From: Smigrodzki, Rafal (SmigrodzkiR@msx.upmc.edu)
Date: Wed Feb 06 2002 - 17:18:22 MST
hal@finney.org [mailto:hal@finney.org] wrote:
Scerir forwards:
> << 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.
Interesting phenomenon. The other obvious question is, even if the
retina can tell that the image is out of focus, how does it know whether
the focal plane is in front of or behind the retina? In other words,
how does it know which way to grow in order to correct the problem?
That sounds even harder than determining whether the image is in focus
or not.
I wonder if they are barking up the wrong tree, and it is a signal from
the lens that does the trick, maybe some subtle chemical wafting through
the vitreous humor.
### The retina contains a number of other types of cells in addition to the
ganglion cells - such as the bipolar and amacrine cells (many types).
Apparently, muscarinic cholinergic activity is needed in the regulation of
ocular growth, but the exact pathways are not known (the cholinergic
amacrines are not its source). Other amacrines (with opiate and dopaminergic
receptrors) are apparently involved, as well as the activity of bFGF-2, VIP,
and possibly other signals. Even with all ganglion cells gone there is
enough circuitry in the retina for an edge detection algorith to hide in.
As far as I know the developing eye does not "know" if the retina is behind
or in front of the focal plane. However, by default the fetal eye is shorter
than it should be, so the focal plane is behind the retina. The eye keeps
growing until the image gets in focus, and then stops. If for some reason
this sharp image does not form, as in partial visual deprivation, or when
the signal from edge detection is not properly received by sclerocytes,
growth continues well beyond the optimal setpoint, resulting in worsening
nearsightedness. As far as I know there is no signal to shorten the eye,
only modulation of its growth rate.
Rafal
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