What have neural networks achieved?

[Jay McClelland]

I reply to Ken's question:

There are huge difficulties associated with the determination of
whether or not hippocampal lesions produce deficits in memory,
expecially when categorical labels are used such as 'recognition
memory' or 'spatial memory' or 'episodic memory' or whatever.  These
difficulties include the fact that there are few lesions that are
totally foolproof in terms of either their selectivity or their
completeness.  The difficulties also include the fact that the
particular parameters of tasks often make a tremendous difference and
labels such as recognition memory, spatial memory, etc really aren't
fully adequate to characterize which tasks do and which tasks do not
show deficits.  

It seems to be pretty well established in the rodent literature,
however, that ibotinate lesions of the hippocampus (which leave fibers
of passage intact) produce profound deficits in many spatial tasks
(e.g., animal must learn to find submerged platform in a tank of milky
water using cues around the room, and starting from a location in the
tank that varies from trial to trial), although again the effects
depend on the details of the experiments (the animal can learn if
there's a lightbulb over the platform or if training and testing are
always done with the same fixed starting place so a fixed path can be
used).

Much less clear is whether such lesions also produce deficits in
non-spatial tasks.  In my view the literature is consistent with the
idea that the hippocampus is crucial for *rapid* learning that depends
on conjuctions of cues; indeed I am one of many who think that the role
of the hippocampus in spatial tasks is secondary to its role in using
a form of coding we call sparse random conjunctive coding, but this
matter is far from settled.

There is an overview of the empricial data through 1994 in the
McClelland, McNaughton and O'Reilly paper (Psychological Review, 1995,
102, 419-457).  I append a couple of other relevant citations.

 -- Jay McClelland

@Article{Jarrard93,
  author = 	 "Jarrard, L. E.",
  title = 	 "On the role of the hippocampus in learning and
		  memory in the rat",
  journal = 	 "Behavioral and Neural Biology",
  year = 	 1993,
  volume =	 60,
  pages =	 "9-26"
}

@Article{RudySutherland9X,
  author = 	 "Rudy, J. W. and Sutherland, R. W.",
  title = 	 "Configural Association Theory and the Hippocampal
		  Formation:  {An} Appraisal and Reconfiguration",
  journal =	 "Hippocampus",
  year =          "199?" %I think it's 95 or 96 -- jlm
}