Compensation for Light Loss Resulting From Filtering by Macular Pigment: Relation to the S-Cone Pathway

Abstract

Purpose. Macular pigment (MP) filters short-wavelength light before it reaches the visual pigments. At peak absorbance (460 nm), transmission of light through MP can range from almost 100% transmission to as little as 3%. As a result of the uneven topographic distribution of MP, spatial nonuniformities in visual perception would result if the visual system did not compensate for filtering differences across the central retina. This study characterizes compensation for different densities of MP. Methods. Sixteen young subjects (aged 24–40 years) with a wide range of MP density were studied. Increment thresholds were measured at 440 and 500 nm in the center of the fovea and at 6° to 7° eccentricity using conditions chosen to isolate the π-1 mechanism. For six of the subjects, increment thresholds were also obtained for eccentricities of 1°, 1.75°, and 3°. MP density was measured using heterochromatic flicker photometry at the same locations as the increment thresholds. Results. Peak sensitivity of the short-wavelength pathway across the central retina was constant despite MP density differences as large as 1.0 log unit. Conclusions. These results suggest that the visual system increases gain of the S-cone pathway to offset light absorption by MP.