This went against Locke’s earlier belief that there was not a separate color center, and Verrey’s findings were discredited.
Semir Zeki
In the 1960’s and 70’s, Zeki implanted microelectrodes into the visual cortex of monkeys and measured the activity of neurons. Zeki found that cells in the Primary visual cortex (V1) responded to wavelength but not color, where cells in the prestriate cortex (V4) responded to color but not wavelength. Further research magnetically stimulated V4, which caused the participant to “see” colored rings. This supported Verrey's earlier finding that there is in fact, a separate center for color.
The Colorblind Painter
One specific instance of cerebral achromatopsia is the case of the “Colorblind painter” by Oliver Sacks. Following a car accident, Jonathan I. saw everything in grayscale which left his world seemingly "dirty". An eye exam determined that all three cone mechanisms in the retina were intact indicating that his colorblindness was a neurological problem. He could, however, differentiate tones, as he was seeing with the wavelength sensitive cells of V1 but unable to use the color sensing cells of V4.
This was determined by administering the Mondarain Test, where abstract geometric shapes are illuminated using three projectors that with red (long wavelength), green (middle wavelength) and blue (short wavelength)filters. In white light, Mr. I. saw most of the shapes, but could not see some boundaries. While switching the filters, Mr. I. saw all the colors (except black) in a different shade of gray. This concluded that he was able to wavelength but could not convert it into color.
He also lost his ability to visualize colors, which undoubtedly frustrated Mr. I., as his previously vividly colorful dreams were now dull and gray.
Years following the accident, Mr. I'.s condition did not bother him as much and he found that he actually had a heightened sense of sight at night.
