Neural networks and brain function

[Michael A. Arbib]

Dear Dr. Harris:

Thank you very much for your very helpful remarks.  I would like to agree
and disagree!! You write:


>After neural networks, we have a different set of analogies.
>We now make neurological models that ascribe a particular
>computational function to a brain structure.  For example:
>
>     "The cerebellum performs supervised learning"
>     "The hippocampus functions as an autoassociative memory"
>
>By talking about a computational function, rather than a
>type of task that a brain structure is needed for, a
>lot of apparent conflict can suddenly be resolved.
>
>In the example of the cerebellum, the evidence that the cerebellum
>is involved in motor control and classical conditioning, and
>even higher cognitive functions does not seem so contradictory.
>It is very plausible that a supervised learning network
>would be useful for all of these functions -- see for example
>the work of Kawato and Thompson.

I agree - and in fact noted that the integration of work on the role of
cerebellum in motor control and classical conditioning was a specific
target of work at USC.  You might reply: "Why is it a target?  The problem
is solved! The cerebellum does supervised learning!  What more needs to be
said?"

And this is where I disagree - the observation that we might use supervised
learning rather than Hebbian or reinforcement learning is only part of the
issue under investigation.  There are 2 complementary concerns:

a) Many many functions can exploit supervised learning.  Thus to show that
A and B both use supervised learning in no way guarantees that the
cerebellum carries out both of them - though I very much accept your point
that the observation that they exploit the same learning mechanism seems a
very useful step in that direction.

b) Again, supervised learning can be realized in simple networks.  We still
have many questions to answer (we do have partial answers) as to why the
cerebellar cortex has the structure it has, what might be the specific
advantages of the actual mix of LTD and "re-potentiation" at the
parallel-fiber-Purkinje cell synapse, and what is the relation between
cerebellar cortex, cerebellar nucleus and inferior olive in a "generic"
microcomplex.  Even when we have answered that, we still have to ask
whether - for posited function A or B - there is a set of cerebellar
microcomplexes appropriately wired up to other brain regions to realize the
supervised adaptation of that function.

>In the example of the hippocampus, work by Michael Recce and
>myself has shown how an autoassociative memory can play a role
>in both episodic memory and spatial function, in particular
>giving an animal localisation ability by performing pattern
>completion on partial egocentric maps.

I very much look forward to reading your paper!  Nonetheless, the above
observations also apply to autoassociative memory - this is not limited to
HC.  Conversely, recent work of ours suggests that, to fully understand its
role in navigation, we must embed HC in a larger system including parietal
cortex and other regions.  It seems unlikely that the same pattern of
embedding will account for episodic memory.

So  ... thank you for a stimulating general perspective.  I look forward to
other messages on brain modeling - both those adding to the stock of
general principles, and those showing how the particularities of a system
(LTP/LTD, neural morphology, embedding within larger networks of networks)
account for its diverse functions.



 *********************************
 Michael A. Arbib
 USC Brain Project
 University of Southern California
 Los Angeles, CA 90089-2520, USA
 arbib at pollux.usc.edu
 (213) 740-9220; Fax: 213-740-5687
 http://www-hbp.usc.edu/HBP/