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From: lyang@sun.uucp (Larry Yang)
Newsgroups: net.arch
Subject: Re: Computing with Neural Circuits:
Message-ID: <6101@sun.uucp>
Date: Wed, 13-Aug-86 14:09:50 EDT
Article-I.D.: sun.6101
Posted: Wed Aug 13 14:09:50 1986
Date-Received: Thu, 14-Aug-86 08:24:07 EDT
References: <1883@ecsvax.UUCP> <64300001@prism> <894@cit-vax.Caltech.Edu>
Reply-To: lyang@sun.UUCP (Larry Yang)
Organization: Sun Microsystems, Inc.
Lines: 51

In article <894@cit-vax.Caltech.Edu> mangler@cit-vax.Caltech.Edu (System Mangler) writes:
>In article <64300001@prism>, brian@prism.UUCP writes:
>> Perhaps I'm being overly simplistic, but doesn't this neural network
>> stuff seem similar to ANALOG networks of non-linear devices?
>
>(Being even more simplistic)  As near as I can tell, neural network
>circuits are made of threshold logic.  When a 'neuron' has more
>excitory inputs true than inhibitory inputs, it fires.	That's
>threshold logic.  The stuff is described in all the old digital
>switching theory books (such as Kohavi) but nobody seems to have
>used it until Hopfield.  Doesn't look very analog to me.

[Being a little less simplistic]

A neuron receives neural input at its dendrites, the receiving end of 
the nerve cell.  Although the incoming excitatory inputs are action potentials,
sort of on-off signals propagating in from the previous nerve's axon, the
signals in the dendrites are actually "graded" (i.e., analog).  It is roughly
safe to say that the faster the action potentials come in, the greater this
graded signal is. (There are probably exceptions to this; there are exceptions
to every rule in biology.) How this dendrite signal behaves is 
modulated by the inhibitory inputs on the dendrites; these
inputs tend to be analog; that is, the greater the inhibit signal, the more
the dendrite signal is modified.  The resulting signal moves through
the cell body and starts down the axon.  If the signal is above a certain
threshold, the axon fires an action potential (spike) and the signal goes
on down the axon to impinge onto the next cell.  If the signal is too weak, it
just dies out (cell walls are VERY resistive).  This threshold can also
be modified by many things; chemical (e.g., drugs) and electrical.

In other parts of the nervous system, analog information is more prevelant.
In the optical system, signals coming off of the retina are in the form
of graded potentials.  Information regarding shapes and motion of objects is 
encoded by different _rates_ of firing of action potentials in nerves, as 
found by Hubel and Wiesel.  In the auditory system, volume is also 
encoded by rates of firing of nerves.

Thus, although signals appear to be digital because of the on-off nature
of the action potentials, it is the amount of these signals (in the
form of both more excitatory inputs and higher rate of excitation) that is the
important factor.  Therefore,  it is not proper to say that the nervous system
is analog or digital, but a happy mixture of both.



-- 


-- Larry Yang
   Sun Microsystems, Inc.
   Mountain View, CA  94043