People can actively suppress memories by dampening activity in specific parts of the brain, according to research published today. The findings could spur development of new treatments for post-traumatic stress disorder (PTSD), either in the form of drugs that target these areas or new behavioral therapies that might boost a person’s ability to suppress certain memories. One drug that appears to act through similar mechanisms has shown early success in patients.
Silencing memories: When volunteers successfully suppressed a memory, they showed less activity in the hippocampus (shown here in blue), which plays a crucial role in memory.
“It does look like there is a brain mechanism for decreasing the potency of a memory,” says Marie Banich, director of the Institute of Cognitive Science at the University of Colorado, Boulder, and leader of the research. “And it seems to be a learning process, which suggests that control of memory could be engaged in therapy.”
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In the experiment, published today in the journal Science, volunteers memorized pairs of pictures, one of a face and one of a disturbing image, such as a car accident or surgery in progress. Researchers then showed the volunteers pictures of the faces while asking them either to remember or to try to suppress the photo that had been paired with it. Using functional magnetic resonance imaging (fMRI) to analyze the brain’s response, scientists found that when people successfully “forgot” the memory, as evidenced in later memory tests, activity in several parts of the brain was dampened. These areas included the hippocampus and the amygdala, parts of the brain crucial for memory and emotion.
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Researchers also found that the pattern of brain activity changed over time as volunteers were asked repeatedly to try to suppress the same memory. The hippocampus–the brain’s memory center–became less active in later trials, suggesting that “the memory has to come back a bit before you can suppress it,” says John Gabrieli, a neuroscientist at MIT.
And the prefrontal cortex–which has been implicated in cognitive control–became more active in later trials, suggesting that the frontal cortex is learning to exert control over the memory parts of the brain, Gabrieli says.
The findings could aid development of new therapies. “If you know what the neurobiological systems are, it helps you to better target treatments for intrusive memories,” says Michael Anderson, a neuroscientist at the University of Oregon in Eugene. “We know the prefrontal cortex is involved in voluntary memory suppression. Armed with that new fact, perhaps we could develop drug treatments that facilitate the ability to suppress unwanted memories.”
The findings could also help shape nondrug treatments. If memory suppression is a skill that can be improved with practice, as the study suggests, PTSD patients may be able to learn to better control their unwanted thoughts.
Suppressing memories for therapeutic reasons is a contentious issue among scientists. For many, the idea itself–to block access to part of one’s past, as depicted in the 2004 movie Eternal Sunshine of the Spotless Mind–is somewhat disturbing. Some psychiatrists believe, as Freud most famously did, that it’s emotionally harmful, contributing subconsciously to everyday anxiety. Most psychologists who treat PTSD patients emphasize that their goal is not to erase memories themselves, but to dampen their emotional charge. “You can never erase a serious trauma from someone’s memory,” says Katherine Putnam, a psychologist at Boston University Medical School. “The issue is to reframe [the memories], to get some kind of detachment.”
Preliminary studies of one drug, currently used to treat heart problems and anxiety, show that it seems to do just that. Researchers administered the drug, known as propranolol, to volunteers after they recalled a traumatic event. The drug appeared to subtly change the way they remembered the event: while they could still recall the details, the emotional response that normally accompanied the memory was dulled. Scientists haven’t yet studied the effect of this drug on the brain with fMRI.
While it’s not yet clear how the drug works, scientists believe it interferes with the way memories are stored after being recalled. Recent research has shown that memories are not set in stone. Unlike a video, every time a memory is “replayed,” it needs to be organized and stored anew, making it vulnerable to change. “We believe during that process, some kind of interference occurs, and the memory is degraded,” says Alain Brunet, a psychologist at McGill University, in Canada, who led the study, published last week in the Journal of Psychiatric Research. (The mechanism behind the memory reconsolidation process appears to be different than that behind memory suppression, but both involve reactivating the memory and then reshaping it in some way.)
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While both studies are exciting, scientists caution that they are still far from clinical practice. In the case of propranolol, larger studies are needed to assess how well the treatment works and for how long. In the case of nondrug therapy, it’s not yet clear whether the techniques used in the memory-suppression study will be applicable to trauma victims. “The effects are small, about 10 percent, so they are not on scale of what we imagine true clinical suppression of a traumatic event would have to be,” says Gabrieli. “But that doesn’t mean they don’t give us hints. Making a patient 20 percent happier is significant.”
