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From: bill@boulder.Colorado.EDU
Newsgroups: comp.ai.neural-nets
Subject: Re: Harmony theory - looking for references and clues
Keywords: Harmony theory, probabilistic units, symmetry
Message-ID: <18636@boulder.Colorado.EDU>
Date: 20 Mar 90 18:07:18 GMT
References: <2693@ruuinf.cs.ruu.nl>
Sender: news@boulder.Colorado.EDU
Reply-To: bill@synapse.Colorado.EDU (Bill Skaggs)
Organization: University of Colorado, Boulder
Lines: 42

In article <2693@ruuinf.cs.ruu.nl> jagversm@praxis.cs.ruu.nl 
(Koen Versmissen) writes:
>  Now neurons are not (or at best hardly) probabilistic, right? 

	I can't claim any particular expertise with respect to Harmony
theory, but I can tell you that most neurons (and all the ones thought
to be involved in cognition) are extremely probabilistic.  There are
a number of factors (variable amount of neurotransmitter released by
a synapse; random opening and closing of channels; etc.) combining to
make it unpredictable when a given cell will fire -- in addition,
because of the all-or-nothing character of firing, any moderately
complex system of neurons will form a chaotic system (in the dynamical
systems sense), so that quantum uncertainties will quickly be
amplified into global randomness.
	There was a book published a few years ago devoted to analyzing
the stochastic behavior of neurons; the title was something like
"Stochastic Analysis of Neural Activity."  (I don't have the reference,
and I don't know the author; it's not in our library.)

>                                                                  How
>  then can harmony theory claim to provide a _micro-level_ description
>  of the mind? Shouldn't it rather be called an _intermediate level_
>  description?

	I believe you are right that the units of Harmony theory are
not necessarily intended to correspond to biological neurons:  the
theory is "micro" level in comparison to the "macro" level of explicit
rules, not in any physiological sense.

>- The same question as the above, but now concerning symmetric
>  connections in Harmony theory, as opposed to asymmetry in the brain.

	The question of symmetry in the brain remains open.  If indeed
the brain makes use of Harmony-style relaxation-calculations, they
probably take place in restricted subnetworks confined to relatively
small portions of the cerebral cortex -- and there are in fact classes
of local connections in cortex which are roughly symmetric.  (Of course
there are also a great many classes of asymmetric connections.)
	The "style" of computation in the brain is something neuroscience
has at present few effective ways of investigating.

	-- Bill Skaggs