Walsh & Hoyt: Preserved Color Vision

Occasionally color discrimination is preserved in patients with other visual perceptual defects, most notably form agnosia. One of these cases provided evidence for impaired brightness discrimination with relatively preserved hue discrimination: in other words, the converse defect in color space to that of achromatopsic patients. A common feature of these reports is carbon monoxide poisoning. Zeki hypothesized that this may reflect a differential toxic or hypoxic vulnerability of the color and form pathways in cortex, although whether this occurs in striate or prestriate cortex is unknown. None of these cases provided anatomic detail; however, a follow-up report of Adlers case with CT and MR imaging many years later showed only occipital atrophy without a focal lesion.