Researchers Show “Proof of Principle” that Memories Can Be Erased

Scientists have advanced their understanding of how memories are encoded, researchers said during an Annual Meeting press briefing. | Carla Schaffer/AAAS

Using exquisitely precise methods to measure how memories are embedded in brain cells in mice, scientists have shown how fear-based memories prompted by the sound associated with an electric shock can be activated and erased.

The research, described at a news briefing during the 2017 AAAS Annual Meeting, suggests, in principle, that scientists may someday be able to erase fearful memories associated with post-traumatic stress disorder (PTSD) or disordered memories associated with drug use.

“Memories really define who we are,” said Sheena Josselyn, senior scientist in the department of physiology of the Hospital for Sick Children at the University of Toronto, Canada. “Only by really understanding how the brain normally functions can we hope to get some new insights into treatments and hopefully, one day, cures for these brain disorders,” she said. 

The ability to erase traumatic memories in humans, as shown in the film “Eternal Sunshine of the Spotless Mind,” remains science fiction for now and any attempt to do so would raise serious ethical considerations.

But neuroscientists, using animal studies, have been making substantial progress in understanding how individual memories are encoded in the brain.

Josselyn described her work tracing the firing patterns of small collections of neurons as they compete to embed memories. Over short periods of time – just a few hours – a collection of neurons may successfully compete to store the memory of two successive events, which can then be linked within the brain for later recall.

That may explain why two emotional events can remain closely connected in our minds. Josselyn vividly recalls March 9, 2009, for example, as the date when one of her scientific papers was published and also the day she gave birth to her daughter through a cesarean section.

Alcino Silva, distinguished professor of psychology at UCLA, has been using genetic markers and a highly miniaturized microscope to zero in on sets of brain cells in mice with such interconnected or “linked” memories.

As the time between events lengthens, memories are recruited and stored by different collections of neurons and remain separately stored within the brain, Josselyn said.

Howard Eichenbaum, director of the Center for Memory and Brain at Boston University, has been studying the function of “time” cells in the hippocampus of mice. There are particular neurons that have a “capacity to tell time,” he said. Along with “place cells” that represent where specific events occur, such neurons have the capacity to organize memories within time and space.

Such basic research is giving scientists a rudimentary ability to target where a memory is stored in the brain, Josselyn said, and they also can manipulate the activity of these targeted neurons to turn them on or off. When memory neurons are deactivated, she said, “The animals don’t remember… It’s as if the memory has been erased.”

Asked about the implications of her research for humans, Josselyn said it does offer a “proof principle” for the very specialized, emotionally salient form of memories she has been studying. She did not see the methods she is now using in mice ever being applicable to humans.

But the proof of principle does suggest that for memory problems in humans, “We don’t have to target the entire body, we don’t have to target the entire brain,” Josselyn said. Just specific cells could be targeted “sort of like a heat-seeking missile or heat-seeking drug that would somehow operate on just the cells we know are important” for a particular memory.

There are important ethical questions in any effort to manipulate human memory, Josselyn and Eichenbaum agreed. We learn from our bad memories as well as our good ones, Josselyn said, and the sum of our memories helps make us who we are.

Still, “for something that really tends to interfere with your everyday life, I would think that a treatment that targets just these cells could be appropriate,” she said.

Eichenbaum cautioned that there are a limited number of brain cells involved in encoding memories that are reused in different ensembles. Destroying one memory might affect others, he said. However, he agreed that if a memory were particularly severe and is “destroying your life,” it might be a reasonable compromise to erase it.

[Associated image: Carla Schaffer/AAAS]