Scientists have found evidence that people can actively suppress disturbing memories by choosing to not think about them, a finding that could lead to improved therapies for post-traumatic stress, whose sufferers are haunted by scary memories they can't control.
By scanning the brains of 16 healthy adults who had been shown gruesome photographs, researchers from the University of Colorado discovered subjects' memory circuits slowed when they were instructed to push mental images of the photos from their minds.
"You can train yourself to remember something, and you can train yourself to forget it," said University of Colorado graduate student Brendan E. Depue, lead author of the study published today in the journal Science.
MIT neuroscientist John Gabrieli, who was not connected to the research, called the study "a big step forward."
Previous experiments by Gabrieli and others have shown that subjects can suppress memories of neutral words and images, material viewed as more forgettable than gory scenes or personal trauma.
"The great issues for memory suppression are emotionally intense experiences," he said, cautioning that no lab experiment can duplicate the trauma of real military combat or physical abuse.
In the latest experiment, researchers trained test subjects to recognize 40 image pairs, each of which had an expressionless face and a murder scene, car crash or other disturbing picture.
After they memorized the pairs, participants were shown only the faces and asked to "think" or "not think" about the corresponding image as a scanner recorded their brain activity.
When subjects were told to block the disturbing image, the scanner recorded reduced activity in the brain regions that process and store memory. When asked to think about the images, activity in those brain regions increased.
Researchers also conducted the test without the scanner by asking participants to write down whether they remembered or forgot the photo paired with each face. They were shown each face 12 times.
When subjects tried to block the negative picture, they remembered it 53.2% of the time. But their recall rose to 71.1% when they tried to remember the disturbing scene.
But some researchers questioned the study's conclusions.
Craig Stark, an assistant professor of psychology at Johns Hopkins University School of Medicine who studies memory, said that deliberate memory suppression involved hard brain work.
"You would expect brain activity to go up, not down," he said.
Stark said the study demonstrated not memory suppression, but an ordinary sort of forgetfulness.
"As you prevent yourself from thinking about something, you will keep yourself from reinforcing and consolidating that memory," he said. "At that point, a perfectly normal process of memory decay goes to work."
James L. McGaugh, a UC Irvine professor who also studies memory, noted that the study examined retention of newly formed memories.
"It would be a stretch to imagine this would apply to being able to suppress well-established, long-term memories," he said.