The above figure shows the schematic configuration of our adaptive optics system for the human eye. A superluminescent diode (790 nm) forms a point source at the retina. The reflected light from this beacon is delivered to the wavefront sensor, and the wave aberration of the eye is calculated. The 97-channel deformable mirror is then shaped to compensate for the eye's aberrations, resulting in a nearly planar wavefront. Fig. 7 shows the performance of the adaptive optics system. The residual wave aberration of the eye (6.8 mm pupil), measured with the wavefront sensor, is reduced after adaptive optics compensation (Fig. 7a), as illustrated by the nearly planar (flat) wavefront maps. We can also obtain a tighter point spread function (PSF) after using adaptive optics, as show in Fig. 7b, improving the Strehl ratio by an order of magnitude.

High Resolution Images of the Cone Mosaic

After the eye's wave aberration has been corrected, we can acquire high resolution retinal images. We use a Krypton flash lamp to illuminate the retina, and an imaging CCD camera captures the light reflected from the retina. Miller, Williams, Morris, & Liang (1996), who did not use adaptive optics, had previously shown that in eyes with exceptional optical quality, single cones could be resolved in highly magnified images of the human retina. As shown in Fig. 8, however, the retinal images are far clearer with adaptive compensation. Fig. 8a shows one of the best images that could be obtained when only defocus and astigmatism were corrected in this subject, taken at an eccentricity of 1 degree from the fovea (6.0 mm pupil). Fig. 8b shows a single image obtained when we use adaptive optics to correct all of the eye's aberrations. In this single image, we can now distinctly resolve individual photoreceptors in the living eye. By registering and averaging many images together, we can further improve the quality of our retinal images, as shown in Fig. 8c (Roorda & Williams, 1999; Williams & Roorda, 1999), where each circle represents a single cone photoreceptor. The dark line running vertically across the middle of all three images is an out-of-focus capillary that lies in a plane that is above the in-focus photoreceptor layer.

Directional Sensitivity of Individual Human Cones

The Stiles-Crawford effect, measured psychophysically by stimulating larger numbers of cones, is a combination of the waveguide properties of single photoreceptors and the disarray in individual cone pointing direction. However, it has not been possible to disentangle these factors with direct measurements in the human eye until recently. Austin Roorda used our adaptive optics system to measure for the first time the angular tuning properties of individual human cones and the disarray in individual cone axes that contributes to the angular tuning properties of the retina as a whole (Roorda & Williams, 2001). Fig. 9a shows images of the same patch of cones when they are illuminated with light entering different locations of the pupil. The image at the center corresponds to illumination at the pupil center and the surrounding images correspond to oblique illumination 1.7 mm from the pupil center. We can nicely see the waveguide properties of the photoreceptors in this figure, as the photoreceptors preferentially reflect light when illuminated on-axis instead of obliquely.

By comparing the intensities of each cone under the different illumination conditions, we can determine the directional sensitivity of each cone. Fig. 9b shows the pointing direction of each cone relative to the center of the pupil for two subjects. For each subject, all the cones are tuned to approximately the same direction. The disarray in cone pointing direction is very small compared with the width of the tuning function for a single cone. This shows that the Stiles-Crawford effect is a good estimate of the angular tuning of single cones.

Miller D.T., Williams D.R., Morris G.M, & Liang J., (1996). Images of cone photoreceptors in the living human eye. Vision Research, 36, 1067-1079.

Roorda A., & Williams DR, (1999). The arrangement of the three cone classes in the living human eye. Nature, 397, 520-522.

Williams DR, & Roorda A., (1999). The trichromatic cone mosaic in the human eye. In K.R. Gegenfurtner & L.T. Sharpe (eds.), Color Vision: From Genes to Perception (pp. 113-122). New York: Cambridge University Press.

Roorda A., & Williams DR, (2001). Retinal imaging using adaptive optics. In S. MacRae, R. Krueger, & R.A. Applegate (eds.), Customized Corneal Ablation: The Quest for SuperVision (pp. 11-32). Thorofare: SLACK, Inc.

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