High-resolution retinal imaging with adaptive optics was used to record the position of a light stimulus on the cone mosaic, with an error at least five times smaller than the diameter of the smallest foveal cones. We discuss the factors that limit the accuracy with which absolute retinal position can be determined. In five subjects, the standard deviation of fixation positions measured in discrete trials ranged from 2.1 to 6.3 arcmin, with an average of 3.4 arcmin (about 17 microm), in agreement with previous studies (R. W. Ditchburn, 1973; R. M. Steinman, G. M. Haddad, A. A. Skavenski, & D. Wyman, 1973). The center of fixation, based on the mean retinal position for each of three subjects, was displaced from the location of highest foveal cone density by an average of about 10 arcmin (about 50 microm), indicating that cone density alone does not drive the location on the retina selected for fixation. This method can be used in psychophysical studies or medical applications requiring submicron registration of stimuli with respect to the retina or in delivering light to retinal features as small as single cells.

Adaptive optics is a relatively new field, yet it is spreading rapidly and allows new questions to be asked about how the visual system is organized. The editors of this feature issue have posed a series of question to scientists involved in using adaptive optics in vision science. The questions are focused on three main areas. In the first we investigate the use of adaptive optics for psychophysical measurements of visual system function and for improving the optics of the eye. In the second, we look at the applications and impact of adaptive optics on retinal imaging and its promise for basic and applied research. In the third, we explore how adaptive optics is being used to improve our understanding of the neurophysiology of the visual system.

Purpose To characterize the rod and cone photoreceptor mosaic at retinal locations spanning the central 60°in vivo using adaptive optics scanning laser ophthalmoscopy (AO-SLO) in healthy human eyes. Methods AO-SLO images (0.7 × 0.9°) were acquired at 680 nm from 14 locations from 30°nasal retina (NR) to 30°temporal retina (TR) in 5 subjects. Registered averaged images were used to measure rod and cone density and spacing within 60 × 60 μm regions of interest. Voronoi analysis was performed to examine packing geometry at all locations. Results Average peak cone density near the fovea was 164 000 ± 24 000 cones/mm 2 and decreased to 6700 ± 1500 and 5400 ± 700 cones/mm 2 at 30°NR and 30°TR, respectively. Cone-to-cone spacing increased from 2.7 ± 0.2 μm at the fovea to 14.6 ± 1.4 μm at 30°NR and 16.3 ± 0.7 μm at 30°TR. Rod density peaked at 25°NR (124 000 ± 20 000 rods/mm 2 ) and 20°TR (120 000 ± 12 000 rods/mm 2 ) and decreased at higher eccentricities. Center-to-center rod spacing was lowest nasally at 25°(2.1 ± 0.1 μm). Temporally, rod spacing was lowest at 20°(2.2 ± 0.1 μm) before increasing to 2.3 ± 0.1 μm at 30°TR. Conclusions Both rod and cone densities showed good agreement with histology and prior AO-SLO studies. The results demonstrate the ability to image at higher retinal eccentricities than reported previously. This has clinical importance in diseases that initially affect the peripheral retina such as retinitis pigmentosa.

AO fundus imaging is a reliable technique for assessing and quantifying the changes in the photoreceptor layer as disease progresses. Furthermore, this technique can be useful in cases where visual function tests provide borderline or ambiguous results, as it allows visualization of individual photoreceptors.

Aims-It is well established that glaucoma results in a thinning of the inner retina. To investigate whether the outer retina is also involved, ultrahigh-resolution retinal imaging techniques were utilised.Methods-Eyes from 10 glaucoma patients (25-78 years old), were imaged using three researchgrade instruments: (1) ultrahigh-resolution Fourier-domain optical coherence tomography (UHR-FD-OCT), (2) adaptive optics (AO) UHR-FD-OCT and (3) AO-flood illuminated fundus camera (AO-FC). UHR-FD-OCT and AO-UHR-FD-OCT B-scans were examined for any abnormalities in the retinal layers. On some patients, cone density measurements were made from the AO-FC en face images. Correlations between retinal structure and visual sensitivity were measured by Humphrey visual-field (VF) testing made at the corresponding retinal locations.Results-All three in vivo imaging modalities revealed evidence of outer retinal changes along with the expected thinning of the inner retina in glaucomatous eyes with VF loss. AO-UHR-FD-OCT images identified the exact location of structural changes within the cone photoreceptor layer with the AO-FC en face images showing dark areas in the cone mosaic at the same retinal locations with reduced visual sensitivity.Conclusion-Losses in cone density along with expected inner retinal changes were demonstrated in well-characterised glaucoma patients with VF loss.

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Ophthalmic instrumentation equipped with adaptive optics offers the possibility of rapid and automated correction of the eye's optics for improving vision and for improving images of the retina. One factor that limits the widespread implementation of adaptive optics is the cost of the wave-front corrector, such as a deformable mirror. In addition, the large apertures of these elements require high pupil magnification, and hence the systems tend to be physically large. We present what are believed to be the first closed-loop results when a compact, low-cost, surface micromachined, microelectromechanical mirror is used in a vision adaptive-optics system. The correction performance of the mirror is shown to be comparable to that of a Xinetics mirror for a 4.6-mm pupil size. Furthermore, for a pupil diameter of 6.0-mm, the residual rms error is reduced from 0.36 to 0.12 microm and individual photoreceptors are resolved at a pupil eccentricity of 1 degrees from the fovea.

The contributions of optical and neural factors to age-related losses in spatial vision are not fully understood. We used closed-loop adaptive optics to test the visual benefit of correcting monochromatic high-order aberrations (HOAs) on spatial vision for observers ranging in age from 18 to 81 years. Contrast sensitivity was measured monocularly using a two-alternative forced-choice (2AFC) procedure for sinusoidal gratings over 6 mm and 3 mm pupil diameters. Visual acuity was measured using a spatial 4AFC procedure. Over a 6 mm pupil, young observers showed a large benefit of AO at high spatial frequencies, whereas older observers exhibited the greatest benefit at middle spatial frequencies, plus a significantly larger increase in visual acuity. When age-related miosis is controlled, young and old observers exhibited a similar benefit of AO for spatial vision. An increase in HOAs cannot account for the complete senescent decline in spatial vision. These results may indicate a larger role of additional optical factors when the impact of HOAs is removed, but also lend support for the importance of neural factors in age-related changes in spatial vision.