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Re: An eye exam



I may have jumped to an unwarranted conclusion. The web site refers to the
increase in binocular visual acuity. The statement might be an indication
that the effect was already well known, but not well measured. In either
case they made no mention of historical papers on the subject. I suspect
there may be some older papers mentioned in their references.

The mechanism behind the effect may not be well known, but similar things
can be done with telescopes. It is possible to link several telescopes to
produce sharper images, and astronomers are now doing this.

If the information presented by both eyes is identical one can not do
anything to sharpen the image since neither eye gives information that can
be used to correct the other. Since each eye is looking at a different
angle the information is not exactly the same. The nonuniformity of the
eyes probably makes this effect larger. Apparently the brain eye system is
able to take advantage of this and in some fashion figure out what things
really look like. A good example of this effect is the fact that you do not
normally see colored fringes around objects. Yet you know that optical
systems produce such fringes. Just look through a simple optical system
such as a Viewmaster and you will see fringes around many of the things you
are viewing. The eye has a lens that is probably not better than that in
such simple optical devices, yet the brain/eye combination gets rid of most
of these visual problems. Eyeglasses are certainly successful in treating
blurred vision, which the brain can only partially fix.

Since the eye is intimately connected to the brain, it is difficult to
disentagle the contributions of the retina from those of the brain.
Apparently the brain in some simple fashion does a convolution to partially
reverse the distortions introduced by the optical system. Likewise it
combines the information from both eyes to get a sharper image. I think a
better analogy would the way you can take advantage of wider bandwidth in a
signal to reduce the signal to noise ratio. This is a problem that would
have been ideas for investigation by Edwin Land. His retinex theory is a
neat model of how the eye can perceive the "true" color of objects in
varying light conditions. I suspect he could come up with a good model for
how the brain might process 2 images to improve resolution. The optometrist
explanation "synergetic effect" sounds like explaining why objects fall by
labeling it "gravity". There is a name for the effect, but has someone
modeled it?

John M. Clement
Houston, TX


John Clement wrote:

... I am surprised that the effect [vision with two eyes being
better than with each eye alone] was discovered only in the 90s...

Ludwig Kowalski wrote:
Me too. But more surprising is the effect itself, it seems to
contradict the common "blurred retinal image" explanations.
If one retinal image is strongly blurred (due to astigmatism,
myopia or hyperopia) while another is sharp then how the
two-eyes vision can lead to a better recognition of details?
The blurred image explanation of visual defects can not be
wrong; it is validated by successful eyeglass industry. (If
I recall correctly eyeglasses were already used in Europe
in 12th century.)

But some secondary effects, not accounted by geometrical
optics, may dominate when both eyes are nearly the same.
What is the mechanism of the two eyes cooperation? I do
not know. The obvious analogy is a weight suspended by
two strings; each string would break under the load but
two strings are able to hold it. At which extreme is the
two-eyes cooperation more pronounced; when both retinal
images are very good or when they are both very bad? I
suspect that the level of illumination (day vision versus
evening vision) is likely to be a factor in this effect. An
"extra-credit" project for a group of students? Why not.
Ludwik Kowalski

This URL leads one to expect an 11% or a half line
improvement on a visual chart on average for binocular
versus monocular testing.

<http://www.lighthouse.org/research_acuity.htm>

Brian Whatcott