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Billy R. Hammond; Compensation for macular pigment: Color appearance and sensitivity regulation. Journal of Vision 2008;8(17):24. doi: 10.1167/8.17.24.
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A long-standing question in color vision research is how the visual system is able to compensate for the significant absorbance of short wave light by the crystalline lens and macular pigment (MP). The significant attenuation of short-wave light due to prior filtering must require subsequent enhancement of the blue signal in order to maintain color constancy across the retina where MP levels are changing quickly and dramatically. We studied this compensation mechanism by measuring MP spatial density profiles, S-cone sensitivity, and hue cancellation functions across the central retina. Despite large variations in MP across the retina, hue cancellation values for the Y-B system across the central retina were constant. For example, one subject's MP density declined from a central peak of 0.99 to near zero at 7° (near 95% transmission difference) yet thresholds for the Y-B system were unaffected. In contrast, the G-lobe of the R-G system was directly correlated with MP density. Absolute sensitivity levels between foveal and parafoveal locations were also very similar for the peak of the s-cone pathway (440 nm). The results for sensitivity and color appearance are consistent. The intense yellow-background used in increment thresholds, for instance, probably adapts the yellow lobe (which receives its input from mid-and-long wave cones) of the Y / B system so strongly that it is the B-lobe of the system exclusively mediating the increment threshold response (i.e., isolating just the S-cone input to the system). Taken together, the results suggest that the visual system compensates for differential filtering of SW-light by MP across the retina by regulating the Y / B opponent system.
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