Have you ever had a particular place or event ruined because of a bad memory or painful association? Ever left a bad first impression you wish could be reversed? Well, in a feat that sounds closer to science fiction than science fact, scientists may soon be able to change a bad memory into a good one with nothing more than a "flip of the switch," reports the BBC.

How is this possible? It turns out that the positive or negative emotional associations we have to certain memories are entirely malleable. They're so malleable, in fact, that a simple neurological procedure can potentially turn a bad memory into a good one instantly.

Or at least, it's possible in lab mice. But scientists say a similar procedure could theoretically work on humans too.

"Emotion is intimately associated with memories of past events and episodes, and yet the 'valence' — the emotional value of the memories — is malleable," said the study's senior author, professor Susumu Tonegawa, from the Riken-MIT Center for Neural Circuit Genetics in Massachusetts.

Development of this technology began two years ago when the researchers showed they could label the cells storing a new, fearful memory in a mouse's brain, and turn them on again at a later date to cause fearful behavior. Then last year, they figured out how to label a memory of a place and then later reactivate it while giving the mice small electric shocks. This allowed them to directly cause a negative emotional association with the place. 

This led researchers to wonder if they could pinpoint specific memories and artificially change the emotional associations that go along with them from one state into another. 

For the experiment, researchers installed a "switch" sent into the mouse's brain by an optical fiber, consisting of a blue light. Researchers were able to install this switch directly into neurons pertaining to a mouse's particular negative memory of a place. Because the blue light stimulated the negative memory, the mice preferred the blue light to be left off. 

But later they stimulated these same neurons while also giving the mouse an unrelated positive emotional cue, and sure enough the mouse altered its preference. It then preferred the blue light to be left on. More importantly, though, when the mouse was returned to the original place it had learned to despise, it was no longer fearful of the place. In other words, the procedure had transformed the negative memory into a positive one.

Researchers were also successful at transforming a positive memory into a negative one using the same methods. 

So can this procedure be performed on humans? It's possible, say researchers, but there are ethical hurdles.

"Nobody's going to be queuing up to have light guides inserted into their brain and have blue light shone down into it," said Professor Richard Morris, a memory researcher from the University of Edinburgh. "But [these results will] help us understand the proportion of cells that are involved when you have to change a memory from being a bad one into a good one. Are we dealing with changing 50% of the cells in the amygdala, or are we dealing with 1% or even less?"

"I think that's valuable, that's important — to move beyond just a behavioral understanding, but to have a deeper understanding of the clinical task in front of us."

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