How Far Can We Extend the Limits of Human Vision ?

Methods to correct the optics of the human eye are at least 700 years old. Spectacles have been used to correct defocus at least as early as the 13th century1,2 and to correct astigmatism since the 19th century.3 Though it is well established that the eye suffers from many more monochromatic aberrations than defocus and astigmatism, aberrations we will refer to as higher-order aberrations, there has been relatively little work on correcting them until recently. In 1961, Smirnov, an early pioneer in the characterization of the eye’s higher-order aberrations, suggested that it would be possible to manufacture customized lenses to compensate for them in individual eyes.4 Recent developments increase the probability that Smirnov’s suggestion may soon be realized. More rapid and accurate instruments for measuring the ocular aberrations are available, most notably the Shack-Hartmann wavefront sensor, first applied to the eye by Liang et al.5 Moreover, there are new techniques to correct higher-order aberrations. Liang et al showed that a deformable mirror in an adaptive optics system can correct the eye’s higher-order aberrations.6 This study was the first to demonstrate that the correction of higher-order aberrations can lead to supernormal visual performance in normal eyes. Presently, the visual benefits of adaptive optics can only be obtained in the laboratory due to the relatively large size and high cost of conventional deformable mirrors. Alternative wavefront correctors that are less expensive and more compact, such as Micro Electrical Manufacture Systems (MEMS) technology and liquid crystal spatial light modulators, offer the exciting possibility of developing new diagnostic tools incorporating adaptive optics that every clinician would ultimately be able to afford. Nevertheless, the success of adaptive optics encourages the implementation of higher-order correction in everyday vision through customized contact lenses, intraocular lenses (IOLs), or laser refractive surgery. Lathing and laser ablative technologies now exist that can create arbitrary surfaces on contact lenses, offering the possibility of truly customized contact lenses. It has also been shown that conventional IOLs do not produce optimal retinal image quality after cataract surgery, and an IOL that is designed to compensate for the corneal aberrations of the eye would yield better visual outcomes.7 Finally, there is a major ongoing effort to refine laser refractive surgery to correct other defects besides conventional refractive errors.8-11 Ultimately, the visual benefit of attempts to correct higherorder aberrations depends on two things. First, it depends on the relative importance of these aberrations in limiting human vision, and second, on the finesse with which these aberrations can be corrected in everyday vision. The emphasis of this chapter is on the first of these issues: How large are the visual benefits that will accrue from correcting higher-order aberrations and under what conditions will they be realized? We review what is known about the fundamental limits on visual acuity and provide theoretical and empirical evidence concerning the visual significance of higher-order aberrations. There are optical, cone mosaic, and neural factors that limit the finest detail we can see, and an understanding of all three is required to appreciate how much vision can be improved by correcting higher-order aberrations in addition to defocus and astigmatism. For example, as we will see later, improving the eye’s optics is not always a good thing. Due to the nature of the limits on visual resolution set by the cone mosaic, improving the optical quality of the eye too much can actually lead to a decline in visual performance on some tasks. Before tackling this apparent paradox, however, we need to review some fundamental aspects of the optical quality of the retinal image.