Loss of color vision and Stiles' II1 mechanism in a patient with cerebral infarction.

A 70-year old man developed achromatopsia with bilateral loss of superior visual fields and an inability to recognize familiar faces (prosopagnosia). Ophthalmologic examination results were normal. Visual acuity was 20/25 in either eye. Computerized axial tomography of the brain revealed infarction of the inferior aspect of the temporal occipital cortex in both hemispheres. The patient's complaint that objects appeared only in shades of gray was supported by large errors made throughout the spectrum on the Farnsworth-Munsell 100 hue test and by matches over the entire red/green range on the Nagel anomaloscope. Although absolute scotopic and photopic thresholds were unremarkable, the increment thresholds to a 482-nm test on a red background increased monotonically as if the II1 mechanism were absent. In addition, the spectral sensitivity to large test flashes on an intense red background peaked in the middle rather than in the short-wave portion of the spectrum, as is normally found. We speculate that the chromatic channel is compromised. The patient's residual vision is mediated by a luminance channel that is subserved by the middle and long--but not the short--wave cone mechanisms.