Bill Skaggs bill at
Thu Sep 28 13:30:20 EST 1995

skeptic at (Chuck Easttom) writes:

   > 1.  An inital study showed LTP saturation in the Dentate Gyrus to
   > interfere with passive avoidence learning tasks.  However several
   > subsequent studies could not confirm this.  Has there been solid 
   > confirmation or refutation of this issue?

A recent issue of the journal Hippocampus was devoted to this question
(but not specifically related to passive avoidance tasks).  The
consensus is that in most cases LTP saturation does not seriously
disrupt learning.  One possible reason is that only a subset of
synapses are actually being saturated; but there are other

   > 2.  What of laboratory induced LTP short time frame (a few
   > days/weeks).  This makes it an unlikely vehicle for Long Term
   > Memory (LTM).  Has any conclusive study supported the following: 

There is no such thing as a conclusive study.  Any result has numerous
possible explanations, though they may differ considerably in apparent
likelihood.  You are asking for more than science can provide.

   >    a) Link LTP specifically to short term memory and perhaps
   >    identify other potential mechanisms for LTM?

No.  These remain to be done.

   >    b) Demonstrated other versions/forms of LTP which have a
   >    sufficient duration to account for LTM?

Gary Lynch claims that LTP in the CA1 region of the hippocampus
persists indefinitely, though some are skeptical.  In addition, there
is an interesting form of plasticity in the neocortex, called
"kindling", that appears to persist for years; but as yet there is
little evidence to connect it with LTM.  (It has mostly been studied
in connection with epilepsy.)  I think it is fair to say that the
physiological basis of LTM is not yet understood.

   >    c) provided confirmation for the memory indexing view point of
   >    hippocampal involvement in memory?

This is a pretty indefinite question, but I'm sure the answer is that
there is nothing that would convince you.  For a more or less
comprehensive discussion of these issues, I recommend the paper, "Why
there are complementary learning systems in the hippocampus and
neocortex:  insights from the successes and failures of connectionist
models of learning and memory", J. L. McClelland, B. L. McNaughton,
R. C. O'Reilly, Psych Rev 102:419-457 (1995).

	-- Bill

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