The Human Eye Can Self-correct Some Optical Faults

Results of the study, "Internal compensation for corneal astigmatism and high-order aberrations of the eye," were reported to the fourth International Congress of Wavefront Sensing and Aberration-free Refraction Correction, Feb. 14-16 in San Francisco, by Howard C. Howland, Jennifer E. Kelly and Toshifumi Mihashi. Howland is a Cornell professor of neurobiology and behavior and director of the university's Developmental Vision Laboratory; Mihashi is the chief scientist at the research institute of the Tokyo-based Topcon Corp., manufacturer of a wavefront analyzer used in the study; and Kelly is a Cornell senior who used the wavefront analyzer as part of her honors thesis by testing the vision of 20 other undergraduate students.

Wavefront analysis is a recently developed technique for "seeing," with computer-based mathematical simulation, more precisely what the eye perceives. A beam of harmless laser light shines through the eye's optics (the transparent cornea, which begins to focus light, and the lens, which completes the focusing) toward the retina, where millions of photoreceptor cones and rods line the rear surface of the eye.

As the light rays are reflected back through the internal optics and exit the eye, the wavefront analyzer measures and computes deviations from a perfectly formed light beam or test pattern a short distance in front of the eye. Light rays exiting an optically perfect eye should be perfectly parallel, but irregularities in the thickness or shape of the cornea or a less-than-perfect lens can cause the exiting light rays to become nonparallel. A test pattern (produced by light passing though regularly spaced lenslets to form a grid, something like the lines on graph paper) should form a regular array of luminous points in an optically perfect eye, but a distorted pattern can tell the wavefront analyzer a great deal about irregularities in the cornea and lens.

The Cornell study, which was funded, in part, by Topcon Corp., and built upon earlier research from Spanish colleagues, looked for ways the eye might compensate internally for several kinds of optical faults. Among them:

* Corneal astigmatism, which is caused by irregularities in the topography of the cornea and can produce a distorted image;

* lateral coma, a so-called high-order aberration that is caused by the line of sight not being along the axis of symmetry of the eye and produces comet-shaped images of points of light; and

* spherical aberration, also a high order aberration, is what afflicted the Hubble Space Telescope because its main mirror was too flat on the edge. Spherical aberration in the human eye is caused by a spherical-shaped cornea and produces blurring of the retinal image.

Howland, who for more than 20 years has studied the development of vision defects in children, has been particularly interested in the possibility of "feedback loops" by which the brain might direct parts of the eye to change shape and compensate for optical aberrations. Reporting on the wavefront analysis of internal compensation, he says: "We found compensation by internal optics for three kinds of corneal aberrations of the 12 different aberrations we investigated. We found no evidence of developmental compensation for spherical aberration, but we did find some evidence for developmental compensation for corneal astigmatism. We're beginning to think that compensation for lateral coma is genetic, not developmental."

He comments that all human eyes, even those that manage to produce perfect vision, have some deviations from the optically ideal properties in their constituent parts. "We're talking about living, biological tissue here. The form and function of anything that's living is a combination of its genetic background and the environment in which it is born, grows, lives and ages," he says, noting that many optical aberrations become more pronounced with advancing age.

"With a system as complex as vision, where so many things can go wrong, it's a wonder we can see at all. Now we're coming to realize," Howland says, "that visual acuity is a result of various component parts 'wanting' to see better, if you will. They seem to be able to sense aberrations and to change shape and function, to some extent, to produce a better result. Some of this compensation occurs early in life, as our visual system is developing -- and to a lesser degree throughout our lives. Other compensations occurred long before we were born, as our distant ancestors evolved more perfect senses.

"And when our eyes can't compensate internally," the neurobiologist says, "that's when we look to those astronauts of the medical world -- the optometrists and ophthalmic surgeons -- to fix what nature couldn't."