A7 Dyschromatopsias: Deficiencies in Color Vision

Topic

In the retina, in addition to the cells responsible for vision in low-light conditions, there are three types of cones, each with a distinct spectral sensitivity: the S-cone responds best to short wavelengths, the M-cone to intermediate wavelengths, and the L-cone to long wavelengths. The brain does not perceive a color by comparing the spectral sensitivity of a single cone type against a fixed reference, but rather by comparing the relative activation levels of the three cone types in response to the same light; it is this comparison among the three cone types—rather than the isolated response of any one of them—that ultimately determines the perceived color.

Of the three cone types, the M-cone and L-cone have spectral sensitivities that overlap significantly with each other—far more than the S-cone’s sensitivity overlaps with that of either of the other two. Consequently, the comparison between the M-cone and L-cone—while requiring the detection of a more subtle difference than the comparison involving the S-cone—is what enables the brain to distinguish colors between red and green. The smaller the spectral separation between the M-cone and L-cone sensitivities, the more subtle the difference the brain must detect to distinguish these colors, and the more likely it is that the difference will go unnoticed.

When one of the three cone types is completely absent or fails to respond to any wavelength, the brain can no longer compare that cone type with the other two, and the comparison relying on it ceases to provide information; this condition is known as dichromacy. If the absent cone type is the L-cone, the condition is called protanopia; if the absent type is the M-cone, it is called deuteranopia. In both cases—without either the L-cone or the M-cone available for comparison with the remaining cone—the brain loses the ability to distinguish colors that relied specifically on the comparison between the M-cone and the L-cone.

Conversely, when all three cone types are present and responsive to light, but the spectral sensitivities of the M-cone and L-cone—which already overlap significantly—shift even closer together than in a typical eye, the spectral separation between them narrows without disappearing entirely; this condition is known as an anomaly. Since spectral separation between the M-cone and L-cone still exists (albeit reduced), the brain can still compare the two and distinguish some differences in colors dependent on that comparison, though with much lower sensitivity than in an eye with typical spectral separation between these cones. If the L-cone's spectral sensitivity is the one that shifts closer, the anomaly is called protanomaly; if it is the M-cone's, it is called deuteranomaly.

Because both dichromacy and anomaly depend on the spectral sensitivity of the cones themselves—whether absent in one case or too similar to another cone's sensitivity in the other—and because this sensitivity is genetically determined before birth, there is no pharmacological correction capable of altering it. The available correction consists of lenses with spectral notch filters—marketed, for example, under the brand EnChroma—which deliberately block a narrow band of wavelengths precisely where the spectral sensitivities of the M-cone and L-cone overlap the most, thereby artificially widening the effective spectral separation between the two cones for the light that passes through the lens. These glasses with spectral notch filters are more helpful to individuals with anomalous trichromacy, because they still possess some spectral separation between the M-cone and L-cone that the filter can enhance; conversely, they are far less effective for those with dichromacy, as there is no functional M-cone or L-cone whose spectral separation the filter could amplify.

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