Cornell Study: Human Eye Can Self-correct Some Optical Faults

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Cornell study proves The Human Eye Can Self-correct Some

Optical Faults

 

http://www.sciencedaily.com/releases/2003/02/030218085515.h

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First, the homeopathy findings, now this. It's been a good week...

 

Fran

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Source: Cornell University

 

Date:

2003-02-18

 

The Human Eye Can Self-correct Some Optical Faults

 

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.h

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