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Home > Archive > Lasik Eyes Surgery > September 2005 > Stiles-Crawford effect
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Stiles-Crawford effect
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| Pauli Soininen 2005-08-15, 5:55 pm |
| Stiles-Crawford effect may have something to do with how severely spherical
aberration is perceived. Now, does someone here understand exactly how the
theory goes? Are there some images (of rays passing through the eye optics)
to explain how this theory might actually work?
Not just how the receptors are angled generally, but how an image of a lamp
for example would form on the retina and how the rays would be drawn in that
situation (and in what angle does the "spherically aberrated" rays hit the
receptors).
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| EyesRBAD 2005-08-17, 10:57 pm |
| Yes, the Stiles Crawford effect determines how much spherical aberration you
will have.
LASIK lasers were never made to treat something round like an eye. The laser
is very close and high
energy at the center of the eye where it is highest, but when the eye falls
away toward the periphery,
energy falls away and the treatment is less. This causes an undertreated
periphery (undersized effective
optical zone) and the unfocused light entering from the untreated and
undertreated periphery is spherical
aberration. Get it?
The rays coming in on the untreated area are not focused on the retina. They
contribute to blur.
"Pauli Soininen" <no@spam.com> wrote in message
news:r%4Me.350$383.90@read3.inet.fi...
> Stiles-Crawford effect may have something to do with how severely
> spherical aberration is perceived. Now, does someone here understand
> exactly how the theory goes? Are there some images (of rays passing
> through the eye optics) to explain how this theory might actually work?
>
> Not just how the receptors are angled generally, but how an image of a
> lamp for example would form on the retina and how the rays would be drawn
> in that situation (and in what angle does the "spherically aberrated" rays
> hit the receptors).
>
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| Pauli Soininen 2005-08-18, 8:59 am |
| > The laser is very close and high energy at the center of
> the eye where it is highest, but when the eye falls away
> toward the periphery, energy falls away and the
> treatment is less.
I'm pretty sure the little difference in length between the laser device and
the eye's center and peripheral areas have absolutely nothing to do with the
amount of the energy the laser has as it arrives on the eye.
Of course the ablation is calculated beforehand and the calculated ablation
is exactly what the laser will do. It is already calculated to ablate less
in the peripheral, there should be no question about it. Then it is another
question how the end result will be as the flap placement etc. will have
drastic effect on the actual refractive power in different locations of the
cornea.
> the unfocused light entering from the untreated and
> undertreated periphery is spherical aberration.
I was trying to get information related to Stiles-Crawford effect, in-depth
information that is.
> The rays coming in on the untreated area are not focused
> on the retina. They contribute to blur.
Yes, that's the first thing to know about refractive surgery, in my opinion.
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| Pauli Soininen 2005-08-18, 5:56 pm |
| I could make an easier question here so that it's easier to answer.
Does Stiles-Crawford effect reduce or grow the unwanted visual distortion
that is caused by lasik (or lasek/prk) induced spherical aberration?
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| Glenn - USAEyes.org 2005-08-18, 5:56 pm |
| The retina cones will move in response to diffused light, seeking more
direct light. This can often reduce visual effects caused by what
shows in Hartman-Shack wavefront aberrometers as spherical aberration.
When someone with virgin eyes and a poor optic system has good vision,
this natural adjustment in direction of the cones is often cited as a
possible explanation. When someone with induced poor vision after
LASIK has improved vision after the healing period, this is also often
attributed to the Stiles-Crawford effect. There is nothing to
scientifically back that up, but it is a reasonable theory.
Changes in the Stiles-Crawford effect happen naturally, however there
is no way to promote the change or predict if it will or will not
occur.
Glenn Hagele
Executive Director
USAEyes.org
"Consider and Choose With Confidence"
Email to glenn dot hagele at usaeyes dot org
http://www.USAEyes.org
http://www.ComplicatedEyes.org
I am not a doctor.
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| Pauli Soininen 2005-08-18, 5:56 pm |
| That is easily the best theory so far (to how improvement can occur). It is
only very recently that I heard about this. Is it new to you also?
If the receptors' angle would be measured (ultrasound? photograph?) pre and
post (6 months or more) RS, then that would certainly help a lot to backing
this theory up. This with a patient who noticed gradual improvement that is.
In one source*, the effect of Stiles-Crawford was said to be about four
times (or more than four) of more sensitivity. That combined with
Weber-Fechner Law (a worsening effect from the spherical aberration point of
view) and some moderate neural adaptation I don't know if it's still enough
to explain total elimination of starburst/halo induced by spherical
aberration. But definitely it's an acceptable theory.
