Unwanted memories can be driven from awareness, according to a team of researchers who say they have identified a brain circuit that springs into action when people deliberately try to forget something.
The findings, published today in the journal Science, strengthen the theory that painful memories can be repressed by burying them in the subconscious, the researchers say.
In the study, people who had memorized a pair of words were later shown one of them and asked to either recall the second word or to consciously avoid thinking about it.
Brain images showed that the hippocampus, an area of the brain that usually lights up when people retrieve memories, was relatively quiet when subjects tried to suppress the words they had learned. But at the same time, another region associated with motor inhibition, called the dorsolateral prefrontal cortex, showed increased activity.
The scientists also found that the more the subjects were told to resist thinking about a word, the more likely they were to have trouble recalling it later.
"This suggests a neurological basis for how people can actually shove something out of mind," said Dr. Michael C. Anderson, a professor of cognitive neuroscience at the University of Oregon and lead author of the study. "There's no question that we're tapping into something that's relevant to the experiences of people who survive trauma and find the memories become less and less intrusive over time."
Dr. Anderson said the burst of activity in the prefrontal cortex, an area that manages higher-order cognitive skills like planning, could represent an overriding mechanism, in which the hippocampus is prevented from dredging up unwanted memories.
Over time, continued suppression of those memories by the prefrontal cortex, he said, can push them from awareness.
"We could predict how effectively people would forget these words just by how much activation they showed in their prefrontal cortex," Dr. Anderson said. "I think this explains why the tendency to be reminded of something horrific, for example, eventually diminishes."
Dr. Larry Squire, a professor of psychiatry and neurosciences at the University of California at San Diego, who did not participate in the study, said it was difficult to say exactly what the brain images meant. Still, concluding that the activity in the prefrontal cortex points to a brain circuit that can block memories, particularly emotional ones, he said, might be too narrow an interpretation.
"This is a much debated issue," Dr. Squire said. "It's possible the subjects are simply directing their attention elsewhere and using a lot of energy and brain resources to think of something different. I don't think it is necessarily an indication of active repression."
But Dr. David Spiegel, professor of psychiatry at the Stanford University School of Medicine, said diverting thoughts away from something was the first step to forgetting about it completely. And the study, he added, supported the notion that people could suppress traumatic memories and still regain them later.
"People have to manage vast amounts of information by keeping most of it out of mind, which is true of emotional memories and all others," said Dr. Spiegel, who was not involved with the study. "At any given moment you couldn't remember most of what you know or you'd be overwhelmed. But the memories are there, and you can still recover them down the line."