Q about artificial and biological neural-nets

William Calvin wcalvin at stein.u.washington.edu
Sun Feb 28 00:09:48 EST 1993


lamontg at u.washington.edu writes:

>recently, i was having a disucssion with a friend of mine over a 
>hypothetical model i have of a small amount of "neurotoxicity" effecting
>specifically axons, possibly being actually functional in terms of
>producing a more efficient, and well-adjusted neural net.  the hypothesis
>runs something along the lines of that the destruction of the connections
>between the neurons promotes the growth of new connections, and that the
>connections that are not re-established could be seen as being relics of
>a previous state that the neural net was in, while the ones that are
>re-established could been seen as being only the ones that are 
>applicable to the environment that the neural net is in now.  and the
>net result is that it functions better now than before.

Certainly real nervous systems have a lot of "pruning" during the juvenile
years.  I don't mean cell death (though some of that happens too) but
simply the breaking of synapses and the retraction of whole axon side
branches.  Connections peak at about 8 months after birth in humans and
then start a slow decline, totaling about 30-50% in cerebral cortex before
puberty.  In some axon pathways it is even more dramatic, e.g., monkey
corpus callosum has counts that fall 70% between birth and six months of age.
	But we don't know anything except the totals.  We could be losing
20% every month, and making 19.5% new synapses -- it would give a new decline.
There's a graph at p.190 in my book THE ASCENT OF MIND (Bantam 1991) of
the childhood time course, and some discussion of the problem.
    William H. Calvin   WCalvin at U.Washington.edu
    University of Washington  NJ-15
    Seattle, Washington 98195 FAX:1-206-720-1989



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