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D. Rativa, B. Vohnsen; Guided Light Spectral Properties of Foveal and Parafoveal Cones. Invest. Ophthalmol. Vis. Sci. 2010;51(13):2931.
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© ARVO (1962-2015); The Authors (2016-present)
The cone cells are known to show waveguide attributes altering the luminous efficiency of light obliquely incident on the retina. An increased directionality with distance from the fovea, has been reported in earlier studies using 540nm light (G. Westheimer, J. Physiol. 1967). However, differences in shape, size and structure of cones might change the radiation coupling efficiency in a different way across the visible spectrum. Here, our purpose is to gain insight into the dependence between the visibility of coupled light and its wavelength for cones at the fovea and parafovea regions.
Experiments of the first Stiles-Crawford effect (SCE) were performed using a white light source and a set of 10 nm bandpass filters across the visible spectrum (400nm-700nm) to examine the variation with wavelength of its characteristic directionality parameter and visibility dependence on pupil point. The measurements were realized both at the fovea and at the parafovea (approximately 4 deg eccentricity), repeated four times at each pupil entrance point and the average visibility was used to estimate the characteristic directionality parameter. The experimental results obtained were compared with numerical modeling based on cone photoreceptor waveguiding.
The parafoveal cones exhibit significantly higher directionality (approximately doubled) in the red spectral region (550-700nm) than foveal cones. In the blue region (400-500nm) the foveal and parafoveal cones have approximately the same directionality value but smaller than compared with the red region for both cases. For the red region, the observed increase in directionality with distance from the fovea may be explained by an increase in mode and waveguide width and thus with foveal distance in a fundamental mode propagation regime. Finally the fact that cones have lower directional of the the blue spectral region is due to the higher number of allowed modes making the SCE more uniform across the pupil.
We have performed new experiments of the SCE to study its spectral characteristics in more detail for the fovea and parafovea region. Our observations agree with a waveguide-based explanation of the effect. The directionality variation can be related to a spectral variation in coupling strength to individual photoreceptors. The model gives fundamental insight in to how the directionality is related with the modes of light propagation in the cones. Such insight is essential to improve vision simulations taking the biophotonics of the retina into account.
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