Synaptic modification rules ?

Matt Jones jonesmat at physiology.wisc.edu
Wed May 19 16:00:46 EST 2004


nettron2000 at aol.com wrote in message news:<ec29a509.0405161715.46916f1f at posting.google.com>...
> Ive bin recently reading about a synaptic modification rule discovered
> by Donald Hebb ( Im assuming this is related to the Pavlovian
> conditioning experiments?) in which a synapse is modified depending on
> whether a pre-synaptic spike occurs before or after a post-synaptic
> spike ( still somewhat unclear about that one), but are there other
> "rules" that govern synaptic modification ?



Hi,

Yes there are a number of other rules that have been proposed, and
some tested experimentally. The spike-timing one you mention is a more
recent embellishment referred to as "spike-timing dependent
plasticity" (STDP).

But first, it should be noted that the roots of the idea attributed to
Hebb go back much farther than 1949.

In  Principles of Psychology (1890), William James wrote:
"When two elementary brain-processes have been active together or in
immediate succession, one of them, on re-occurring, tends to propagate
its excitement to the other."

A few years later Freud sugggested, in Project for a Scientific
Psychology, that "neurons" are permanently altered by excitation.

I think Hebb's main contribution wasn't the Hebbian synapse idea, but
rather the "cell assembly" idea (i.e., population coding of
information). I believe he proposed the synaptic modification
mechanism because it seemed like the simplest plausible way of getting
a population to organize itself to do something useful, without
needing an external supervisor. But now that I think of it, I'm not
sure whether Hebb was the true originator of the population coding
idea either (but I don't know who was).

As for other learning rules, the neural network literature is full of
them, as Allan Barker pointed out.  In addition to the Kohonen book, I
would recommend Gluck and Myers' :Gateway to Memory: An Introduction
to Neural Network Modeling of the Hippocampus. This is a biologically-
and behaviorally-based approach to different ways of modeling memory,
including descriptions of several different learning rules that one
might try to apply. The math is introduced very gently.

On the more detailed biological side, the first really strong proposal
for a biochemical mechanism behind synaptic modification, phrased as a
formal rule, is probably in

Bear, Cooper & Ebner (1987) A physiological basis for a theory of
synapse modification. Science 237:42-48



Cheers,

Matt



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