Brain may block out undesired memories.

Memory requires collaboration between different brain structures. So does forgetting, a new study suggests.

Two neural regions join forces to enable people to suppress unwanted memories, say psychologist Michael C. Anderson of the University of Oregon in Eugene and his colleagues. The team has scanned the brains of volunteers who were asked to forget previously viewed words. As volunteers try to do so, tissue near the front of their brains, in the prefrontal cortex, dampens activity in the hippocampus, an inner-brain structure required for memory retrieval, Anderson's group finds.

These findings provide a potential brain mechanism for the voluntary form of memory repression originally proposed by Sigmund Freud, the scientists conclude in the Jan. 9 Science. Freud regarded repression as a process in which the motivated forgetting of disturbing or threatening information occurs either unconsciously or with an intentional push.

"Our new findings help to demystify how repression might occur in the brain," Anderson says. The same section of prefrontal cortex now linked to intentional forgetting was previously implicated in the inhibition of learned physical responses.

In the new work, 24 adults studied a long series of written word pairs, such as ordeal and roach. Functional magnetic resonance imaging monitored blood flow in each volunteer's brain as he or she saw one word from each pair and was asked either to recall and think about the associated word or to avoid thinking about it.

A subsequent memory test indicated that for most people, efforts at suppressing words impaired recall for those words, a result that bolsters an earlier study by the same researchers (SN: 3/17/01, p. 164). The effectiveness of this willful forgetting varied among participants. Some forgot one-third of the words they tried to suppress, whereas several participants forgot only a few words, despite their best efforts.

While trying to suppress their memories, the best forgetters displayed particularly intense blood flow--indicating elevated neural activity--in the prefrontal cortex and unusually little blood flow in the hippocampus.

The new study provides an intriguing peek at brain processes that correlate with intentional forgetting, remarks psychologist Jonathan W. Schooler of the University of Pittsburgh. However, this phenomenon doesn't necessarily qualify as repression, which is a hazy concept, he contends. Repression has often been described as the forgetting of emotionally traumatic experiences, whereas Anderson's study involved relatively neutral words, he notes.

Psychologist Endel Tulving of the University of Toronto says he awaits independent confirmation of the results in larger trials before accepting Anderson's theory that the brain fosters a conscious form of memory repression.

For anyone expecting that brain imaging is about to quell scientific debate over repression--forget it.