Synaptic modification rules ?

Allen L. Barker alb at datafilter.com
Thu May 20 01:16:43 EST 2004


nettron2000 at aol.com wrote:
> "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.
> 
> 
> ...and im not enough of a mathematician or programmer to thoroughly
> understand Kohonen networks. Maybe i'm asking too much, but i'd really
> like to know how these rules translate unto biological networks. Is
> there any clinical or experimental proof of these concepts that you or
> anyone can point out ? In my o.p. i brought up Hebb's concept because
> it was the only one i knew anything about. After some exhaustive
> searching/reading (and a headache to boot) via Google, found that
> modification takes place not exactly when spikes occur at the same
> instant in time but rather during a certain time window, dont quote me
> on this but something like 50 ms or so ? unless of course, one accepts
> 50 ms as occurring at the same time.
> 

Let me first mention mathematical modeling in general.  You can
mathematically model a neuron and its synaptic connections with
other neurons to almost any desired level of precision that you
can figure out (from scientific experiments).  You could model
all the chemical processes, giving rise to the electrical spikes,
and so forth.  That's actually useful in some contexts.

Another approach is to deal with simplified neurons.  The above
approach is useful in some contexts, but for mathematical analysis
or even simulation studies of large networks of neurons it tends
to fail.  This is just like how simplifying assumptions are used
in physics.  If you actually had a complex mathematical model
like the one posited above you could presumably prove that under
certain conditions certain simplified models provide accurate
large-scale predictions.  Or you can just postulate some plausible
synaptic rules changes and analyze how large networks of such
neurons would function.  Both approaches can be useful.  From
an engineer's viewpoint, the brain "works" and so finding similar
sorts of systems which give rise to suggestive collective behavior
can be illuminating.  This is not to say that these are going to
be exact models, in the sense of the low-level model above.
But if the low-level model doesn't "work" when simulated, then it,
too, still needs some refinements.

The neurons that Kohonen analyzes are typically simplified sigmoidal-
response neurons.  The simplifying assumption here is that for the
most part you can ignore the lower-level chemical reactions and spikes
and consider the signal essentially integrated over a time window.
This smooths out the process.  The synaptic weights are considered
linear and multiplicative.  Maybe think of it like taking a
single-cell recording from a couple of neurons, averaging over a time
window, and fitting to a simple electrical V=IR sort of model to find
dw/dt for the weight w.  That is, there *is* going to be some
averaged-out curve, subject to the assumptions.  (Hopfield has shown
that for spin-glass models of neurons the sigmoidal assumption is
equivalent to a mean-field approximation.)

That is basically what Kohonen networks are.  The area of artificial
neural networks tends to split into those who care about biological
plausibility, and those who don't.  Kohonen actually has models in
both camps, from biologically inspired systems like topological maps
to practical pattern recognition algorithms.

It all comes down to the approach, and what you're interested in.
I'm far more interested in large-scale complex-systems analysis than
I am in the chemical pathways involved in low-level synaptic change.
Obviously many neuroscientists are heavily interested in such details.
That works out fine for me.  Many neuroscientists are also
interested in the solid, mathematical study and modeling of large
systems of neurons.

Once you get a few million neurons interacting, like many-particle
physical systems, you can probably safely introduce a few simplifying
assumptions -- just to get anywhere.  Of course if you have the time
and a fast enough computer you can run simulations of even extremely
complicated mathematical neural models (perhaps to verify that
certain of the simplifying assumptions are reasonable).


>  
> 
>>>>>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.


-- 
Mind Control: TT&P ==> http://www.datafilter.com/mc
Home page: http://www.datafilter.com/alb
Allen Barker




More information about the Neur-sci mailing list