back to school

From: scerir (
Date: Wed Feb 06 2002 - 13:45:06 MST

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)]

This archive was generated by hypermail 2.1.5 : Fri Nov 01 2002 - 13:37:38 MST