X-Message-Number: 15849
Date: Mon, 12 Mar 2001 22:12:34 -0500
From: Thomas Donaldson <>
Subject: a bit on how memory works

Hi everyone!

This is a very brief reply to both of Dave Pizer's question messages. I
have decided to put in my reply mainly because I do have something to
say on both, but it wasn't clear from Dave's message whether or not 
he wanted a reply from me specifically.

On memory:
I will have to say that we still don't have a good idea about how 
our memories (of the several different kinds so far identified) work.
The discovery that we DO have several different kinds, that depend on
different brain centers to work, is itself significant. Among other 
points it tells us that understanding one kind does not automatically
help us understand another ... though in practice, if we DID come to
understand one kind, it would probably help a lot with the others.

Among the different kinds we have:
1. memories of facts independent of us (the State of California's
   rough history, WWII, the landing on the moon).
2. memories of facts of our own life. This deserves more attention than
   it's gotten, because evidence suggests that we don't really remember 
   such facts in detail, but just an outline. This means that 2 people
   can quite honestly disagree about events in their personal pasts,
   and that disagreement can increase with time.
3. short term memory for events which happened very recently. Generally
   we forget such events over time,  but the process of such memory
   plays an important role in 1 and 2.
4. short term memory for facts and events we are thinking about. The
   issue with 3. seems to include our hippocampus; the issue with 
   these other memories, some of which may have come from the distant
   past, is that it seems to take place in our forebrain, as part of
   our thinking.

If I understand properly, the most common theory for how our brain 
changes in response to NEW memories is that new connections grow between
different nerves. However recent work on the actual behavior of neural
spines (which hold one side, the receiving side, of a connection between
two nerves) suggests that our neural spines are far less stable than
neuroscientists once thought. At one time neuroscientists thought thta
such connections were very stable and lasted a long time. Recent
experiments have so far found quite the contrary: adult brains have
many changes in their synapses. 

The major problem with these experiments is that they have often been
done only on cultures of neurons, often only from the hippocampus. (It
is not easy to follow synapses in living brains!). However this work at
a minimum raises some strong questions about how our memories work. It
is important to understand here that two nerves can have MANY contacts
with one another, not just one. Just what's really going on here will
take a while to work out.

A few very simple facts may tell us why these ideas are probably true.
First, we know that the hippocampus does not control all kinds of memories
because people with damaged hippocampuses do not forget EVERYTHING, but
only particular kinds of memory. Again, work going back to the 1970's 
has shown that synapses are much less stable than thought. The problem
with such work has been that it could study only neurons on the 
periphery of our brain. The latest work looks at neurons inside our
brain. It's also been known for some time that hibernation and (of all
things) menstruation cause a decrease in the number of synapses, with
no obvious decrease in what the hibernating (animal) or female human
could remember. These results make us think that synapses may well
not store memory, though they may be one form by which it is expressed.

I shall discuss what we know about colors and sight in a separate
message.

		Best wishes and long long life for all,

			Thomas Donaldson

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