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Triggering happy memories, scientifically


Chronic stress is a leading cause of depression.

Susumu Tonegawa, a Nobel-winning molecular biologist at MIT, recently completed work that might eventually help people bounce back.

Tonegawa recently showed that he can cure mice of this type of malady — by manipulating their brains to trigger a happy memory.

Normally, when we make a happy memory, engram cells undergo physical and chemical changes to encode it, Tonegawa explained. We can retrieve it when those cells are triggered.

If you once had a wonderful vacation in Paris, a picture of the Eiffel Tower can trigger your brain to recall that experience.

Depression blocks the action, however. We take less joy in things that used to give us pleasure. That picture of the Eiffel Tower becomes just a picture.


Hoping to better understand both depression and memory, Tonegawa and his colleagues gave a mouse some happy memories. (For a male mouse, happiness isn’t a trip to Paris, but a female housed in his cage.)

During those good times, the researchers tagged the engram cells encoding the happy memories and added a special protein, so the cells would fire when exposed to blue light.

Then, researchers repeatedly stressed the mice, making them depressed. A depressed mouse won’t try to climb its captor’s hand when held by the tail — a sign that it sees struggle as futile. Depressed mice also drink sugar water and regular water in equal amounts, while a normal mouse prefers the sweet stuff.

When researchers turned blue light on the rodents’ happy engram cells, the depressed mice immediately started to struggle again; they started drinking more sugar water.

Artificially triggering the pleasant memory overrode depression’s block on their happiness. And the effect lasted for a couple of days at least.

(Giving a depressed mouse happy experiences doesn’t work, Tonegawa said. Nor does simply reminding it of nicer times. Though in people, psychotherapy that repeatedly reminds someone of the good in their life might have the same benefit, he said. In mice, only artificially forcing the engram cells to fire seems effective.)


So far, this treatment has only been attempted in mice.

“The question is, is this of any use for medicine?” Tonegawa asked. “The honest answer is we don’t know yet.”

But he said he’s optimistic that eventually someone will be able to build a device that has similar effect on the human brain — artificially triggering someone’s happy memories to treat their depression.

“When that kind of technology is invented,” he said, “it could potentially become very powerful therapy.”