Scientists have shown for the first time how the brain can intentionally blot out unwanted memories.

The discovery lends weight to Sigmund Freud’s century-old theory about the existence of voluntary memory suppression.

It also raises questions about cases of childhood sexual abuse in which the accounts of victims have been challenged.

United States researchers conducted a study which shows that people are capable of blocking thoughts or experiences they do not want to remember until they can no longer be recalled.

Freud, the father of psychoanalysis, proposed that the mind could choose to bury traumatic memories and keep them out of the consciousness. The suppressed memory was lurking somewhere, and could have effects that an individual was unable to explain.

But testing the theory is difficult, since experimental terrorising of volunteers is obviously not acceptable.

Professor John Gabrieli, a psychologist at Stanford University, in California, said: “The big news is that we’ve shown how the human brain blocks an unwanted memory, that there is such a mechanism and it has a biological basis.

“It gets you past the possibility that there’s nothing in the brain that would suppress a memory – that it was all a misunderstood fiction.”

For the study, 24 volunteers, aged 19 to 31, were given 36 pairs of unrelated nouns such as “ordeal-roach”, and “jaw-gum” to remember. The volunteers were tested on the word pairs until they got about three-quarters of them right.

Next, the participants were tested while having their brains scanned using a technique called functional magnetic resonance imaging.

The researchers randomly divided the word pairs into three sets of 12.

Volunteers were asked to look at the first word in one group of pairs, presented by itself, and recall and think about the second word.

In another set, volunteers were asked to look at the first word of a pair and not to recall or think about the second word.

The third group of 12 word pairs was memorised as normal but not during brain scanning. Subsequent testing showed that volunteers remembered fewer of the word pairs they had actively tried not to think of than the baseline pairs.

The scans showed that controlling unwanted memories was associated with increased activation of the left and right frontal cortex of the brain.

This in turn led to reduced activation of the hippocampus, the region used to remember experiences.

“For the first time we see some mechanism that could play a role in active forgetting,” said Prof Gabrieli.

“That’s where the greatest interest is in terms of practical applications regarding emotionally disturbing and traumatic experiences, and the toxic effect of repressing memory.”

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