London: Imagine being plagued with bad memories, images of such terrible trauma - an accident or an instance of abuse — that they produce an uncontrollable emotional reaction. Now imagine being able to wipe away the pain of those memories. Scientists are working on a way to do just that. By studying how we lay down our memories, research shows it is possible to select and alter the way memories are stored in our minds.

Roger Pitman, a psychiatrist at Harvard University, has shown that giving certain drugs to victims of trauma when they were brought in to hospital meant they were less likely to develop conditions such as post-traumatic stress disorder (PTSD). Now he believes he can even cure PTSD sufferers years after the event.

"Our theory about PTSD is that there's an excess of stress hormone that sears the memory too deeply into the brain," said Dr. Pitman. "If we can block the effects of those stress hormones, we may be able to prevent people from getting these overly strong memories that can become PTSD."

The work takes advantage of the way memories - essentially networks of brain cells that each store information on a single event or object — are formed.

"When you form a new memory, it's not immediately stored in the brain," said Karim Nader of McGill University, Montreal. These new memories exist as temporary modifications to already-existing networks of brain cells that, over the course of a few hours, stabilise into networks of their own.

To get into the long-term memory, the temporary modification has to stabilise or fix into a new network of brain cells. The traditional view among neuroscientists has been that, once these memories become fixed, they are difficult to change.

But this view was challenged when Dr. Nader carried out experiments on rats. He trained them to be fearful of particular stimuli, such as heat. He later made them remember those fearful memories by exposing them to the stimuli again. Dr. Nader found that calling up a fixed memory from deep in the brain made that memory go back to an unstable state, the same as a new memory, which then has to be re-stabilised if it needs to be stored. "If you block it from being restabilised, then the memory is essentially no longer there," said Dr. Nader.

"Each time the synapse is activated, as it is memory, there are processes going on that could result in a strengthening or a weakening," said Dr. Pitman.

How memories are fixed also depends critically on any emotional response we might have had at the time of the incident. "We can all better remember things that have emotional meaning to us than things which are neutral, so your first date, the first time you were in love, your first divorce," said Dr. Nader.

- © Guardian Newspapers Limited 2004

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