“Not a Memory Failure, but a Function”: Why the Brain Needs the Process of Forgetting
Memories make us who we are. It’s no surprise that scientists have long tried to understand how memories are formed and stored. However, today, research is shifting focus from memory formation to the process of forgetting. What can studies on forgetting tell us about ourselves and our memory? Why does the brain need to forget? What challenges or advantages do people with exceptional or impaired memory face? Is it possible to stop forgetting? And how can understanding the mechanisms of forgetting help treat anxiety, phobias, PTSD, depression, and even Alzheimer’s disease in the future?
The Importance of Forgetting
Memories shape our understanding of the world and help us predict what’s coming. For the past century, scientists have been trying to figure out how memories are formed and stored so that we can recall them days, weeks, or even years later. But all this time, researchers were only seeing half the picture. To understand how we remember, we also need to understand how and why we forget.
About ten years ago, most scientists considered forgetting a passive process, where unused memories simply faded away over time, like a photograph left in the sun. But one group of memory researchers found results that contradicted this long-held belief. They began to propose a radical idea: the brain is designed to forget.
New research shows that memory loss is not a passive process. On the contrary, forgetting is a very active process that is constantly happening in the brain. In fact, for all living beings, the brain’s default state may not be remembering, but forgetting. If we better understand this state, it could lead to breakthroughs in treating anxiety, PTSD, and even Alzheimer’s disease.
“What is memory without forgetting?” asks Oliver Hardt, a cognitive psychologist studying the neurobiology of memory at McGill University in Montreal, Canada. “It’s impossible for memory to function properly without forgetting—you have to forget.”
The Biology of Forgetting
Each type of memory is created in its own way and stored in different parts of the brain. While researchers are still studying this, we know that autobiographical memories—memories of events we’ve personally experienced—begin to take long-term form in the hippocampus, a process that starts in the hours and days after the event. Neurons communicate through synapses—tiny gaps where nerve impulses are transmitted chemically. Each neuron can connect to thousands of others. Through a process called synaptic plasticity, neurons constantly produce new proteins to rebuild parts of the synapse, especially the receptors for chemical transmission, allowing neurons to selectively strengthen their connections. This creates a network of cells that together encode a memory. The more often information is recalled, the stronger its neural network becomes. Over time, with repeated recall, the memory is encoded in both the hippocampus and the cortex. Eventually, it remains only in the cortex for long-term storage.
Neurobiologists often call this memory function an “engram.” They believe each engram consists of a set of synaptic connections, sometimes across multiple brain regions, and that each neuron and synapse can be involved in several engrams.
Much is still unknown about how we create and access memories. Scientists need more time to solve this puzzle. Also, until recently, little attention was paid to how the brain forgets. Michael Anderson, a professor at the University of Cambridge who studies cognitive neuroscience, notes:
“This is a serious oversight. Every species with memory forgets. It doesn’t matter how simple the organism is—if it can learn from experience, it can also forget. Given this, I’m amazed that neurobiology has treated forgetting as a secondary process.”
Active Forgetting in Fruit Flies
This wasn’t Ron Davis’s main goal when he conducted an experiment in 2012 and found evidence of active forgetting in fruit flies (Drosophila melanogaster). Davis, a neurobiologist at the Scripps Research Institute in Jupiter, Florida, was studying how memories form in the mushroom bodies of flies (dense networks of neurons in insect brains that store olfactory and other sensory memories). He was especially interested in the role of dopamine-producing neurons that connect to these structures. Dopamine is a neurotransmitter involved in many behavioral responses in the fly’s brain, and Davis suspected it might also play a key role in memory formation.
Interestingly, Davis found that dopamine is necessary for forgetting. He and his colleagues trained genetically modified flies to associate electric shocks with certain odors, teaching them to avoid those smells. Then, by activating the dopaminergic neurons, they saw that the flies quickly forgot the association. But when those neurons were blocked, the memory remained. Davis notes:
“These neurons regulated how memories would be expressed, mainly by sending a ‘forget’ signal.”
Further research using methods that allowed scientists to control neuron activity in live flies showed that dopamine neurons are active for long periods—at least in flies.
“The brain is always trying to forget information it has already learned,” says Davis.
From Flies to Rodents
Several years later, Hardt found something similar in rats. He studied what happens in the synapses of neurons involved in long-term memory storage. Scientists know that memories are encoded in the mammalian brain when the strength of connections between neurons increases. This strength is determined by the number of a certain type of receptor found in the synapse. These structures, called AMPA receptors, must be maintained for the memory to remain intact. “The problem,” says Hardt, “is that none of these receptors are stable. They’re constantly moving in and out of the synapse and can be replaced within hours or days.”
Hardt’s lab showed that a special mechanism continuously stimulates the expression of AMPA receptors in synapses. Yet, some memories are still forgotten. Hardt suggested that AMPA receptors can also be removed, indicating that forgetting is an active process. If that’s true, then preventing the removal of AMPA receptors should prevent forgetting. When Hardt and his colleagues blocked the removal of AMPA receptors in the hippocampus of rats, they found that the rats didn’t forget the location of objects. It seemed that, to forget, the rat’s brain had to actively break down synaptic connections. In Hardt’s view, “forgetting is not a memory failure, but a function.”
Paul Frankland, a neurobiologist at the Hospital for Sick Children in Toronto, also found evidence that the brain is programmed to forget. Frankland studied the production of new neurons (neurogenesis) in adult mice. This process was long known to occur in young animals, but was only discovered in the hippocampus of mature animals about 20 years ago. Since the hippocampus is involved in memory formation, Frankland and his team wondered if increasing neurogenesis in adult mice would help them remember better.
