memory (and SOMs)

yan king yin (dont spam) y.k.y at lycos.com
Tue Feb 26 13:58:55 EST 2002


"mat" <mats_trash at hotmail.com>
> > According to one study, there are about 32 visual areas in the macaque
> > visual cortex, connected by 305 connections. Now it seems that these
> > visual maps cannot be the result of self-organization, so they must be
> > genetically specified. If this can happen to the visual cortex, the rest of
> > the cortex might be organized this way too. By simple division we can
> > estimate that there could be 100 to several hundreds of  "maps" in the
> > cortex. That is the number of circuits we have to figure out!
>
> I think you should be weary in any part of biology of using the term
> 'genetically specified'.  What do you actually mean by this?  Sure the
> growth factor peptides are of course genetically specified, but to
> suggest that the genome has much control over what happens to the
> peptide is very debatable.  DNA encodes proteins and is regulated by
> proteins and as such is not much more than a 'library' accessed at
> intervals.  Further, the actions of proteins themselves is
> inextricably linked to the macro and microenvironments of the
> organism.  Though DNA contains the code, in my view environment plays
> at least as much if not greater role in shaping the organism,
> including its brain.  Therefore there will always be some element of
> self-organisation, though not perhaps not in the strict Kohonen map
> sense.

Braitenberg and Schuz have done some statistical analysis of the cortex,
and they found that most parts of the neocortex are organized into
discrete "patches" with parallel projections connecting them. Therefore
one thing for sure is that cortical connections are not diffuse (as least
in the neocortex). My point is that these discrete connections between
patches are specified by genes, ie not altered by experience under
normal circumstances. For example in the visual cortex you have areas
V1, V2, V4 and MT connected by specific pathways that do not vary
from individual to individual (as least such has not been reported). As
pointed out by Matt Jones, there are exceptional situations where
cortical reorganization does take place -- see my reply to him.
Personally I am inclined to the view that large scale cortical pathways
connecting the "patches" are genetically determined, I'll explain this
in Matt Jones thread as well.

Also Im still writing my reply to your SOM idea, please be patient.






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