function of hippocampus

[Ali Minai]

Ken Miller writes:

	With respect to recent postings about models of hippocampus and
	memory, I'd like to toss in a cautionary note.  A recent report
	(Elisabeth A. Murray and Mortimer Mishkin, "Object Recognition and
	Location Memory in Monkeys with Excitotoxic Lesions of the Amygdala
	and Hippocampus", J. Neuroscience, August 15, 1998, 18(16):6568-6582)
	finds no deficit in tasks involving visual recognition memory or
	spatial memory with lesions of hippocampus and amygdala....

	I'm not in any way an expert on this literature -- only a very distant
	observer -- but I worry that, based on lesion studies that also
	involved lesions of overlying cortex, both the neuroscience and
	connectionists communities may have jumped to a wrong conclusion that
	the hippocampus has a special role in episodic and/or spatial memory.
	I'd be interested to know if there's still good reason to believe in
	such a role ...

The concern expressed here is certainly warranted --- and not just on
theories of hippocampal function. I think a lot of us are increasingly
skeptical about theories of a simple, unitary function for the hippocampus.
Indeed, different people often mean different things when they use the
term ``hippocampus''.

That having been said, I do think (and others can marshall the evidence
better than I can) that a preponderance of evidence favors a hippocampal
involvement in episodic memory and, at least in rodents, spatial cognition.
The data from lobotomy patients such as H.M. and the extensive series of
results from Squire's group provide convincing evidence that the hippocampus
and its surrounding regions are involved in certain types of memory. Whether
this role is central or peripheral (but important) is not clear, and I
agree that most theories about the CA3 or the hippocampus as the site of
associative storage --- temporary or otherwise --- are driven primarily
by the intriguing structural analogies with recurrent neural networks.
However, that is not necessarily a bad way to proceed. A major problem
with experimental neuroscience is its tendency to produce oceans of data
based on very narrowly focused experiments. Addressing this with formal
large-scale theories provides a valuable --- if imperfect --- means of
thinking about the big picture, and we need more of such theorizing.

The issue of hippocampal involvement in spatial cognition in rodents
is based on a very large body of lesion studies, but is given overwhelming
credibility, in my opinion, by the undeniable existence of place cells and
head-direction cells. The systematic study of sensory, behavioral,
mnemonic, and other correlates of this organized cell activity provides
convincing evidence that the hippocampus ``knows'' a great deal about
the animal's spatial environment, is very sensitive to it, and responds
robustly to disruptions of landmarks, etc. Recent reports on reconstructing
physical location from place cell activity (Wilson and McNaughton,
Science, 1993; Zhang et al., J. Neurophysiol., 1998; Brown et al.,
J. Neurosci, in press) clearly show that very accurate information about
an animal's spatial position is available in the hippocampus. It must
be used for something. Similar results are available about head direction
cells.

I do not think we really understand what role the rodent hippocampus plays
in spatial cognition, but it is hard to dispute that it plays some ---
possibly many --- important roles. I think that, as theories about
hippocampal function begin to place the hippocampus in the larger context
of other interconnected systems (e.g., in the work of Redish and Touretzky),
we will move away from the urge to say, ``Here! This is what the hippocampus
does'' and towards the recognition that it is probably an important part
in a larger system for spatial cognition. Indeed, it is quite possible
that, when we do arrive at a satisfactory explanation of hippocampal
function, we will have no name for it in our current vocabulary (though
I have no doubt that psychologists will invent one:-).

Finally, one issue that is particularly relevant to hippocampal theories
is the possibility that the categories of memory (e.g., episodic, declarative,
etc.) or task (DNMS, spatial memory, working memory, etc.) that
we use in our theorizing may not match up with the categories relevant to
actual hippocampal functionality. Perhaps we are trying to build a science
of chemistry based on air, water, fire, and earth. The good news is that
the chemistry experiment was eventually successful and we did find our
way to the correct elemental categories.

Ali

-----------------------------------------------------------------------------
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/