Bad memories made good at the flick of a switch

memory encoded in hippocampus and amygdala
3D image of the brain. Hippocampus is in blue, amygdala is green. Image by lifesciencedb.BrianMSweis from Wikimedia Commons

How quickly our memories of people, things and places can change from good to bad if something like a break up happens. Now scientists have worked out how these memories change. And perhaps more importantly, they have turned bad memories into good memories. This builds on previous work showing it is possible to create and delete false memories.

Two parts of the brain, the amygdala and the hippocampus, work together to process emotional memories. The amygdala encodes the emotion and the hippocampus encodes the context i.e. the people, things and place. We are more likely to remember information if we feel emotion when we learn it.

Scientists used mice to study how emotional memories can be manipulated. The mice were genetically modified to have a light-sensitive protein in neurons in either their hippocampus or their amygdala. These light-sensitive neurons also expressed a red protein so scientists could see exactly which neurons they had activated. Then a tiny optical fibre was inserted into the mouse brains. This fibre could activate the light-sensitive neurons.

Scientists first taught one group of mice to be afraid by giving them small electric shocks on their feet while pulsing their brain with light (fear mice). A second group of mice were allowed to hang out with a lady mouse while their neurons were activated with light (reward mice).

Then the mice were put in a cage where they could turn blue light on or off. The mice that had been shocked were afraid of the light, whereas the reward mice liked the light. The light itself was enough to trigger memories of foot shocks or lady mice.

The scientists then tried to switch the mouse memories. The fear mice were allowed to spend time with lady mice while their brain was pulsed with blue light. The reward group had their feet shocked while their neurons were activated with light.

The mice were put back in the cage where they could turn the light on or off.  There was no change in the mice that had light-sensitive neurons in their amygdala. The fear-to-reward mice were still afraid of the light. The reward-to-fear mice still liked the light. So these mice still had their original memories of either foot shocks or lady mice.

But there was a really obvious change in the hippocampus group. The fear-to-reward mice were now much more keen to turn the light on, but the reward-to-fear mice were now scared of the light. This response was even stronger after a few days, with the fear-to-reward mice spending more time in the light and the reward-to-fear mice being even more afraid of the light when the scientists re-tested them four days later.

This is a really cool result because it shows that memories made in the amygdala, core emotional memories, cannot be changed. However, memories made in the hippocampus, in this case, memories about a place, can be altered.

The scientists then used the red protein in the light-sensitive neurons to work out what was happening during this memory shift. The brain was literally rewiring to make different circuits between neurons. And that is why seeing an ex who once made us happy can now make us sad.

This technology can’t be used to erase painful memories yet because humans don’t have light sensitive neurons and not many people would agree to have an optical fibre implanted in their brain. But while this specific approach won’t work, it does show that these sort of memories can be manipulated.

The Scientific Paper

Redondo et al. Bidirectional switch of the valence associated with a hippocampal contextual memory engram. Nature. 2014

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