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Cortical Architectures

Leslie Kay lmk2 at garnet.berkeley.edu
Thu Mar 3 23:41:10 EST 1994

In article <2l4krv$jtc at truffula.fp.trw.com>,
Harry Erwin <erwin at trwacs.fp.trw.com> wrote:
>Does anyone have a clue on why the paleocortex has 3 layers and the
>neocortex, 6? In the work I've been doing with models of the olfactory
>system, the three layers appear to have input, processing, and output as
>their functions, but then I'm a dumb engineer...

I'm not sure what you mean by "why," since the brain wasn't designed
by an engineer ( ;-) ) . One can only speculate on the evolutionary
advantage of developing the six-layered cortex, which could fill
books.  Entorhinal cortex, also known as mesocortex (Lopes da Silva,
Witter, and so on, I don't have the refs handy), is considered to
be a transitional cortex, as it is six-layered, but not quite as
highly differentiated as neocortex.  It has the three layers of
paleocortex (layers I-III), than the lamina dissecans, which may be
seen as the remnant of the boundary of paleocortex, and then a
not very organized layers IV, V, and VI.  In rats the area is
much less differentiated functionally (by region) than in "higher"
mammals, such as cats and monkeys.  All cortex has, from an 
engineering perspective, input, processing, and output, functionally,
but each cortex performs different functions, more or less complicated.
The entorhinal cortex and neocortex receive more highly processed
activity from many differesources, probably more than the
pyriform cortex (although I can't be sure about this).  So, I
would guess that more and more differentiated input and output
demands, as well as the more complicated demands of the newer
senses (olfaction being evolutionarily the oldest in mammals),
would demand more layers, more cells, more processing capabilities.

Leslie Kay
lmk2 at garnet.berkeley.edu

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