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Max Snodderly, Randy Hammond, James Stringham, Bill Wooten; Compensation for light loss due to filtering by the macular pigment: Specificity of the mechanism. Journal of Vision 2007;7(9):670. doi: 10.1167/7.9.670.
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Our past data have shown that the visual system compensates for filtering of short-wave light by macular pigment (MP). Compensation is evident from the fact that S-cone sensitivity and yellow-blue color perception are unaltered when comparing foveal responses, where MP is dense, to parafoveal responses where MP density (MPOD)is unmeasureable.
Yellow (575 nm)-blue (440 nm) and red (600 nm) - green (501 nm) cancellation functions, as well as increment thresholds (using conditions that were designed to isolate the S-cone pathway) were obtained at 0,1, 1.75, 3 and 7°. MPOD was measured using heterochromatic flicker photometry. Ten young subjects were studied.
When using conditions that isolate the S-cone pathway, the S-cone system behaved univariantly and scaled sensitivity to offset differences in MPOD across the retina. Since the spectral shapes of these components differ, the effects of compensation were a function of wavelength. Adjustment of the S-cone pathway predicts overcompensation at 520 nm, and undercompensation at 460 nm for foveal (FOV) compared to parafoveal (PF) sites, respectively. We tested these predictions at multiple wavelengths based on a theoretical compensation mechanism specific to the S-cone pathway. S-cone sensitivity changed with eccentricity consistent with our predictions based on a gain mechanism. The PF S-cone curve (based on classic p-1 curves or Stockman's fundamentals) is different from the broader FOV curve, which includes MP absorbance. The shapes of subjects' PF functions were very similar across subjects, but shapes of the FOV S-cone functions differed depending on individual differences in MPOD. Hue cancellation values for the Y-B system did not change significantly across the retina. R-G sensitivity, in contrast, changed as a function of MPOD. These results are consistent with the idea that the visual system increases gain of the S-cone system, but not other systems, to compensate for light absorption by MP.
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