While the old saw says that it is easier to forgive than to forget, new research reveals that the latter may still be possible, even for very emotional memories.
Researchers at the University of Colorado worked with 16 subjects to see if they could actively suppress their own memories.
To do this, the researchers had subjects memorize 40 pairs of pictures, which were chosen because they provoked emotional responses.
The subjects were then shown the pictures again, this time instructed by the researchers to either remember or forget specific ones. At the same time, the researchers ran brain scans on the subjects to see which areas of the brain became active when the subjects were told to remember or forget a picture.
Finally, subjects were shown one picture from each of the pairs to see if they remembered its mate.
What researchers found was that subjects had much better memories of the pictures they were instructed to remember than of those they were told to forget.
The research appears in this week's issue of the journal Science.
"The big step forward is they showed how memory suppression in the brain can operate successfully with emotionally intensive material," said John Gabrieli, a professor in cognitive neuroscience at MIT who was not involved with the study.
In 2004, a team of researchers led by Michael Anderson, head of the Memory Control Lab in the psychology department at the University of Oregon, had first shown that the brain had mechanisms to suppress memories.
But, as Anderson explained, this experiment is the first to show that the brain can apply those same mechanisms to suppress complex, emotionally intense scenes.
The findings could have implications for those suffering from posttraumatic stress disorder or others whose traumatic memories have a negative impact on their lives.
"In the future, this will hopefully guide therapeutic approaches," said lead researcher Brendan Depue, a neuroscience doctoral student at the University of Colorado.
Gabrieli, who works with patients in addition to his lab work, sees two potential clinical applications for this research.
First, he said, using a brain scan, a clinician could determine early on whether a certain therapy was effective in helping patients control their disturbing memories, rather than waiting for outward signs of improvement or crisis.
"You don't wait with Lipitor to see if a patient gets a heart attack," Gabrieli said.
Second, he said, the patients might even be able to learn how to activate the parts of their brain that would allow them to forget unwanted memories by viewing their active brain scans.
Based on the brain scans from the study, Depue concluded that the brain has two steps in forgetting.
First, the part of the brain responsible for recalling the senses associated with the memory blocks those out. Then, the brain blocks the actual memory retrieval and the emotions associated with the memory, he explained.
However, Depue said that it is unlikely that the memory is actually forgotten, and more likely that the brain simply blocks the memory from being accessed.
It has been a matter of some debate whether memories are suppressed by directly blocking the memory or by recalling other memories instead, to prevent the undesired memory from surfacing,