A new case of unilateral dichromatism is described. She is a young woman with normal color vision in one eye and dichromatic vision of a primarily deuteranopic type in the other.
Critical fusion frequency functions for a centrally fixated 28-min field were determined in ten spectral regions, ranging from one having a spectral centroid at 452 mμ to one with a spectral centroid at 682 mμ, on both eyes of this unilaterally dichromatic subject. Determinations were also made with white light. Measurements extended over a range of approximately 3.5 log millilamberts. For all colors except red, the curve of critical fusion frequency vs log luminance for the color-blind eye is displaced downward on the critical fusion frequency axis with respect to the curve for the normal eye. For any given spectral region, the displacement is approximately constant over the luminance range tested and the two curves do not reach the same maximum fusion frequency. The magnitude of the shift varies with wavelength. It is greatest in the green, next in the blue-green, yellow-green, blue, and yellow; there is a slight loss in the orange and no detectable loss in the red. The data for white light data also show reduced critical fusion frequencies for the color-blind eye.
These findings are taken to reflect a reduction in the dichromatic eye of the number of receptors (of a type especially sensitive to green) available for excitation by the spectral range from about 450 mμ to 625 mμ.
Some measurements of critical fusion frequency with a 1° green and a 2° white field are also reported. They display the same general trends as the data for small fields, but the extent of the downward shift of the color-blind function is in each case less than that for the corresponding pair of curves with the 28-min area. The reduction in the amount of downward displacement with larger test fields comes about through a proportionately greater increase in critical fusion frequency with area for the color-blind than for the normal eye. The results are formulated in terms of a nonuniform distribution of color receptors across the fovea.
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