By Merritt McKinney

NEW YORK, Mar 14 (Reuters Health) - In a finding that bolsters Freud's theory that people can suppress painful or otherwise unwanted memories, researchers at the University of Oregon report that when people are instructed to try to forget certain words, they are less likely to remember them.

The study provides "very clear evidence" that people can suppress memories, according to the study's co-author, Dr. Michael C. Anderson, of the University of Oregon in Eugene. It is possible to "actively push memories outside of the mind," he told Reuters Health in an interview.

Dr. Daniel Schacter, chairman of the department of psychology at Harvard University in Cambridge, Massachusetts, concurs. The study, he said, "provides strong evidence that people can willfully suppress memories" at least in a laboratory setting. "We still don't know whether this applies to emotionally-laden experiences," he added in an interview with Reuters Health.

Although the findings do not have any immediate clinical implications, future research may lead to a better understanding of why people suppress some sorts of experiences, according to Schacter, who is the author of an upcoming book entitled, "The Seven Sins of Memory."

Anderson got the idea for the research after a colleague detected differences in how adults who had been sexually abused as children remembered the abuse. People who had been abused by a parent or someone else they knew were much more likely to have forgotten the experience for several years than people who had been abused by a stranger, according to Anderson.

Since people abused by someone they know would most likely have to face that person repeatedly, they might have an incentive to forget the experience, he said.

"Maybe what you're doing is training your mind to not go there," Anderson explained.

To see whether the brain can selectively forget memories, Anderson and a colleague conducted a set of experiments in which people were shown a series of pairs of unrelated words, like "ordeal-roach." When the participants were shown the first word, they were instructed to either recall and say the second word in the pair or to not think about the second word.

Writing in the March 15th issue of the journal Nature, Anderson and colleague Collin Green report that participants were less likely to remember the words that they had been told to forget. And the more times they were exposed to these "suppressed" words, the less likely they were to remember them. In contrast, the ability to remember the other words improved with practice.

After several variations of the experiments, participants were instructed to remember all words, even those they had previously suppressed. Even when offered money for each word they correctly remembered, the participants still had a more difficult time remembering suppressed words.

According to Anderson, one possible explanation is that people who are trying to forget an experience become accustomed to thinking of something else every time they are faced with a situation that triggers the memory. Over time, this response may become automatic so the person is not even aware of it, he said.

But there is still a lot to learn about how the brain suppresses memories, Anderson said in the interview. He noted that the experiment was conducted in a laboratory using everyday words, and the process could be different for memories that involve real emotions. Also, the researchers tested memory after a brief delay, not months or years.

The findings do not necessarily mean that recovered memories so often revealed on television talk shows are necessarily the whole truth.

It is important to investigate what happens to memories themselves when they are suppressed, Anderson said. It is conceivable that suppression could disrupt the memory in some way so that it is somewhat fragmented, he suggested.

Just because a person can remember something previously forgotten "doesn't mean that the memory is just fine," Anderson said.

The findings "may ultimately shed light on how repression comes about," according to Dr. Martin A. Conway, of the University of Bristol in England.

The researchers "have now shown that, even in the innocuous setting of the laboratory, and with stimuli as trivial as randomly paired words, powerful inhibition (of memory) can be evoked," Conway writes in an editorial that accompanies the study.

"How much stronger must this inhibition be for objects central to our thoughts and emotions," he concludes.