The correction of higher-order aberrations can also increase our ability to see "out", thereby offering the possibility of increased visual acuity, perhaps beyond the typical limit of 20/15. In theory, diffraction-limited optical cutoffs for 3 mm and 8 mm pupils would be high enough to yield retinal images of letter targets as small as 20/6.7 and 20/2.5, respectively. To illustrate what the retinal image would be like with super-normal optics, imagine yourself viewing the Statue of Liberty at a distance of 3 kilometers from a boat in the New York Harbor. At this distance the statue subtends almost 0.9 degrees, which is equivalent to that of a US quarter at 5 feet away. Under optimal viewing conditions and 20/15 vision (i.e. a normal 3 mm pupil), your retinal image of the statue would look like Figure 4A. If you view the statue through adaptive optics, programmed to fully correct all ocular aberrations across your 3 mm pupil, then the retinal image of the statue would look like Figure 4B. Notice the finer detail and higher contrast in the retinal image when the eye's aberrations are corrected. This illustrates that retinal image quality can be noticeably increased even for pupil sizes as small as 3 mm. Maximum retinal image quality can be obtained with the largest physiological pupil diameter (8 mm) and full correction of all ocular aberrations. This situation is depicted in Figure 4C, which shows that the theoretical maximum optical bandwidth that can be achieved with the human eye is six times greater than the optical cutoff of a normal eye with a 3 mm pupil (i.e. 20/2.5 versus 20/15).

Figure 4. Simulation demonstrating the improvement in retinal image quality expected by correcting the optical aberrations of the eye. (A) Normal optics, 3mm pupil. (B) Corrected optics, 3mm pupil. (C) Corrected optics, 8mm pupil. All three images were simulated under best refraction of sphere and cylinder. To avoid ocular chromatic aberrations, a narrow-band chromatic filter, centered at 555 nm, is placed in front of the eye. The wave aberration used in the simulation of a. is that of my eye, which has corrected 20/15 vision. The wave aberration measurement was collected by Larry Thibos and his graduate student Xin Hong.

Improving the quality of the retinal image is an important first step towards achieving super-normal visual acuity, but it may not be sufficient. This is because when optical limitations have been removed, visual performance now becomes constrained by neural factors. Specifically, the spacing between retinal photoreceptors represents a neural limitation to visual resolution which is only slightly higher than the normal optical limit. Consequently, increasing the quality of the retinal image will probably not yield a major increase in resolution acuity, although it could have a large impact on detection acuity.

Although there are many potential benefits of super-normal visual optics, there is at least one expected penalty. Given a dramatic increase in optical quality of the retinal image, the photoreceptor mosaic will appear relatively coarse by comparison, as shown in Figure 5. As a result of this mismatch, very fine spatial details in the retinal image will be smaller than the distance between neighboring cones and therefore will not be registered properly in the neural image. This mis-representation of the image due to neural undersampling by a relatively coarse array of photoreceptors is called "aliasing". However, for everyday vision the penalty of aliasing is likely to be outweighed by the reward of higher contrast sensitivity and higher detection acuity. Therefore we anticipate that correcting the eye's optical aberrations will yield a net increase in the quality of the patient's visual experience and therefore is worth pursuing. Indeed, our preliminary observations indicate that stimuli seen through adaptive optics have a strikingly crisp appearance expected of an eye with supernormal optical quality, which is consistent with the sixfold increase in contrast sensitivity we have measured experimentally.

Figure 5. An enlargement of the image of Figure 4C is overlaid with an hexagonally-packed mosaic of circles that represent the foveal cone mosaic. This neural mosaic is relatively coarse compared to the retinal image, which may introduce artifacts into the neural image and ultimately cause a kind of mis-perception called "aliasing".

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