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John D. Mollon, S. Mary Hood, Lisa Purves, Gabriele Jordan; Color discrimination in carriers of color deficiency. Journal of Vision 2004;4(11):9. doi: 10.1167/4.11.9.
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Some 15% of women are carriers of X-linked deficiencies of color vision. Do they share a little in the disability of their sons, in the way that mothers of hemophiliacs bruise more readily than other women? We argue that carriers of protan deficiencies must be considered separately from deutan carriers. If the long- and middle-wave cones are normally present in a ratio of 2:1 and if X-chromosome inactivation occurs randomly within the retina, then protan heterozygotes are expected to exhibit an average ratio (1:2) that is no further from unity than is the normal ratio, whereas the average ratio for deutan heterozygotes should be more extreme (5:1).
We tested 55 obligatory carriers of color deficiency and 55 controls. Square-wave, red-green gratings of 2 cycles per degree were displayed on a calibrated monitor using a graphics card with a resolution of 15 bits per gun. Discrimination thresholds were measured by a two-alternative temporal forced-choice task: The subject was required to report in which interval a minimal red-green modulation was present. To prevent the use of luminance cues, sensation luminance for the red and green guns was equated for each subject in a preliminary test and in addition, a random luminance variation was superimposed on the stimulus.
Deutan, but not protan, carriers had significantly poorer discrimination than did male or female controls. Note that this finding is independent of the question of whether carriers of anomaly are tetrachromatic: The number of dimensions of color vision is distinct from the fineness of discrimination along any given dimension.
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