The influence of filtering by the macular carotenoids on contrast sensitivity measured under simulated blue haze conditions

PURPOSE Distant objects are often obscured as a result of wavelength-dependent scattering in the atmosphere. This scattered light, which is mostly short-wave, effectively forms a veiling luminance (or background light) against which a target must be detected and discriminated. The macular pigment (MP) carotenoids could reduce the effective background intensity by selectively filtering out short wavelengths which would increase the contrast of the object in the retinal image, thus improving visibility. This Visibility hypothesis was originally posited by Wooten and Hammond (2002). This study represents a first empirical test of the hypothesis. METHODS Five young healthy subjects were evaluated. MP optical density (OD) was measured using HFP. Visibility was assessed by measuring contrast sensitivity thresholds at 8 cycles/deg (CST) using an optical system that passed xenon-light through the sine-wave grating. Blue haze was simulated using an ecologically valid broad-spectrum filter. Changes in MP density were simulated using a variable path length filter with an oil-based carotenoid solution that mimicked the absolute absorption spectrum of MP. RESULTS The average baseline CST was 0.004. Adding 0.25OD of simulated MP lowered the average threshold to 0.003 (25%). An additional 0.25OD decreased thresholds an additional 10% and the effect reached a plateau at about 0.50. DISCUSSION The largest improvement (about 25%) in contrast occurred with the initial, and relatively modest, addition of 0.25OD units of simulated MP suggesting that the largest improvements may be linked to initial increases in MPOD.