Spectral sensitivity and color vision tests were performed on a subject who was totally color blind at low photopic luminances, but exhibited hue discrimination at relatively high photopic levels. Chromaticity confusion loci showed that the residual color vision was abnormal; a tritan defect was superimposed on the generalized reduction of cone sensitivity. Although cone function was markedly depressed, a photopic spectral sensitivity curve was obtained for the light-adapted fovea. Dark-adapted foveal measurements, on the other hand, gave a scotopic function almost identical to that of the dark-adapted periphery. However, it was possible to demonstrate objectively that this subject shifts fixation to an eccentric position under scotopic conditions, i.e., the dark-adapted “foveal” results were, in fact, for a para-macular region. Irrespective of the degree of light adaptation, cone function was not detectable in the peripheral measurements.
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