From: Michael M. Butler (email@example.com)
Date: Thu Feb 07 2002 - 05:45:50 MST
Is this phenomenon still observed in late(r) life? I seem to have developed a bit of nearsightedness in my one
LASIKed-to-better-than-20/20 eye, and have been wondering if some adaptation effect related to my uncorrected eye might
be "pulling" the other back into elongation. The new nearsightedness only became noticeable after the one year warranty
> That matter influences light is an obvious fact -
> if it were not so, we could not see objects.
> The influence of light on matter is, however,
> a more subtle effect .....
> [a friend wrote ....]
> << In order for the eye to be able to focus an image on the retina,
> the axial length of the eye needs to be just right. If the eye is too
> long (or too short), then the image is perpetually focused in front
> of the retina (or behind it). The result is nearsightedness (or
> farsightedness). It turns out that the retina of the eye plays a big
> role in making sure that the axial length of the eye is just right.
> How? Well, the retina has the ability to determine whether an image
> is "in focus" or "out of focus." If an image is "out of focus" then
> the retina emits chemical signals that control the growth of the eye,
> making it either longer or shorter, as needed, so that the image can
> be focused properly.
> But there's a big unanswered question: How does the retina determine
> whether an image is "in focus" or not? The obvious answer is that an
> image that's "in focus" is clear and has sharp edges, while an "out
> of focus" image is blurry. So one idea is that the retina can
> distinguish between "fuzzy" and "not fuzzy." But the photoreceptor
> cells (the cells that directly absorb the photons) cannot do this by
> themselves. Instead, a little bit of image processing is required.
> The photoreceptor cells synapse onto bipolar cells, and these synapse
> onto retinal ganglion cells. A single ganglion cell receives input
> from multiple photoreceptor cells. One function of ganglion cells is
> in "edge detection." An edge is defined as a sharp boundary
> between "light on" and "light off." So if the ganglion cells detect
> lots of sharp edges, then they might be able to conclude that an
> image is focused on the retina.
> The problem with the above hypothesis is that the ganglion cells
> appear *not* to be involved with the ability of the retina to
> determine whether an image is in focus. That is, if you kill off the
> ganglion cells (while keeping the photoreceptor cells intact), then
> the retina is *still* able to guide the development of the eye in
> such a way as to maintain the proper axial length. The experimental
> evidence is still a little bit sketchy, iiuc; but an argument can be
> made that the photoreceptor cells, all by themselves, can determine
> whether an image is focused or not on the retina. And as far as I
> know, there's no way to explain, in classical terms, how that might
> work. >>
> [see also - Crewther, DP. The role of photoreceptors in the control of
> refractive state. Progress in Retinal and Eye Research. Vol 19, no 4,
> pp 421-457 (2000)]
-- butler a t comp - lib . o r g Wm. Burroughs said it best: "After a shooting spree, I am not here to have an argument. they always want to take the guns away I am here as part of a civilization. from the people who didn't do it."
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