| Summary: | Initial experiments examine the performance of heterozygous females on standard tests of colour vision. The results of these tests do not provide convincing evidence to support the hypothesis that carriers share in the colour deficiencies of their fathers and sons. Colour vision is thought to depend upon two subsystems that remain parallel and independent in the early stages of visual processing. I examine the relative organizing power of these two subsystems and find that carriers of a colour deficiency cannot be considered as one population. For small element separations, the relative organizing power of the L-M subsystem is reduced for deutan carries in comparison to both colour normals and protan carriers. In contrast, for large element separations, there is no difference between colour normals and carriers of any colour deficiency. Individuals with more symmetrical L: M cone ratios are often supposed to exhibit better red-green chromatic discrimination. As heterozygosity will not lead to symmetrically extreme L: M cone ratios in protan and deutan carriers, I propose that the L-M subsystem of deutan carries is less sensitive owing to the more extreme skewing of the L: M cone ratio in such carriers; the reduction in sensitivity will be most evident at high spatial frequencies. Red-green chromatic spatial Contrast Sensitivity Functions (CSF) are measured to test this hypothesis. At spatial frequencies comparable to those of the smallest element separation used in examining the relative saliency of the two subsystems of colour vision, deutan carriers have lower red-green contrast sensitivity than both protan carriers and colour normals. Further experiments were conducted on a new subject population to verify these findings. The results confirm that deutan carriers exhibit poorer red-green contrast sensitivity than colour normals and suggest that individuals with more symmetrical L: M cone ratios do exhibit better red-green contrast sensitivity.
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