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
>>now.
>>
>>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
memory.

>>>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 
>>>a recommendation.
>>>
>>>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.
>>>
>>>      Cheers,
>>>
>>>         Matthew.


-- 
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Home page: http://www.datafilter.com/alb
Allen Barker




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