Researchers have long believed that once memories are formed, they remain relatively stable in the brain. However, research by New York University psychologist Joseph LeDoux, PhD, presented at APA’s Annual Convention, suggests that every time a memory is retrieved, the brain breaks some of the chemical bonds that make up its physical foundation.
As a result, memories become unstable for a brief time after their retrieval — and this fact, said LeDoux, could have implications for therapists working with people who have phobias or post-traumatic stress disorder.
“Memories become fragile during the retrieval process,” he said.
As a case in point, LeDoux shared findings from a study he and his colleagues published in Nature (Vol. 406 No. 6,797). In that study, rats learned to fear a tone that always preceded a foot shock. One to 14 days after the shock training, the researchers reactivated the memory by playing the tone and then injecting the lateral and basal nuclei of the rats’ amygdalas — a major site involved in memory storage for fear learning — with anisomycin, a protein-synthesis inhibitor. The anisomycin prevented the brain from re-forming memory connections after breaking them during the retrieval process. As a result, those rats did not show fear responses, such as freezing in place, when they heard the tone in subsequent trials.
In comparison, rats that received the anisomycin without having the fear re-activated showed persistent fear responses. Also, rats that received anisomycin injections more than six hours after fear re-activation didn’t show the attenuated fear response since the brain had reconsolidated the fear by that point, LeDoux said.
Because anisomycin is toxic to humans, this particular procedure can’t be used to help people overcome phobias or PTSD. However, some researchers have used the blood-pressure medication propranolol to interrupt the memory reconsolidation process in human participants, LeDoux said.
Nonetheless, new research by LeDoux and his colleagues, published in Nature (Vol. 463, No. 7,277) has found that one can exploit the fragility of recently retrieved memories without drugs. In the study, the researchers conditioned participants to associate colored squares with a mild electric shock. The researchers gave the participants a day to let the fear memory consolidate and then reactivated the memory by showing them the colored squares (this time, without the electric shock).
Here’s the important part: The researchers then allowed either 10 minutes or six hours to elapse before showing the participants the colored square again. For those participants in the 10-minute condition, their retrieved memory was still fresh and fragile, and they later showed little fear response to the colored square. Participants that went through the extinction procedure six hours after memory retrieval, on the other hand, still showed a strong fear response to the colored square. That’s because their memory had plenty of time to reconsolidate, LeDoux said.
LeDoux isn’t yet sure how these results might translate to clinical settings, but it’s possible that therapists would have longer-lasting results if they performed exposure therapy during that critical time period — 10 minutes to six hours following memory retrieval, said LeDoux. “If you’re doing exposure therapy, don’t do it all at once,” he said. “That’s regular extinction,” and it can leave people vulnerable to spontaneously recovering fears.
Though taking advantage of animal researcher’s new understanding of memory consolidation could provide therapists with a powerful new tool, it won’t, for example, erase a soldier’s memory of war. That’s because the details of a memory appear to be stored elsewhere in the brain than the amygdala, which records a memory’s emotional pull.
“You’ll still have a memory of trauma, but you won’t be quite as upset about it,” he said.
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