TR: Hippocampal Conjunctive Encoding, Storage, and Recall

[Randall C. O'Reilly]

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	HIPPOCAMPAL CONJUNCTIVE ENCODING, STORAGE, AND RECALL:
			 AVOIDING A TRADEOFF

			 Randall C. O'Reilly

			 James L. McClelland
	
		      Carnegie Mellon University
		    Technical Report PDP.CNS.94.4
			      June 1994
	
The hippocampus and related structures are thought to be capable of:
1) representing cortical activity in a way that minimizes overlap of
the representations assigned to different cortical patterns (pattern
separation); and 2) modifying synaptic connections so that these
representations can later be reinstated from partial or noisy versions
of the cortical activity pattern that was present at the time of
storage (pattern completion).  We point out that there is a tradeoff
between pattern separation and completion, and propose that the unique
anatomical and physiological properties of the hippocampus might serve
to minimize this tradeoff.  We use analytical methods to determine
quantitative estimates of both separation and completion for specified
parameterized models of the hippocampus.  These estimates are then
used to evaluate the role of various properties and of the
hippocampus, such as the activity levels seen in different hippocampal
regions, synaptic potentiation and depression, the multi-layer
connectivity of the system, and the relatively focused and strong
mossy fiber projections.  This analysis is focused on the feedforward
pathways from the Entorhinal Cortex (EC) to the Dentate Gyrus (DG) and
region CA3.  Among our results are the following: 1) Hebbian synaptic
modification (LTP) facilitates completion but reduces separation,
unless the strengths of synapses from inactive presynaptic units to
active postsynaptic units are reduced (LTD).  2) Multiple layers, as
in EC to DG to CA3, allow the compounding of pattern separation, but
not pattern completion.  3) The variance of the input signal carried
by the mossy fibers is important for separation, not the raw strength,
which may explain why the mossy fiber inputs are few and relatively
strong, rather than many and relatively weak like the other
hippocampal pathways.  4) The EC projects to CA3 both directly and
indirectly via the DG, which suggests that the two-stage pathway may
dominate during pattern separation and the one-stage pathway may
dominate during completion; methods the hippocampus may use to enhance
this effect are discussed.

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