Researchers found that by manipulating neural circuits in the brains of mice they could alter specific memories.

The Howard Hughes Medical Institute and MIT investigators revealed connections between the part of the brain that stores contextual information of an experience and the region involved in emotional memory are malleable. Altering these connections could potentially morph a negative memory into a positive one.

"There is some evidence from pyschotherapy that positive memory can suppress memories of negative experience. We have shown how the emotional valence of memories can be switched on the cellular level," said Howard Hughes Medical Institute investigator Susumu Tonegawa.

In a past study the team found artificially activating small sets of cells that stored a specific memory could create a new false one; they did this by making the cells light-sensitive. They found switching on cells of a pleasant memory from a safe place while subjecting the animal to a mild shock in a new environment caused the mouse to fear the original location of the memory, even though it had never had an unpleasant experience there.

In the more recent study the team placed male mice in a shock chamber, which caused them to fear the "dangerous" location. The team waited a few days and then switched on the bad memories, causing the mice to stop exploring and freeze in place.

The researchers placed the mice in a new environment where instead of being shocked they were allowed to interact with female mice. The team then activated the neurons storing the fearful memories.  Reactivating the amygdala (the  portion responsible for emotional associations) did not evoke a response in the mice, but when the researchers reactivated memory cells in the hippocampus the mice acquired a new emotional association, causing them to seek out the environment that triggered the memory.

"So the animal acquired a pleasure memory," Tonegawa said. "But what happened to the original fear memory? Is it still there or is it gone?"

When the team put the animals back in the chamber in which they had experienced the shock they showed less fear and more exploratory and reward-seeking behavior.

 "The original fear memory is significantly changed," Tonegawa concluded. 

The findings were published in the Aug. 28, 2014, issue of the journal Nature.