Synaptic modification rules ?

nettron2000 at aol.com nettron2000 at aol.com
Tue May 18 16:29:51 EST 2004


"Allen L. Barker" <alb at datafilter.com> wrote in message news:<1hfqc.17550$KE6.7993 at newsread3.news.atl.earthlink.net>...
> nettron2000 at aol.com wrote:
> > "Allen L. Barker" <alb at datafilter.com> wrote in message news:<10_pc.10522$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.



 Matthew,could you please explain further what you meant by "As far as
"rules" go, there are no rules, just consequences of particular firing
patterns for cells which have particular membrane properties and
biochemistry."
 Are you saying that the spike firing pattern determines the mechanism
( or means) used for modification ? In otherwords, if a particular
firing pattern is present in a cell assembly then some , as yet,
imagined molecular mechanism "kicks-in" and modification takes place ?
If not ,well, sounds interesting. :)

BTW, thanks, eagerly await your reply.



 

> >>>
> >>>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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