ITHACA, N.Y. -- While the vision-impaired Hubble Space Telescope needed
optical doctoring from shuttle astronauts, vision researchers back on Earth were
wondering if the human eye was clever enough to fix itself. Now a neurobiology
study at Cornell University suggests that internal parts of the eye indeed can
compensate for less-than-perfect conditions in other parts -- either
developmentally (during the lifetime of one individual) or genetically (over
many generations).
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."
Editor's Note: The original news release can be found
here.
Note: This story has been adapted from a news
release issued for journalists and other members of the public. If you wish to
quote any part of this story, please credit Cornell University as the
original source. You may also wish to include the following link in any
citation:
http://www.sciencedaily.com/releases/2003/02/030218085515.htm
