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J.M. Stringham, B.R. Hammond, Jr., B.R. Wooten, D.M. Snodderly; Evaluation Of The Compensation Mechanism For Macular Pigment . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3809.
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© ARVO (1962-2015); The Authors (2016-present)
Past data indicate that the π–1 system adjusts its gain to offset the differential filtering of short–wave light by MP across the central retina. When using the conditions necessary to isolate the π–1 system, the S–cone pathway appears to behave univariantly and scales sensitivity to directly offset differences in MP density across the retina. Such an adjustment is probably largely based on the relative peaks of MP and the π–1 mechanism. Since, however, these curves are not similar in shape, the effects of compensation on wavelengths outside of these peaks do not tend to be constant across the retina. For example, adjustment near the peak of π–1 predicts hypersensitivity at 520 nm, and hyposensitivity at 460 nm for foveal (FOV) compared to parafoveal (PF) sites, respectively. This study was designed to test such predictions at multiple wavelengths.
For one experiment, five normal, young (age range = 22–41 yrs.) subjects participated. MP was measured using HFP and ranged from 0.1 to 0.62 (using the standard 1–deg stimulus). A 17º, 3.3 log–td yellow background was used to isolate the short–wave pathway. For FOV π–1 testing, a 1º target of wavelengths ranging from 400–520 nm (in steps of 10 nm) was flashed for 200 msec every 3 secs. For PF testing, a 2º target was presented 7º in the temporal retina. The method of adjustment was used. In a second experiment, 16 normal, young (age range 23–40) subjects were tested, under the same conditions as described above, except that only 440 nm and 500 nm test wavelengths were used.
Our results show that the shapes of subjects’ PF π–1 functions was very similar across subjects. In contrast, the shapes of the FOV π–1 functions were different across subjects. These differences could largely be accounted for by differences in MP absorption. Consistent with the results from experiment 1, π–1 sensitivity to 440 nm was almost identical for FOV and PF conditions. For 500 nm, however, sensitivity was shown to be approximately 0.10 log unit greater in the FOV compared to the PF. This finding is also consistent with the prediction of hypersensitivity in the FOV for wavelengths outside the predominant absorption band of MP.
These results are consistent with the idea that the visual system increases gain of the SWS system to offset light absorption by MP. Our data suggest this gain occurs by simply increasing the sensitivity of an effectively univariant mechanism.
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