Selective Loss of Sensitivity to Colour Contrast With No Observable Cerebral Lesions

Abstract

Abstract: : Purpose: We report and investigate a unique case of total achromatopsia and absence of chromatic sensitivity following severe whiplash, with no associated scotomas or agnosia. Methods: The patient NF was investigated with standard neuropsychological tests, brain imaging, modern psychophysical techniques and visual evoked potentials. Results: NF was diagnosed densely achromatopsic, although CAT and MRI brain scanning revealed no cortical lesions to visual or any other brain regions. Six years after the trauma, we tested this patient extensively and showed that he lacks the mechanisms of colour opponency necessary to perceive equiluminant gratings, and that this is the only measurable visual deficit. Contrast sensitivity to equiluminant red-green gratings of low temporal frequency is deeply impaired, while visual acuity, perimetry, luminance contrast sensitivity for form and motion over a wide range of spatial and temporal frequencies are all perfectly normal. In a task where colour perception impedes performance (colour camouflage) NF performs better than controls. Visual evoked potentials (VEPs) to equiluminant chromatic contrast gratings are non-existent, while those to luminance gratings are normal. Conclusions: These results indicate a selective loss of the chromatic contrast neural system at an early level, probably V1 or more peripherally, without any measurable anatomical effects. Although it is difficult to ascertain the specific neural sites compromised, the present results show that sensitivity to chromatic contrast can be selectively damaged without altering any other visual performance.

Keywords: color vision • contrast sensitivity • visual cortex