Aberrations of the Eye - Crude Flaws or Ecological Design?

Dear Editor: The recent editorial by Professor Rafael Navarro1 in the Journal of Optometry entitled “Darwin and the Eye” celebrates the bicentennial of Charles Darwin's birth. The editorial invites discussion of the influence of creationism and evolution on our views of the optics of the eye, and provides a thoughtful critique. Professor Navarro is puzzled by the mixture of “smart solutions and crude flaws” that seem to coexist in the eye's design. The words we use routinely to describe aberrations reveal a deep zeitgeist regarding the photographic “camera” model of the “chambered” eye and the role that aberrations play in vision science. Zeitgeist refers to ideas that are prevalent in a period and place and that are expressed in literature, philosophy, and religion. Creationism and Darwinism both nurture the notion that aberrations are defects that distort and impair the retinal image; they are removed in part by accommodation, emmetropization, neural adaptation, spectacles and contact lenses as well as laser surgery. Whether by intelligent design or by evolution, the idea of a perfect aberration-free eye, like a Leica or a Canon camera, forming a perfect image of the world on the retina is appealing. James Gibson2 contributed to the Zeitgeist by insisting that the optics of the eye and the resulting retinal image are irrelevant for ecological optics. In Gibson's view, information from the environment is structured into the optic array, and the eye simply “picks up” invariant relationships in the array that specify the environment. In this model the “perfect” eye all but disappears! Ecological optics2 started a revolution in visual optics and perception sixty years ago. Gibson described an exquisite solid geometry termed “ambient optic array” that represents the environment from an observer's point of view. The geometry is a solid visual “cone” comprised of an array of solid angles, having all a common apex at the point of observation. Surfaces and objects in the environment serve as bases of the solid angles, and the solid angles are separated by “transitions of brightness and color” that represent salient features of the environment, like edges. The geometry of the ambient optic array has its base in the environment and apex at the pupil centre or nodal point. The solid geometry is reversed in geometrical optics, where the area of the pupil is a common base for a large array of solid angles (composed of pencils or bundles of rays), each having a separate apex located in the environment. The two geometries are congruent when the ambient optic array is extended to every point of observation across the area of the extended pupil. Once pupil area, refraction and dispersion are included in the model, diverging rays and wavefronts with optical vergence enhance Gibson's transitions of brightness and color at edges, and the retinal image is overlaid with a polychromatic blur that specifies relative depth3 and that drives accommodation.4-6 Vergence, refraction and chromatic dispersion across the pupil for an accommodating eye are illustrated in figure 1. Figure 1 Accommodating eye illustrating vergence, refraction and dispersion across the pupil and effects across the retina. (Link to movie in electronic PDF version, available at: www.journalofoptometry.org) In the prevailing model of the eye, aberrations are imperfections, distortions, crude flaws or noise that blurs the retinal image, especially at high spatial frequencies (30 c/deg or 6/6 Snellen acuity). But defocus and chromatic aberration provide strong signals with ecological validity at intermediate spatial frequencies7,8 (3 c/deg), where their effects are moderate. The eyes of fishes, chicks, guinea pigs, tree shrews, cats, marmosets and monkeys compensate for positive and negative vergence by changing its axial length.9-11 Hyperopic defocus accelerates the rate of elongation and thins the choroids of chicks, while myopic defocus slows the rate of elongation and thickens the choroid.12 Emmetropization and accommodation both operate in monochromatic light, and accommodation responds without feedback from defocus4 or higher-order aberrations.13 This suggests a sensitivity to the wavefront vergence across the exit pupil of the eye, rather than to blur of the retinal image alone. While our idealized model of the eye remains constrained by zeitgeist and dogma, Darwin's eye, less encumbered by word or name or myth, embraces nature's “crude flaws” as distance and relative depth…