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.