What have neural networks achieved?

[Ali Minai]

 David Redish writes:

	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.

To add to David's very interesting comments on how self-localization and
replay of learned sequences are not incompatible, I would point out that
the hippocampal system has a variety of recurrent connection pathways at
various hierarchical levels (e.g., CA3-CA3, dentate-hilus-dentate,
entorhinal cortex-dentate-CA3-CA1-entorhinal cortex, etc.), and a variety
of time-scales at which processes occur (e.g., a theta cycle, a gamma cycle,
etc.) It is quite possible for functions requiring symmetric and asymmetric
connectivities to coexist in the hippocampus if they occur in different
subsystems and/or at different time-scales.

I often find that apparent conflicts or trade-offs in modeling result from
our neglecting hierarchical considerations (in both space and time) and
the possibility of multiple modes of operation. There is plenty of evidence
for both in the brain, but a lot of neural modeling still focuses on single
time-scales, single (or perhaps 2 or 3) modes, and ``compact'' systems.
The hippocampal models developed by Redish and Touretzky are especially
interesting because they place the hippocampus in a larger, multi-level
context with other systems. Others are starting to address issues of
temporal hierarchies in models of memory recall, phase precession, etc.

Also, on the point of episodic memory vs. cognitive mapping, it is
possible to think of frameworks in which the two may appear as aspects (or
even parts) of the same, more abstract, functionality.

-----------------------------------------------------------------------------
Ali A. Minai
Assistant Professor
Complex Adaptive Systems Laboratory
Department of Electrical
   & Computer Engineering
               and Computer Science
University of Cincinnati
Cincinnati, OH 45221-0030

Phone: (513) 556-4783
Fax:   (513) 556-7326
Email: Ali.Minai at uc.edu

Internet: http://www.ececs.uc.edu/~aminai/
-----------------------------------------------------------------------------