The majority of memories are linked with some form of emotion, much like the recalling of the death of a loved one evokes feelings of sadness, while thinking about times spent with a friend may evoke feelings of happiness.

Neuroscientists from MIT have discovered the brain circuit that controls the link between memories and specific emotions, both positive and negative. In addition, the researchers have also discovered how to manipulate brain cells in order to reverse such emotional associations with certain memories through a technique called “optogenetics,” which uses light to control neuronal activity.

Senior study author Susumu Tonegawa, the Picower Professor of Biology and Neuroscience, and colleagues found a neuronal circuit connection between the hippocampus and the amygdala of the brain, which plays a key role in associating certain emotions with memory. This circuit may help scientists develop drugs to treat mental health conditions, including post-traumatic stress disorder (PTSD).

Previous studies have found that many factors about memory are malleable, which is what psychotherapists have taken advantage of when helping patients suffering from PTSD or depression. For this study, the researchers sought to explore this malleability with an experimental technique that permits them to find neurons encoded with a certain memory, or engram, in lab rats. The researchers then “incepted” false memories by reactivating engrams while the rats were going through a different experience.

“In the future, one may be able to develop methods that help people to remember positive memories more strongly than negative ones,” said Tonegawa in a statement.

The findings of the study are published in the journal Nature.