May 18, 2019 - Science

Deep Dive: The future of forgetting

Illustration of hole floating in space shown through alternating green and black lines in a style of optical illusion.

Illustration: Aïda Amer/Axios

We are desperate to fight forgetting — it scares us, it annoys us, and it can cost us. Yet there are also memories we want to forget.

Why it matters: Rather than being a flip side or failure of memory, forgetting is now being studied by neuroscientists as a brain process in its own right. They're starting to understand how neurons forget information, in hopes of developing new treatments for degenerative diseases that cause debilitating forgetting and for helping those who need to forget.

Day to day, we're able to learn, make decisions and move through life because the brain balances forgetting irrelevant information and experiences, with remembering important ones.

“Forgetting is not a nuisance or flaw. It is a virtue of our system.”
— Lili Sahakyan, associate professor of psychology, University of Illinois

Psychologists have studied forgetting for decades (and contemplated it for more than a century), including how we control our memory by substituting thoughts or by directing some attention, but not too much, to an unwanted memory.

Now, neuroscientists are starting to figure out how the brain forgets.

How it works: Neurons represent memories as patterns of firing between themselves in the short term. In the long term, they're etched in connections between neurons that strengthen with repeated firing.

  • Like remembering, there may be different processes underlying forgetting — for example, memories can fade over time if the connections between neurons weaken, or similar memories may interfere with one another.

A flurry of recent papers and media suggest the brain also actively forgets — when we recall memories or learn new information that overrides old memories.

  • Humans and rats appear to use similar brain mechanisms to forget a memory that distracts them from the one they need to achieve a goal, according to recent research by Michael Anderson at the University of Cambridge.
  • The finding opens the door to studying the process at the level of cells and molecules, he says.

Forgetting could even be the default state of the brain, says Ronald Davis, a neurobiologist at the Scripps Research Institute.

  • Studying fruit flies, his team found the neurotransmitter dopamine is involved in both forming a memory and forgetting it.
  • When cells are blocked from continuously releasing dopamine, flies remembered much better at later times to avoid a particular odor associated with an electric shock.
  • Davis proposes forgetting is a default system for the brain: soon after a new memory is formed, dopamine acts as a "forget" trigger in a cascade of chemical changes known to affect the connections between neurons. Memories deemed important are stabilized, others aren't.

It's not known if the mechanism exists in humans but Anderson agrees it makes sense. "It's actually more miraculous when we keep something around for a long period of time because your brain is getting rid of things at a pretty constant rate," he says, pointing out that some of the forgetting mechanisms seen in fruit flies are also found in rats.

More recently, Davis found the dopamine system drives the formation of a new memory when learning while disrupting the traces of existing ones.

  • This is "just the tip of the iceberg with this one mechanism of forgetting," he says.
  • Researchers recently described another in rats as they learned to make their way through a maze.

What's next: As Alzheimer's disease drug trials fail and the projected costs of dementia rise, researchers want to home in on new molecules to target for treatments.

  • Davis' lab screens for new drug candidates but he cautions it is early days for aspirations of altering forgetting.
  • For example, it's not known what mechanisms the human brain uses, or how many neurons constitute a memory.

Meanwhile, psychologists want to study the brain's forgetting strategies to improve therapies for post-traumatic stress disorder and other conditions.

"If we can better map how the human brain discards information and commandeer that process, we can better facilitate forgetting in a therapeutic setting," says Jarrod Lewis-Peacock, a professor of psychology at the University of Texas at Austin.

  • This isn't "Eternal Sunshine of the Spotless Mind" zapping memories out of people's heads, he says.
  • Rather it is figuring out how to help people put their memories into a mushy state where they can be updated or reassociated with more positive things.

Ultimately, says Lewis-Peacock, "a better understanding of forgetting clarifies our understanding of remembering."

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