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Saturday, September 08, 2007

Losing Your Past

Do you remember the names of your elementary-school teachers? How about the name of the bully in middle school? Or names of your friends when you were a kid? These are all things you remembered well at one time and remembered for a long time. But you may well have forgotten by now.

Scientists like to talk about "long-term" memory, but even long-term memory has its limits. A recent study on rats suggests what it takes to sustain longer term memories. Rats in the study learned a "bait shyness" task. Rats were given a drink of saccharin-flavored water, and then shortly afterwards injected with lithium, which made them nauseated. This was a typical conditioned learning situation, as with Pavlov's dogs. In this case, rats typically remember to avoid such water for many weeks. This is the basis for "bait shyness." If rats survive a rat poisoning episode, they will avoid that bait in the future. However, in this experiment,one group of rats received an injection directly into the part of the brain that holds taste memories.This injection contained a drug that blocks a certain enzyme, a protein kinase. These rats lost their learned taste aversion. The bad memory was lost irrespective of when the injection was made during the 25 days after learning occurred.Giving the enzyme blocker before learning had no effect on the ability to learn to avoid the flavored water.The protein kinase thus seems to be necessary for sustaining a long-term memory. It is possible that other long-term memories the rats may have had were also wiped out by the enzyme-blocking drug, but this was not tested.

So what is the practical importance of these findings? I suggest that even "long-term" memories have to get rehearsed once and a while or they may eventually fail to remember (see my earlier post on "reconsolidation" of memories. Or if you do remember, there is a good chance that the memory is corrupted, that is, not totally correct. The consequence is that things that happened long ago may be either forgotten, misremembered, or so buried in memory that it takes a great deal of cuing to retrieve the memory. I have a whole chapter in my book, Thank You Brain for All You Remember on the subject of false memory.

The study showed that a certain enzyme has to be present to preserve a memory. Without the enzyme, the memory disappears. What sustains the enzyme? I suspect it is rehearsal and periodic reactivation of the memory, although this possibility has not been tested yet. Some scientists are excited about the possibility of developing a drug that would eliminate the enzyme or insure its presence. The problem with that, however, is that the drug could abolish old memories that you might not want to forget (like your name) or may cause you to remember too much that is now irrelevant.


Shema, R., Sacktor, T. C., and Dudai, Y. 2007. Rapid erasure of long-term memory associations in the cortex by an inhibitor of PKM. Science. 317:951-953.

For more summaries of recent research with practical application, click here.