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Tom Holroyd tomh at BAMBI.CCS.FAU.EDU
Fri Nov 20 11:27:26 EST 1992

Nice post, but I have some comments...

>From: erwin at trwacs.fp.trw.com (Harry Erwin)
>Stimulus invariance is associated with semantic invariance, but _not_ with
>syntactic invariance (at the level of the olfactory nucleus). The _same_
>pattern in the nucleus _means_ different things from month to month. I've
>speculated that the cortex downloads a "semantic" model to the olfactory
>nucleus, the nucleus then monitors the stimulus using the model, reporting
>minor novelties (discrepancies between the model and the stimulus) and
>major novelties (unmatchable stimulus patterns) to the cortex (in a
>distributed manner).

I have a problem with the term "downloads".  Does this mean that the
cortex flips a switch in the olfactory nucleus and then dumps a semantic
model into it?  What is the switch?  What causes it?  What does the model
look like?  How is it transmitted to the nucleus?  How is the model
created in the cortex?  How is it stored in the nucleus?  How long does
the transfer take and what's the baud rate? :-)

Kidding aside, the concept of a "model" as an entity that can be transported
around the brain leads to all sorts of problems.  Homunculii, question
begging, etc.  The brain does not have a von Neumann architecture.

The discussion of the chaotic dynamics of the olfactory system was wonderful!
For example, you say:

>Since they operate in a state space with hyperbolic fixed points, it's easy
>to perturb them slightly and send them whereever you want. ...
>you can perturb them as they approach a hyperbolic fixed point and send
>them anywhere on their manifold.  The energy needed to control a chaotic
>process is many orders of magnitude less than the energy needed to control
>a process that has a fixed point or stable limit cycle.

This is a great picture, and shows that the system's behavior can be altered
by small, subtle changes in the interactions of the nucleus with its

So we have a picture of a complex dynamical system being driven by
patterned inputs and interactions with other cortical regions.  Then you
go and download a model into it, as if it were a RAM chip. :-) It seems much
clearer to me to continue with the dynamical reasoning:  slow parametric
changes in the olfactory nucleus alter the attractor layout - these are
internal to the nucleus itself (like changes in synaptic strength).
The changes can be both 'autonomous' structural changes in the nucleus
itself, or as a result of coordinated pattern changes between the nucleus
and the cortical regions it communicates with.

Remember that the olfactory system has to cope with many problems during
development and over the course of evolution.  It is not known a priori
which scents will be meaningful.  It is not known a priori what the detailed
structure of the olfactory system will be.  Also, the sensitivity of the
system requires that it be unstable in the sense of the quote above.  I
suggest that variation and selection are necessary properties for such a
pattern recognition system to be able to adapt to the "blooming, buzzing,

The patterns resulting from the interactions are stabilized (selected) by
the higher cortical structures as a result of behavioral information which
supplies the semantics for the patterns.  Changes in the patterns resulting
from growth, receptor damage, synaptic change, etc. result in new activity
patterns in the olfactory cortex, which, being an equally sensitive system,
uses the new patterns and the recent behavioral information to stabilize
the new pattern.  The variability seen in the nucleus/cortex system enables
it to act as an adaptive filter, isolating the rest of the brain from the
irrelevant details.

In summary, I think it makes more sense to think in terms of the coordinated
patterns of the whole system rather than isolating subsystems which send
"models" back and forth.

Tom Holroyd
Center for Complex Systems and Brain Sciences
Florida Atlantic University, Boca Raton, FL 33431 USA
tomh at bambi.ccs.fau.edu

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