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How the eye’s natural adaptive
mechanism can compensate for corneal aberrations
Daniel
Keller PhD
in San Francisco
THE natural adaptive mechanism of the human visual system can compensate
for deficiencies in its own optics, including some common types
of corneal aberrations, researchers say.
Neurobiologist Howard C Howland PhD and colleagues used wavefront
analysis to study ocular aberrations in volunteers. Presenting his
results at the fourth International Congress of Wavefront Sensing
and Aberration-free Refraction Correction, he reported that the
visual system could compensate for three of the 12 kinds of corneal
aberrations.
The researchers used a Topcon KR 9000PW Wavefront Analyser to study
the eyes of 20 young, healthy undergraduate students whose refractions
were within +1.0 D to – 2.0 D of emmetropia.
They used the Topcon instrument to measure refraction, corneal topography
using 11 rings and the optical wavefront of the total eye. The topographical
and wavefront measurements were made along the same axis, which
was at the centre of the pupil, and measurements referred to aberrations
in the line of vision.
The degree of compensation was computed by comparing the wavefront
of the total eye to the corneal topography. The 11 infrared illuminated
rings are at a distance of about 30cm from the eye and reflected
off the cornea to measure its topography.
“We did find some evidence for developmental compensation
for corneal astigmatism. We see a good correlation between the amount
of astigmatism you have in the cornea and the amount of compensation
you have in the lens. The internal optics of the eye could compensate
for both corneal with and against-the-rule astigmatism,” Dr
Howland reported.
There was no correlation between the individual amount of spherical
aberration and the individual compensation. This suggests that it
wasn't an individual developmental feedback loop.
Rather, it looked like it was genetically determined. The feedback
loop was present by dint of natural selection in lens structure,
with higher power operating in the centre. Similarly, it appears
that compensation for lateral coma is genetic rather than developmental,
he said.
The Cornell investigators' work built upon research by Pablo Artal
and colleagues in Spain (Artal, P et al: Compensation of corneal
aberrations by the internal optics of the human eye. Journal of
Vision 2001;1:1-18), who had examined the total and corneal aberrations
of the eye.
The Spanish researchers found that the cornea had both spherical
aberration and coma and somehow the eye's internal optics could
compensate for corneal irregularities.
Such compensation may wane with age since aberrations appeared to
worsen, according to findings from McLellan et al (Age-related changes
in monochromatic wave aberrations of the human eye. Investig Ophthal
and Vis Sci 2001;42 (6):1390-1395) and Artal et al (Contribution
of the cornea and internal surfaces to the change of ocular aberrations
of the eye. J Opt Soc Am A 2002;19(1):137-143).
Dr Howland explained that in a system as complex as vision, it appears
that visual acuity is a result of various component parts 'wanting'
to see better. The parts seem to be able to sense aberrations and
then change shape and function.
Some compensation occurred across the evolution of the species,
but others are more individualised, occurring early in life and
even throughout life.
The underlying neurobiology of the developmental compensatory effects
is not yet known with any degrees of certainty. Experiments have
shown a feedback mechanism in lower animals as well as primates
which compensates for defocus. The eye grows in length so that the
image falls on the retina.
Dr Howland gave an example of a weak cornea and lens system, where
the image falls behind the retina and the eye elongates so that
the location of the retina will correspond to the point of focus
of the image.
In the case of astigmatism, the compensation probably comes from
the lens and not by asymmetrical elongation of the eye.
“The lens grows continuously throughout life. A curve of the
weight of the lens with age would be a straight ascending line.
It's continuously laying on new lens fibres. However, the question
is how would it know where to put them and how would the growth
signal get there?,” Dr Howland asked.
He proposed that the ciliary muscle may have a role, but no one
has determined a mechanism. In addition, the inner surface of the
cornea may make slight changes. But since the rear surface of the
cornea is in contact with the vitreous and has a similar refractive
index, it probably does not have much focusing power.
“We know there's compensation. We know it's probably in the
lens. But we don't know how developmental compensatory effects take
place,” he said.
In the case of spherical aberration, Dr Howland explained that the
mechanism is a long-term genetic effect. The immediate clinical
significance of this work may relate to children with irregular
astigmatism which is not correctable with lenses. Clinicians may
be less likely to intervene during childhood since these eyes may
internally compensate over time.
“When the eyes cannot compensate internally, that's when we
look to those astronauts of the medical world, the ophthalmic surgeons,
to fix what nature could not,” Dr Howland said.
Howard
C Howland PhD
Cornell University in Ithaca, New York, US
Email: hch2@cornell.edu
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