In a 2014 paper, the researchers found the opposite: instead of improving memory, increased neurogenesis made the mice forget more. As counterintuitive as this seemed at first, given the assumption that more new neurons would mean better memory, Frankland says it now makes sense:
“When neurons integrate into the adult hippocampus, they become part of an existing, established network. If you have information stored in that network and start changing it, it can make the existing information harder to access.”
Since the hippocampus isn’t the site of long-term memory storage, its dynamic nature isn’t a flaw, but a feature, says Frankland—a result of evolution to help learning. The environment is always changing, and animals must adapt to new situations to survive. This allows new information to overwrite the old.
Forgetting in Humans
Researchers believe the human brain may work in a similar way. Blake Richards, who studies neural circuits and machine learning at the University of Toronto Scarborough, says:
“Our ability to generalize new experiences is at least partly due to the brain’s involvement in controlled forgetting.”
Richards suggests that the brain’s ability to forget can prevent a phenomenon known as overfitting. In artificial intelligence, this happens when a mathematical model memorizes too many specific examples instead of learning patterns, making it less effective with new data it hasn’t seen before.
Similarly, if a person remembered every detail of an event—like a dog attack, including not just the sudden movement that startled the dog, but also the dog’s floppy ears, the color of the owner’s shirt, and the position of the sun—it would be harder to generalize the experience to avoid being bitten again in the future.
“If you erase some details but keep the essence, it helps you use the information in new situations,” says Richards. “It’s very possible that our brain engages in some controlled forgetting to prevent overfitting our own experiences.”
Studies of people with exceptional and impaired autobiographical memory seem to support this. People with a condition called hyperthymesia (HSAM) remember their lives in such incredible detail that they can describe what they wore on any given day. But despite their remarkable memory, these people tend to be more prone to obsessive thinking. “It’s a state where a person can’t extract themselves from specific situations,” says Brian Levine, a cognitive neurobiologist at the Rotman Research Institute at Baycrest Hospital in Toronto.
On the other hand, those with severely deficient autobiographical memory (SDAM) can’t recall specific events from their lives. As a result, they have trouble imagining what might happen to them in the future. Still, in Levine’s experience, people with SDAM do well in jobs that require abstract thinking, likely because they don’t focus on specifics.
“We think people with SDAM, because they constantly face a lack of episodic memory, are able to consider all episodes from their lives at once,” says Levine. “They’re good at problem-solving.”
Studies of forgetting in people without memory disorders also show how important this process is for a healthy brain. Anderson’s team studied active forgetting in people using a combination of functional MRI and magnetic resonance spectroscopy to measure levels of the inhibitory neurotransmitter GABA in the hippocampus. Scanning participants as they tried to suppress certain thoughts, the researchers found that the higher the GABA level, the more the prefrontal cortex suppressed the hippocampus, and the better people were at forgetting.
“We were able to link the process of forgetting to a specific neurotransmitter in the brain,” says Anderson.
Trying to Forget
By understanding how we forget—through the lens of biology and cognitive psychology—Anderson and other researchers are getting closer to improving treatments for anxiety, PTSD, and even Alzheimer’s disease.
Anderson’s work measuring GABA levels in the brain may point to the mechanism behind the effectiveness of calming drugs (benzodiazepines) like diazepam, which have been prescribed since the 1960s. Researchers have long known that these drugs work by enhancing GABA receptor function, helping to reduce anxiety, but didn’t know why. Anderson’s results offer an explanation: if the prefrontal cortex tells the hippocampus to suppress a thought, the hippocampus can’t respond if it doesn’t have enough GABA.
“The prefrontal cortex is like a general sending orders from above to suppress activity in the hippocampus,” says Anderson. “If there are no troops on the ground, those orders go unheeded.”
The crucial role of GABA in suppressing unwanted thoughts also has implications for phobias, schizophrenia, and depression. Various symptoms of these conditions—including intrusive memories, obsessive thoughts, depressive rumination, and trouble controlling thoughts—have been linked to an overactive hippocampus. Anderson notes:
“We think we have a key mechanical structure that ties together all these different symptoms and disorders.”
His group’s research could be significant for treating PTSD, a condition often seen as a problem of remembering traumatic episodes too well, but which is actually a problem of forgetting. Better understanding how to help people make traumatic memories less intrusive could help researchers treat some of the most difficult cases. When Anderson and his colleagues studied what happens when volunteers suppress unwanted memories (a process he calls motivated forgetting), they found that people suffering from trauma are good at suppressing other memories. Understanding the cognitive psychology behind this ability, as well as the mental resilience needed to develop it, could help improve PTSD treatment.
Hardt believes that Alzheimer’s disease could also be better understood as a disorder of forgetting, not remembering. If forgetting is truly a well-regulated, innate part of the memory process, then a breakdown in this regulation could have negative consequences:
“What if what’s really happening is a hyperactive forgetting process that goes awry and erases more than it should?”
This question remains unanswered. But more and more memory researchers are putting forgetting on equal footing with remembering and shifting their focus to studying it.
“There’s a growing understanding that forgetting is a set of processes that should be distinguished from encoding, consolidation, and retrieval,” says Anderson.
In the past decade, researchers have begun to see forgetting as an essential part of the whole.
“Why do we have memory at all? As humans, we like to think it’s important to remember as many autobiographical details as possible,” says Hardt. “And that’s probably completely wrong. Memory primarily serves an adaptive purpose. It gives us knowledge about the world, and then updates that knowledge.”
Forgetting allows us, both as individuals and as a species, to move forward.
“Evolution has achieved the perfect balance between the importance of remembering and forgetting. Thanks to this, we strive for stability and consistency, but also for letting go of things that hold us back.”