Could we reactivate positive memories to fight depression?

Steve Ramirez and Xu Liu spend their days in a lab at MIT, working to manipulate the memories of mice.

Steve Ramirez and Xu Liu spend their days in a lab at MIT, working to manipulate the memories of mice.

Steve Ramirez and Xu Liu are cartographers of the brain, mapping out its intricate memory system and charting the neurological pathways to discover what makes us tick. But some may just call them neuroscientists.

At TEDxBoston in 2013, Liu and Ramirez shared their early research, documenting how exactly one identifies a memory in the brain and the uncharted potential their discoveries hold via tests on lab mice.

Their work involves using an engineered virus to activate brain cells in mice with pulses of light. The virus acts as “a sort of light-sensitive switch that can be artificially installed in brain cells,” Ramirez says in the talk, “to activate or inactivate the brain cell simply by clicking it, and in this case we click it on with pulses of light.”

Through this process, they triggered the memory of fear in a mouse not immediately facing any dangerous or fear-inducing situations, a memory that caused an otherwise calm and curious mouse to freeze in its tracks.

And this year, the pair was invited back to TEDxBoston to discuss their most recent research, involving the relationship between positive memories and depression.

Watch Steve Ramirez and Xu Liu’s 2014 TEDxBoston talk

In their talk, the pair explains that when giving mice an option between sugar water and regular water, healthy mice show a strong preference for the sweeter water 80% of the time, while mice with depression-like symptoms only show a preference 50% of the time — they do not care for one more than the other. These mice exhibit an inability to experience pleasure, Ramirez says, one of the many symptoms associated with depression in humans.

Knowing that light could be used to activate negative memories in mice from their previous research, Liu and Ramirez wondered if the same process could be used to activate positive memories in mice showing depression-like symptoms, and if so, possibly aid in lessening the symptoms of these mice. To test their theory, they gave mice showing depression-like symptoms the choice between sugar water and regular water, while simultaneously activating a positive memory in the brain via light pulses. When the process was used, a depressed mouse’s preference for sugar water immediately rockets to 80%, Ramirez says, which shows a profound antidepressant-like effect.

But not only did the two note behavioral changes in mice, they also saw increased number of neurons in the brain after positive memory activation. “Past animal studies have shown that depression-like symptoms can reduce the number of new neurons in the brain,” he says, “while anti-depression treatments, such as antidepressant drugs and exercise can boost up the number of new neurons in the brain.” To test their process’s effect, they repeatedly activated a positive memory in the brain of a mouse exhibiting depression-like symptoms and found that the number of new neurons in its brain indeed increased.

Though their research continues, Liu and Ramirez are optimistic about their discoveries, likening the process to a “double thick Oreo milkshake multiplied by world peace,” which gives one reason to believe they’re on the road to some sweet memories.


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