Structure of the Optical Nerves ??

Eugene Leitl ui22204 at
Wed Sep 6 12:40:20 EST 1995

> : Hi,
> : I have studied Electronics.
> : A friend of mine is blind on one eye. Isn't it possible
> : To connect light sensors (or a complete optical camera chip)
> : to the optical nerve in the eye ?

The problem is: if the retina's gone, you have to mimick the
its function with a circuit/algorithm (I totally ignore interacing
difficulties here). The information compression factor is 126:1, a lot 
of processing horsepower (well beyond any current/near future 
supercomputer according to Moravec. And he's too optimistic, imo). 

If the retina's still functional: there is a project somewhere (I
can look it up) where a coarsened image will be projected (via
an electrode array) upon the retina. One project I've read of involved an 
array of small vibrating cells worn on the back, projecting a video bitmap.
One learns to interpret the "image" with time.

> : I understood that there are milions of these nerve endings in the 
> : eye.

I don't have the numbers at hand: 16 Mreceptors? (Probably 2-4 times more).

> : I have heard of a similiar connection to nerve cells involved with 
> : hearing.

The bit rate is very much smaller. Interestingly, the cochlea obviously
sorts inputs by frequencies, this is hardware Fourier transform :) and
a quite high-res one at that.

> : The brain automatically sorts out the information stream.

It does not work very well. This is much coarser input, one cannot
understand e.g. speech.

> : I mean with my friend there is still an optical nerve. It's his eye 
> : that doen't work anymore.

Retina's dysfunctional? :(

> : Is this a stupid idea, or is it simply too dificult to do ?

This is certainly not stupid. Far from it. But _very_ difficult.

> : What I'd like to know is how do the optic nerve cells pick up signals, and 
> : how do "bio sensors" in general work.

Since "Psychophysik" we know that receptor signals (frequency-coded) get
logarithmically modulated. The retina functionality is quite well mapped,
even the first stages of the visual pipeline have been charted with good
detail. Beyond, however...

> : Like touch and heat/cold sensations.

This is similiar mapping, alright. However, since the visual input
constitutes about 95% of total senory input (a significant part of
the brain is devoted to visual processing) the problem is orders
of magnitude more difficult.
> : I know it is a electrical/chemical process that starts somewhere.

This very vague. As saying humans are mammals.

> : Fill me in.

I would suggest starting e.g. by reading a basic biochemistry book,
(pick the excitable membrance/photoreceptor stuff)
e.g. Stryer or Alberts and then progress to the specialized retina
processing. Some older Scientific American issues might be instrumental.
-- Eugene

> : Ronald Kaim.

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