To Remember, Your Brain Must Actively Forget


It’s a central principle of owning a human body that if you don’t use it, you lose it. Singers have to practice hitting the high notes to keep vocal range wide and vocal cords limber. Basketballers have to shoot countless hoops to hone their technique. We all have to get out of the easy chair and get a little exercise to keep muscles from getting slack and unresponsive. The idea even extends to psychology — practicing mindfulness or meditation takes rigorous training.

So it makes sense we’ve always thought the same thing about memory, that if the neural maps that form “engrams” (the hypothetical brain states that represent thoughts, dreams, memories, or emotions) aren’t used — they simply fade away. But we might be wrong.

To be fair to a century of neuroscience, it seems self evident. If some sliver of knowledge isn’t going to benefit your reproductive capacity or long-term survival beyond a casual water-cooler discussion with a co-worker it tends to degrade quickly. But those that do benefit us (which bus to catch to work, where the best hunting ground is) become entrenched in the wiring, expressed in our consciousness as easier, quicker, and sometimes even automatic recall.

The traditional mechanisms of memory loss were call interference, like when you can’t remember your new PIN number because your old one is so ingrained, and trace decay, referring to the gradual fading of memory engrams you don’t use anymore.

But several recent studies have built on a decade-old line of inquiry that reveals something new and potentially game-changing about memory. Rather than degrading passively, unused over time, we’ve found that brain chemicals actually do very energy-intensive work to remove memories. Forgetting, it turns out, is as active a process in the brain as remembering.

A New Outlook

Dr. Oliver Hardt, a behavioral and cognitive neuroscientist and cognitive psychologist at Montréal’s McGill University, was interested in previous work by a colleague, Dr. Virginia Migues, whose study revealed one of the mechanisms that provide continuous, ongoing maintenance memories need to persist. If some part of that maintenance process was disrupted the memories vanished completely, often immediately.

However, knowing that long-term memories are still forgotten despite this maintenance process, Hardt realized something organized and purposeful must counteract it naturally. Called “active decay,” it disassembles the changes to the brain brought about by memory formation, removing the connections between neurons required for a given memory to emerge.

At The Hospital for Sick Children, Toronto, neuroscientist Dr. Paul Frankland and his team wondered whether neurogenesis — the promotion of growth in neurons and other nerve cells — in mice could improve their memories. They were actually shocked to find it did the exact opposite, causing the lab animals to forget more stuff.

“The ongoing production of new neurons has two effects,” Frankland says. “On one hand they enable new memories to be encoded more efficiently, the effect we were initially interested in. On the other, as they integrate into established brain circuits they make new connections, which might overwrite old memories already stored in the hippocampus. It’s a trade off — as soon as you encode new information you overwrite the old information. In computational fields it’s sometimes called the ‘stability–plasticity dilemma.’ ”

At Florida’s Scripps Institute department of neuroscience, Dr. Ronald Davis and his team were activating and inhibiting dopamine neurons in test animals when they were led to what he calls an “aha” moment. The eventual conclusion was that dopamine neurons were providing a signal that prompts the brain to forget previously learned information. It led to a hypothesis by Davis that the brain is designed to forget information unless consolidation deems it important. Or as other researchers have put it, the brain isn’t designed to remember at all, it’s built to forget.

“It’s like other homeostatic processes in the body,” Davis says. “We regulate glucose concentration, heartbeat, etc. Biological systems like to operate close to a set point. When we’re inundated with information throughout the day, maybe what the brain is doing is fighting back and trying to erode all that information. When a memory is important, consolidation arises and says ‘remember this piece of information’ it lets other information be whittled away.”

Of course, it’s worth remembering that Frankland’s work was with rodents and Davis’ with fruit flies. But Davis, referring to the similarities between brain structures at the cellular level across the animal kingdom, thinks almost everything we find in the fly is translatable to the rat or mouse, and he’s very confident we’ll find similar “active forgetting” pathways from previous studies in us too.

To that end, Cambridge University cognitive and behavioral neuroscience professor Dr. Michael Anderson has been investigating active forgetting in humans. Using fMRI and magnetic-resonance spectroscopy machines, his lab has been looking at the levels of the inhibitory neurotransmitter GABA (γ-aminobutyric acid) in the hippocampus.

Scanning subjects while they attempted to suppress certain thoughts, Anderson’s lab discovered that if people had higher GABA levels, their prefrontal cortex suppressed activity in the hippocampus, making them “better” at forgetting.

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