Visual resolution decreases rapidly outside the foveal center. The anatomical and physiological basis for this reduction is unclear. We used simultaneous adaptive optics imaging and psychophysical testing to measure cone spacing and resolution across the fovea, and show resolution is limited by cone spacing only at the foveal center. Immediately outside the center, resolution is worse than cone spacing predicts and better matches the sampling limit of midget retinal ganglion cells.
Although imaging of the living retina with adaptive optics scanning light ophthalmoscopy (AOSLO) provides microscopic access to individual cells, such as photoreceptors, retinal pigment epithelial cells, and blood cells in the retinal vasculature, other important cell classes, such as retinal ganglion cells, have proven much more challenging to image. The near transparency of inner retinal cells is advantageous for vision, as light must pass through them to reach the photoreceptors, but it has prevented them from being directly imaged in vivo. Here we show that the individual somas of neurons within the retinal ganglion cell (RGC) layer can be imaged with a modification of confocal AOSLO, in both monkeys and humans. Human images of RGC layer neurons did not match the quality of monkey images for several reasons, including safety concerns that limited the light levels permissible for human imaging. We also show that the same technique applied to the photoreceptor layer can resolve ambiguity about cone survival in age-related macular degeneration. The capability to noninvasively image RGC layer neurons in the living eye may one day allow for a better understanding of diseases, such as glaucoma, and accelerate the development of therapeutic strategies that aim to protect these cells. This method may also prove useful for imaging other structures, such as neurons in the brain.
Morgan and colleagues demonstrated that the RPE cell mosaic can be resolved in the living human eye non-invasively by imaging the short-wavelength autofluorescence using an adaptive optics (AO) ophthalmoscope. This method, based on the assumption that all subjects have the same longitudinal chromatic aberration (LCA) correction, has proved difficult to use in diseased eyes, and in particular those affected by age-related macular degeneration (AMD). In this work, we improve Morgan's method by accounting for chromatic aberration variations by optimizing the confocal aperture axial and transverse placement through an automated iterative maximization of image intensity. The increase in image intensity after algorithmic aperture placement varied depending upon patient and aperture position prior to optimization but increases as large as a factor of 10 were observed. When using a confocal aperture of 3.4 Airy disks in diameter, images were obtained using retinal radiant exposures of less than 2.44 J/cm 2 , which is ~22 times below the current ANSI maximum permissible exposure. RPE cell morphologies that were strikingly similar to those seen in postmortem histological studies were observed in AMD eyes, even in areas where the pattern of fluorescence appeared normal in commercial fundus autofluorescence (FAF) images. This new method can be used to study RPE morphology in AMD and other diseases, providing a powerful tool for understanding disease pathogenesis and progression, and offering a new means to assess the efficacy of treatments designed to restore RPE health. L. Dunaief, J. Z. Baffi, and J. Ambati, "DICER1 deficit induces Alu RNA toxicity in age-related macular degeneration," Nature 471(7338), 325-330 (2011).
Myopic observers may not benefit to the same extent as emmetropes from adaptive optics (AO) correction in a visual acuity (VA) task. To investigate this, we measured AO-corrected VA in 10 low myopes and 9 emmetropes. Subjects were grouped by refractive error. Mean spherical equivalent refractive error was -2.73 D (SEM = 0.35) for the myopes and 0.04 D (SEM = 0.1) for the emmetropes. All subjects had best corrected VA of 20/20 or better. The AO scanning laser ophthalmoscope was used to project ultrasharp stimuli onto the retina of each observer. High-contrast photopic acuity was measured using a tumbling E test with and without AO correction. AO-corrected minimum angle of resolution was 0.61' (SEM = 0.02') for the myopes and 0.49' (SEM = 0.03') for the emmetropes. The difference between groups is significant (p = .0017). This effect is even greater (p = .00013) when accounting for spectacle magnification and axial length, with myopes and emmetropes able to resolve critical features on the retina with a mean size of 2.87 mum (SEM = 0.07) and 2.25 mum (SEM = 0.1), respectively. Emmetropes and low myopes will both benefit from AO correction in a VA task but not to the same extent. Optical aberrations do not limit VA in low myopia after AO correction. There is no difference in the high-order aberrations of emmetropes and low myopes. Retinal and/or cortical factors limit VA in low myopes after AO correction.
Adaptive optics imaging of cone photoreceptors has provided unique insight into the structure and function of the human visual system and has become an important tool for both basic scientists and clinicians. Recent advances in adaptive optics retinal imaging instrumentation and methodology have allowed us to expand beyond cone imaging. Multi-wavelength and fluorescence imaging methods with adaptive optics have allowed multiple retinal cell types to be imaged simultaneously. These new methods have recently revealed rod photoreceptors, retinal pigment epithelium (RPE) cells, and the smallest retinal blood vessels. Fluorescence imaging coupled with adaptive optics has been used to examine ganglion cells in living primates. Two-photon imaging combined with adaptive optics can evaluate photoreceptor function non-invasively in the living primate retina.
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