US scientists shine a light to alter dark memories

The intriguing possibility of changing your bad memories into good ones has been raised by research in the US. Scientists there have managed to flip a mouse's memories, changing its emotional response from a negative to a positive and from a positive to a negative.

We assume our memories are fixed and can’t change, but while the details remain immutable, our emotional response can alter over time. Something bad happening at a place where previously we were happy can cause our memories to turn sour.

Now scientists at the Massachusetts Institute of Technology have shown they can flip memories in mice over like throwing a switch. The memories of where a thing happened are remembered, but the mouse's emotional response has been turned about.

Mice vs humans

It would probably be possible to achieve similar results in the human brain, said Prof Richard Morris of the centre for cognitive and neural systems at the University of Edinburgh.

"I can see no reason why in principle it would not work in the same way in humans, " he said yesterday, though he added that it would be impossible to conduct such an experiment for ethical reasons: "This is not a technique that will be used in humans."

Remarkably it only required shining a blue light on certain cells in the mouse brain to change the memory. The use of light in this way comes via an emerging technique known as optogenetics. The method uses light to control brain cells which have been genetically changed to be sensitive to the light.

The goal was not to find ways to alter memory but to understand the complexity of the brain and its circuitry, according to the authors of a research report published in the journal Nature.

Building on earlier research at MIT's school of brain and cognitive sciences, professor of biology Susumu Tonegawa and colleagues targeted two brain areas that interact when memories form. The amygdala is thought to hold on to your positive or negative response to a memory, while the dentate gyrus of the hippocampus retains hard details such as location.

The modified male mice were either conditioned with fearful memories, the feeling of a small electric shock, or positive ones, being able to interact with a female mouse. This left them either fearful of or liking the location where these events occurred.

When cells in their brains were exposed to the blue light via a fine fibre optic line in their heads, these memories flipped to reverse the emotional response permanently.