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E.A. Rossi, A. Roorda; The Limits of High Contrast Photopic Visual Acuity with Adaptive Optics . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5402.
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© ARVO (1962-2015); The Authors (2016-present)
To examine the extent to which photopic high contrast visual acuity with adaptive optics (AO) correction is limited by optical, retinal and neural factors.
The adaptive optics scanning laser ophthalmoscope (AOSLO) was used to project a high contrast AO–corrected stimulus onto the retina of six observers (four emmetropes and two myopes). High contrast photopic letter acuity was measured using a four alternative forced choice tumbling E test and high contrast photopic grating acuity was measured using a two alternative forced choice truncated square wave test. Visual acuity was assessed under the following conditions: 1) spectacle correction without AO 2) 3 mm pupil with AO correction and 3) 5.81 mm pupil with AO correction.
The range of measured acuities for the three conditions were as follows: 1) spectacle correction without AO: 20/10.25 to 20/18.73 2) 3 mm pupil with AO: 20/6.56 to 20/13.97 and 3) 5.81 mm pupil with AO: 20/7 to 20/13.68. Three of the emmetropic observers consistently achieved acuities below 20/8. There was no significant difference in performance between the 3 mm and 5.81 mm AO conditions. Acuities measured using a truncated square wave grating did not differ significantly from those measured using the tumbling E test. A learning experiment in which observers were trained daily for five days in the tumbling E acuity test with explicit feedback did not show an increase in performance with training. Measurements of the residual aberrations after AO correction showed that optics were not a limiting factor for either AO condition.
Previous studies have found that the peak cone density in normal eyes imposes a sampling limit of around 60 c/deg, which, to first order, converts to an equivalent Snellen acuity of 20/10. However, after AO correction, observers often performed better than that predicted by sampling theory. Although the 3 mm and 5.81 mm conditions perform similarly, there are likely different mechanisms that limit the visual acuity for each condition. The 3 mm pupil may be limited by a combination of low contrast (due to diffraction) and retinal sampling limits. For the 5.81 mm pupil, the benefits of higher contrast at lower spatial frequencies are offset by the presence of higher spatial frequencies beyond the photoreceptor Nyquist limit, which introduce aliasing distortions.
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