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P. De Gracia, C. Dorronsoro, G. Marin, M. Hernandez, S. Marcos; Visual Performance With Combined Astigmatism and Coma: Optical and Neural Adaptation Effects. Invest. Ophthalmol. Vis. Sci. 2010;51(13):6344.
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To determine whether specific combinations of astigmatism and coma can provide better optical and visual performance than astigmatism alone. To study whether adaptation to natural astigmatism may play a role in visual performance under those conditions.
Thru-focus computations of the Strehl Ratio (SR) were performed for a combination of astigmatism ranging from 0.25 to 1 D, coma ranging from 0.03 to 0.71 µm and defocus ranging from -2 to 2 D, for 6 and 4-mm pupils. Visual acuity (VA, using a tumbling E letter test) was measured under specific combinations of astigmatism and coma, astigmatism alone and all high order aberrations (HOA) corrected. Measurements were performed with a custom-developed adaptive optics system on 20 young subjects. Natural astigmatism was < 0.25 D in 10 subjects usually corrected (0.50-1.50 D) in 4 subjects, and usually uncorrected in 6 subjects (0.25-0.75 D).
Simulations show an increase in optical quality (SR) for certain combinations of astigmatism and coma. For example, in the presence of 0.5 D of astigmatism, adding 0.23 µm of coma (at a relative angle of 0 degrees) increased SR by 1.7 over astigmatism alone. In some particular conditions, adding coma from 0.15 to 0.36 µm can also improve SR. All subjects increased VA when HOA aberrations were corrected over natural aberrations, on average x1.23. Combinations of 0.5 D of astigmatism with 0.23 µm of coma produced an improvement of VA over 0.5 D of astigmatism alone in 12 subjects. Subjects with no natural astigmatism improved by 1.12 when coma was added to the induced astigmatism, while subjects with astigmatism (usually corrected) improved only by 1.07. However, astigmatic subjects usually uncorrected decreased VA (x0.90) when coma was added to astigmatism, over astigmatism alone. In fact, VA in those subjects with all HOA corrected was not significantly different to VA with 0.5 D of astigmatism only.
Combining coma with astigmatism can improve visual acuity over astigmatism alone, as predicted purely by the optics. Improvement values are highly correlated with the amount of non corrected natural astigmatism present. The protective effect of coma occurs only in eyes with no natural astigmatism (and to a less extent in eyes usually corrected for astigmatism). Results suggest that adaptation to natural astigmatism may occur.
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