Synaptic modification rules ?
Allen L. Barker
alb at datafilter.com
Mon May 17 22:19:25 EST 2004
nettron2000 at aol.com wrote:
> "Allen L. Barker" <alb at datafilter.com> wrote in message news:<10_pc.10522$zO3.1210 at newsread2.news.atl.earthlink.net>...
>>Matthew Kirkcaldie has provided a useful discussion from the
>>biological viewpoint, below. From a theoretical perspective,
>>it is quite fascinating just what simple Hebbian networks
>>are capable of. For a good and relatively easy-to-read
>>(given the requisite mathematical background) introduction
>>to such analyses I would strongly recommend Teuvo Kohonen's
>>_Self-Organization and Associative Memory_, Springer-Verlag,
>>1984. (I think there is more recent version available.)
>>Grossberg has some very good articles in that area, also,
>>and there is a particular article I'd like to recommend,
>>but I don't have that paper or reference at hand right
>>Matthew Kirkcaldie wrote:
>>>In article <ec29a509.0405161715.46916f1f at posting.google.com>,
>>> nettron2000 at aol.com wrote:
>>>>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 ?
>>>Hebbian learning isn't a rule - it was a concept Hebb thought up to
>>>suggest how synapses might be changed according to the activity of the
>>>cells sending and receiving them, in order that experience would shape
>>>the connections between neurons. The idea is if two cells are usually
>>>active at the same time, this activity would cause the synapses between
>>>them to become stronger. If their activity occurred at different times,
>>>the connection would become weaker. Conceptually, he showed that this
>>>was enough to explain some kinds of behaviour and learning, so he
>>>guessed that a process like this might operate in the nervous system,
>>>without knowing what that process was.
> For clarity i'll post Hebb's concept ( if you will) here:
> "When an axon of cell A is near enough to excite cell B and
> repeatedly or persistently takes part in firing it, some growth
> process or metabolic change takes place in one or both cells such that
> A's efficiency, as one of the cells firing B, is increased."
> Although this idea doesnt account for depression, how did Hebb guess
> this concept ? I know there are other related concepts to this such as
> anti-hebbian and what not, but does anyone know of other "rules" ( i
> use the term loosely) that can account for synaptic modification ?
In a modern analytical context, such rules are expressed as
differential equations. I'm not enough of a historian of
neuroscience to guess at how Hebb came up with the concept.
There are many different synaptic modification rules that
one can consider. I recommended the Kohonen book above
because he explicitly analyzes several different such rules.
Doing the math (and simulations) he shows that large systems
of neurons all operating by Hebb-like rules can give rise to
collective, "emergent" properties such as associative
>>>The nearest known physiological processes to Hebbian learning are
>>>long-term potentiation and long-term depression, which are effects on
>>>synaptic strength caused by patterns of firing and the biochemical
>>>processes which these patterns trigger. LTP and LTD are studied very
>>>widely around the world in all sorts of systems, and are understood
>>>moderately well in terms of receptors moving to and from the synapse
>>>according to activity. There are all kinds of reviews of LTP and LTD
>>>ranging from the conceptual to the severely technical - if you can
>>>indicate what you'd like to know, myself and wiser heads here could make
>>>As far as "rules" go, there are no rules, just consequences of
>>>particular firing patterns for cells which have particular membrane
>>>properties and biochemistry. The people trying to understand these
>>>processes give them names and descriptions, but they're for our
>>>convenience - there's nothing in a neuron which says "well, conditions A
>>>and B are met, so this synapse will be altered." It's more like inputs
>>>A and B trigger events inside the cell, and the interaction of those
>>>events might cause side effects which modify the strength of the synapse.
>>>Recently a very interesting mechanism has begun to be unravelled,
>>>whereby activity at a synapse can cause the synapse to "capture" the
>>>connection by causing DNA to be transcribed in the nucleus to make RNA,
>>>but this RNA only becomes new protein at the synapse which was active.
>>>So that's like another "rule" in that specific patterns of events can
>>>trigger it, such as the receipt of a puff of the transmitter serotonin
>>>at the right time. Other recent studies have looked at how signalling
>>>between presynaptic and postsynaptic membrane can maintain the physical
>>>structure, and the role that glia have in allowing the synapse to exist
>>>instead of pushing in to separate the cells, and how long synapses
>>>typically last (minutes? days? years? nobody knows for sure).
>>>Anyway - nobody really knows how all our synapses are made and
>>>maintained. But that's what makes it all interesting.
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