Neural networks and brain function

[Ken Harris]

Michael Arbib writes:

> 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.

Agreed.

> 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.

Again agreed.  Simple connectionist networks can be no more than an
analogy for the functioning of the brain.  Although sometimes it is possible
to model the function of a brain structure with out modelling its
circuitry.  For example, some of Kawato's simulations model the
cerebellum by a backprop net.

> 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.

I absolutely agree with the need to embed the hippocampus in a larger
system.  But I do think this can be done in a way consistent with a role
in episodic memory.  Without getting too deep into the details of our
model:

We propose that the neocortex is responsible for constructing and
representing an egocentric map of space, i.e. a firing pattern that
codes for the egocentric position of environmental features.  The
hippocampus is an autoassociative memory that performs pattern
completion on egocentric maps, as well as on more general firing
patterns.  This function may explain the involvement of the hippocampus
in certain spatial tasks, as well as in general episodic memory.

In the example of the Morris water maze, a rat introduced to the maze
constructs a partial map from sensory input, that will contain the
positions of observable cues but not the hidden platform.  This will
trigger the recall of a full map stored in the hippocampus during
previous exploration, that also contains a representation of the
platform location.  After recall, the neocortical firing pattern will
contain a representation of the platform location, even though it was
not directly observed.  Neocortical motor systems then allow the rat to
head directly towards the platform.


-----------------------------------------------
Ken Harris
Department of Anatomy and Developmental Biology
University College London
http://www.anat.ucl.ac.uk/~kdh