What have neural networks achieved?

[David Redish]

Michael Arbib wrote:

>So: I would like to see responses of the form:

>"Models A and B have shown the role of brain regions C and D in functions E
>and F - see specific references G and H".

>The real interest comes when claims appear to conflict:

>Can we unify theories on the roles of cerebellum in both motor control and
>classical conditioning?

>What about the role of hippocampus in both spatial navigation and
>consolidation of short term memory?

In terms of the role of the hippocampus, a number of conflicting
hypotheses have recently been shown not to be incompatible through
computational modeling.  The two major theories that have been argued
over for the last twenty-plus years are (1) that the hippocampus forms
a cognitive map for navigation (e.g. O'Keefe and Nadel, 1978) and (2)
that the hippocampus stores episodic memories temporarily and replays
them for consolidation into cortex (e.g. Cohen and Eichenbaum, 1993).

We (David Touretzky and I, see Touretzky and Redish, 1996; Redish and
Touretzky 1997, Redish 1997) examined the role of the hippocampus in
the navigation domain by looking at the whole rodent navigation system
(thereby attempting to put the role of the hippocampus in an
anatomical context of a greater functional system).  By looking at
computational complexities and extensive simulations, we determined
that the most likely role of the hippocampus in navigation is to allow
an animal to reset an internal coordinate system on re-entry into an
environment (i.e. to *self-localize* on returning to an environment).
(From this theory we predicted that hippocampal lesions should not
affect the ability of animals to wander out and return to a starting
point, an ability called path integration which had previously been
hypothesized to be hippocampally-dependent.  This prediction has been
borne out by recent experiments, Alyan and McNaughton, 1997).  It is
straight-forward to extend this idea of self-localization to a "return
to context" which explains a large literature of primate data (Redish,
1999).

In addition to the self-localization role, the hippocampus has been
shown to replay recently traveled routes during sleep (Skaggs and
McNaughton, 1996).  However, the mechanisms that have been proposed to
accomplish these two functions require incompatible connection
matrices.  Self-localization requires a symmetric component and
route-replay requires an asymmetric component.  We showed (Redish and
Touretzky, 1998) that with the incorporation of external inputs
representing spatial cues during self-localization (obviously
necessary for accurate self-localization), self-localization can be
accurate even with a weak asymmetric component, and that the weak
asymmetric component is sufficient to replay the recently traveled
routes (without the external input, which would presumably not be
present during sleep).  This shows that the two roles hypothesized for
hippocampus are not incompatible.

REFERENCES

S. H. Alyan and B. M. Paul and E. Ellsworth and R. D. White and
B. L. McNaughton (1997) Is the hippocampus required for path
integration? Society for Neuroscience Abstracts. 23:504.

N. J. Cohen and H. Eichenbaum (1993) Memory, Amnesia, and the
Hippocampal System, MIT Press, Cambridge, MA.

J. O'Keefe and L. Nadel (1978) The Hippocampus as a Cognitive Map,
Clarendon Press, Oxford.

A. D. Redish and D. S. Touretzky (1997) Cognitive Maps Beyond the
Hippocampus, Hippocampus, 7(1):15-35.

A. D. Redish (1997) Beyond the Cognitive Map: Contributions to a
Computational Neuroscience Theory of Rodent Navigation, PhD Thesis.
Carnegie Mellon University, Pittsburgh PA.

A. D. Redish and D. S. Touretzky (1998) The role of the hippocampus in
solving the {M}orris Water Maze, Neural Computation, 10(1):73-111.

A. D. Redish (in press) Beyond the Cognitive Map: From Place Cells to
Episodic Memory, MIT Press, Cambridge MA.

W. E. Skaggs and B. L. McNaughton (1996) Replay of neuronal firing
sequences in rat hippocampus during sleep following spatial
experience, Science, 271:1870-1873.

D. S. Touretzky and A. D. Redish (1996) A theory of rodent navigation
based on interacting representations of space, Hippocampus, 6(3):247-270.

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A. David Redish		adr at nsma.arizona.edu
Post-doc		http://www.cs.cmu.edu/~dredish
Neural Systems, Memory and Aging, Univ of AZ, Tucson AZ
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