Scientists use lasers to manipulate memories in mice

Neurons are triggered in a mouse’s brain using light
Neurons are triggered in a mouse’s brain using light
Image: Mark Rossi, of Yin Lab at Duke University
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Researchers have rewired the memories in the minds of lab mice by manipulating their brain cells using a technique that involves pulses of light.

Memories carry emotional weight that can change over time, even as the details of the memory remain accurate. Take, for example, a pond where you swam as a child: perhaps it summons up an idyll. Now imagine returning to the pond, as an adult and witnessing the drowning of someone you know. The details of your childhood memories will remain intact, even as the drowning death changes the emotional resonance.

Therapists often help people who are depressed or who have PTSD to reframe their bad memories. But until now it hasn’t been clear how this reframing happens in the brain itself. A study from MIT may change that.

First, the scientists labeled the neurons that encoded specific memories in a part of the brain thought to contribute to memory formation, called the hippocampus, while the mice received an electric shock. The scientists could trigger those tagged neurons with a laser, making the mice behave as if they had been shocked without any external source. They could be made to avoid parts of a cage simply by triggering the memory every time they wandered nearby.

A cartoon illustration of the MIT experiments.
A cartoon illustration of the MIT experiments.
Image: Collective Next and Roger Redondo

The scientists then put the male mice in with females, a pleasurable experience, and triggered the shock memory. Then, they were isolated and the shock memory was stimulated again. This time, the mice spent more time in the areas associated with the shocks.

This isn’t Eternal Sunshine of the Spotless Mind: the memories of the shocks weren’t erased. But positive experiences appeared to have tempered the mice’s fear. The scientists repeated the method with another center in the brain, the amygdala, also associated with memory, but didn’t see the same rewiring effect.

The findings suggest that memories stored in the hippocampus are emotionally neutral, while those in the amygdala provide only the fear or reward responses that shade the emotional part of memory recollection. The researchers plan to see if activating pleasurable memories will alleviate depression. Maybe one day we’ll get to a point where we don’t forget our exes, just that it hurts they’re gone.