However, scientists do not entirely understand the brain mechanisms that enable memories to change and that even allow us to feel different emotions about those memories. Elucidating these mechanisms could help scientists one day develop new therapies for conditions such as depression and post-traumatic stress disorder.
In the new study, the researchers looked at neurons in a brain structure called the hippocampus, which is thought to encode the context of memories, such as where an event happened. The researchers also looked at neurons in another brain structure, the amygdala, which is believed to encode emotions.
The mice in the study were genetically engineered to make tracking their memories easier. As the animals' fearful or pleasant memories were formed, a light-sensitive protein was expressed in the neurons that encoded the new memories. This way, the researchers were able to tag these neurons, and later use light to reactivate the memory those brain cells held.
The new experiment worked because the scientists tackled the contextual and emotional aspects of the memory separately. When the researchers activated the neurons in the hippocampus, it evoked the contextual part of the memory, while new events the mouse was experiencing rewrote the emotional part of the memory. This led to a new memory of the same place but with a different emotional association, the researchers said.
Looking at the cells under the microscope, the researchers confirmed that the relationship between the memory-holding neurons in the hippocampus and those in the amygdala was altered after the scientists' manipulations, suggesting that the connection between the two brain regions is indeed malleable.
The new experiments followed previous studies by the same researchers last year, in which they implanted false memories in mice. In those studies, the researchers activated neurons to make mice remember a previous experience as the animals were undergoing a new and different experience. This made mice form a memory of the mixture of the two experiences, which in real life had never happened.
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