* = http://www.handprint.com/HP/WCL/color1.html (find "Crawford")
To explain it to those who might be interested, the Stiles-Crawford effect
is simply suggesting, that applied to lasik, receptors change their angle to
point directly at the center of pupil, where there is no spherical
aberration. That way, rays coming from the peripheral of cornea are not
hitting directly in the receptor "pipe" and hence are not sensed as strongly
as those hitting directly in the bottom of the "pipe". And it is good to
understand, that basically all points in the natural image we see are
generally reflecting light to all directions (except "backwards") and all
points in the cornea are refracting rays to all points in retina. This of
course in a logical fashion, not just randomly. Because of this, we can for
example read 100% all of text in an image with one eye, even we put an thin
obstacle like a 2mm thick wooden ruler right in front of the eye sideways).
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| Glenn - USAEyes.org 2005-08-18, 5:56 pm |
| Stiles-Crawford effect has been known for quite some time, but it was
about three or four years ago that I first heard it offered as an
explanation for unexplainable improvement in vision after refractive
surgery.
You seem to be on the right track regarding resolution of aberrations
after the healing period. Undoubtedly it is the combination of these
(and probably more) influences that combine to help correct the
problems.
Glenn Hagele
Executive Director
USAEyes.org
"Consider and Choose With Confidence"
Email to glenn dot hagele at usaeyes dot org
http://www.USAEyes.org
http://www.ComplicatedEyes.org
I am not a doctor.
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| George Warner 2005-09-24, 2:40 pm |
| I've never heard of the Stiles-Crawford effect explaining
improvement in vision after refractive surgery.
The effect does explain how people with large pupils can still
get good results with a laser that doesn't fully correct the
full pupil area (i.e. laser optical zone smaller than pupil).
The retina is less sensitive to light rays passing through the
outer ring of the pupil that may not be fully corrected.
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| Glenn - USAEyes.org 2005-09-24, 2:40 pm |
| There is typically a nasal tilting of receptors in the nasal retina of
myopic patients. When the myopia is removed through refractive
surgery, the receptors are effectively misaligned. This can cause poor
vision quality in low light environments. After a period of time these
receptors realign to the new refractive error, thus improving the
quality of vision.
Ophthalmic Physiol Opt. 2003 Sep;23(5):473-6
Shows Stiles-Crawford effect of myopia on anisometropic patient after
PRK in the myopic eye.
Glenn Hagele
Executive Director
USAEyes.org
"Consider and Choose With Confidence"
Email to glenn dot hagele at usaeyes dot org
http://www.USAEyes.org
http://www.ComplicatedEyes.org
I am not a doctor.
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| Pauli Soininen 2005-09-24, 2:40 pm |
| I hope to understand this better. Things I don't understand:
From the study:
"The emmetropic eye showed the normal well-centered SCE-I functions across
the retina, whereas the myopic eye of the same subject showed the nasal
tilting of receptors in the nasal retina, which has been the typical finding
among myopic subjects."
Is this nasal tilting in the nasal retina (on myopic subjects) a thing that
worsens their vision, improves it or doesn't have any effect?
"The eye that underwent the PRK procedure showed results that were typical
of myopic eyes"
And this means what - that the lack of "normalization" (= centralization?)
of the tilt worsens, improves or doesn't have any effect?
Glenn wrote:
> After a period of time these receptors realign to the
> new refractive error, thus improving the quality of vision.
I think the only logical thing the receptors can do to improve spherically
aberrated vision by changing the tilt is to point at the center of the
cornea, where the refraction matches the length from the cornea to the
retina.
Nasal tilting sounds like tilting strongly towards the nose. That should be
worse than towards the center of cornea. What is the actual conclusion of
that study? They should not have us pay for the full versions...
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| Glenn - USAEyes.org 2005-09-24, 2:40 pm |
| Let us say that light passing through the cornea of a normally sighted
person hits the retina at a 30 degree angle. The receptors in the
retina will be aimed at that 30 degree angle to catch the light
precisely and provide the best possible vision.
Let us say that light passing through the cornea of a myopic
(nearsighted) person hit the retina at a 45 degree angle. The
receptors will be aimed to this 45 degree angle because this is the
best angle to catch the light coming into a myopic eye.
When a person who is nearsighted (45 degree angle) has refractive
surgery to remove the myopia, the angle at which the light hits the
retina is changed to 30 degrees, but the receptors are still aimed at
45 degrees. Some of the light is hitting the side of the receptors and
is lost.
This is a gross oversimplification, but as this peer-reviewed study
indicates, the receptors in a myopic eye are aimed differently than
the receptors of a normally sighted eye.
It is important to note that this effect is almost exclusively with
the rod receptors that provide vision in low light environments.
The assumption and early evidence indicates that after a time, the
receptor rods adjust themselves to the new angle of the light to
provide the best vision, but this change takes time and is very
subtle.
Glenn Hagele
Executive Director
USAEyes.org
"Consider and Choose With Confidence"
Email to glenn dot hagele at usaeyes dot org
http://www.USAEyes.org
http://www.ComplicatedEyes.org
I am not a doctor.
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