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 Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=15849