More effective therapies for depressed people and those who suffer from Posttraumatic Stress Disorder, or PTSD, who are haunted by scary and depressing memories and experiences, may soon become available with a new technique developed by neuroscientists from the Massachusetts Institute of Technology.

In a new study "Bidirectional switch of the valence associated with a hippocampal contextual memory engram," which was published in the journal Nature on Wednesday, Aug. 27, researchers have come up with a technique that uses pulses of light to switch neurons on and off.

For the experiment that the researchers conducted on mice, Susumu Tonegawa, from the Howard Hughes Medical Institute of the Massachusetts Institute of Technology, and colleagues labeled the neurons in the brains of mice that encode a specific memory with a light-sensitive protein called channelrhodopsin while the mice were given an electric shock. The researchers could then trigger these labeled neurons with pulses of light to make the mice behave as if they were given an electric shock even without an external source.

The researchers then gave the mice a pleasurable memory by placing the male mice in the company of female mice and triggered their shock memory. After the mice were isolated, their shock memory was once again stimulated but this time, the mice were not no longer as fearful of the parts of the cage where they were conditioned to remember the electric shock and while the memories of the electric shock did not disappear, the positive experience appear to have altered the fear of the mice.

Tonegawa said that the results of the experiment provide a basis for certain types of psychotherapy where patients are urged to associate something pleasurable with the things that they fear. The study has likewise found that the neurons in the hippocampus, the part of the brain that is involved in forming, organizing, and storing of memory, can be altered to make a bad memory less negative and vice versa. The neurons in the amygdala, the part of the brain that is also involved in memory as well as in emotions and aggression, on the other hand, do not change prompting scientists to conclude that these nerve cells are already wired to reflect negative or positive emotions.

Although the experiment was conducted on mice, the study may lead to better therapies for people who suffer from psychological problems.

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

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