Emotions connected to memories can be rewritten, making bad events in the past seem better and good things appear worse, scientists from Japan and the United States have found.
The discovery of the mechanism behind the process helps to explain the power of current psychotherapeutic treatments for mental illnesses such as depression or Post Traumatic Stress Disorder (PTSD), they said, and could offer new avenues for psychiatric help.
"These findings validate the success of current psychotherapy, by revealing its underlying mechanism," research leader Susumu Tonegawa told AFP in Tokyo.
The team, formed from a collaboration between Japan's RIKEN institute and the Massachusetts Institute of Technology in the US, used optogenetics -- a new brain-control technology which utilises light -- to better understand what happens when we reminisce.
They found that warm feelings or intense fear triggered by the interaction between the hippocampus -- the brain's diary room -- and the amygdala -- the place believed to encode positivity or negativity -- are more flexible than previously thought.
"It depends on how strongly the (good or bad aspect) dominates... there is competition between the two circuits' connection strengths," Tonegawa said.
The researchers injected two groups of male mice with light-sensitive algae protein.
This allowed them to identify the formation of a new memory as it was happening and then use pulses of light to reactivate it when they wanted to.
One group of rodents were allowed to play with female mice, creating a positive memory. The other group were given a small but unpleasant electric shock through the floor.
Researchers then artificially reactivated the memory using the light pulses -- effectively making the mice remember what had happened to them.
While the mice were "remembering" their event, they were given the opposite experience -- the mice with the nice memory got a shock, while those with the painful memory were introduced to females.
Tonegawa said his team had discovered that the emotion of the new experience overpowered the original emotion, rewriting how the mice felt about it.
"We did a test in the original chamber and the original fear memory was gone," he said.
However, the over-writing of a memory was only possible by manipulating the hippocampus, which is sensitive to context. The same result could not be achieved by manipulating the amygdala.
Tonegawa said the connection between the contextual memory in the hippocampus and the "good" or "bad" emotions in the amygdala became stronger or weaker depending on what was experienced.
The researchers hope their findings might open up new possibilities for treatment of mood-affecting disorders such as depression, or PTSD, a condition found in people such as soldiers who have undergone life-threatening or particularly horrific events.
"In the future, I would like to think that with new technology we will be able to wirelessly control neurons in the brain, without intrusive tools like electrodes," said Tonegawa, who won the Nobel Prize for Physiology and Medicine in 1987.
"We could possibly augment good memories over bad ones," he said.
The research paper is published in Nature.
In a commentary, also carried by Nature, cognitive researchers Tomonori Takeuchi and Richard Morris at the University of Edinburgh in Scotland said the study broke new ground in exploring the mechanisms of memory, although optogenetics had limitations as a tool for doing this.
"Molecular engineering is nonetheless shedding light on our understanding of the underlying physiological networks of memory," they wrote.