Oliver Hardt, "The brain forgets in order to improve your memory: You need to forget to remember what matters"
Forgetting is usually seen as a flaw in the human brain. But neuroscientist Oliver Hardt argues that forgetting is in fact crucial for some of the most important parts of our brain function. If you remembered every version of every face you'd ever seen, you wouldn't be able to recognize anyone. Your brain needs to erase the clutter to generalize and see patterns. Our capacity to forget isn’t a flaw of human memory, it’s an incredibly useful part of its design.---
Every seven to thirty seconds, Clive Wearing appears in the here and now from nowhere, suddenly regaining consciousness out of nothingness. He cannot explain these extraordinary events, and so he records them in his diary immediately: “9:06 Now I am completely, overwhelmingly awake (for the first time).” But then he sees a previous line and reads “8:31 Now I am truly fully awake (for the first time)”—recognizing his handwriting, he realizes that he had just documented that very experience, but that is impossible because he has no memory of it, and he crosses out the earlier entry, as it must have been a mistake. So it goes, page after page.
When his wife Deborah visits him in his hospital room, he greets her effusively because he cannot remember when he last saw her—years might have passed. And when she leaves the room briefly and returns a few minutes later, this moving scene repeats itself with the same intensity as before, as Clive has no memory of it. Like the townspeople of Punxsutawney in Groundhog Day, he completely forgets everything he experiences, but the reset happens every waking minute of the day, not once every 24 hours.
Clive once worked at BBC Radio 3, played several instruments, and conducted an amateur choir, a renowned expert on early music, specializing in the Renaissance composer Orlando di Lasso. But in 1985, herpes simplex virus caused a brain inflammation that damaged several parts of his brain and completely destroyed the hippocampus. Since then, Clive has lived perpetually in a moment that is no longer than a minute, and, stripped of past and future, he requires permanent care.
Clive’s diary is like a desperate attempt to replicate the function of a failed hippocampus, which normally would continuously store what just happened and where. We are completely unaware of this, cannot consciously control it, yet it is this built-in logbook of experiences that details why we are where we are and what we have done before, so that we can make sense of our current situation, firmly linked to our past.___As essential oils concentrate what defines a scent, forgetting distills the gist of memories by removing what is irrelevant.___
If no lasting memory is formed when we experience an event, it is as if it never happened. One cannot decide after an experience whether it may be important and should be remembered, because that only works if there is a memory in the first place. Therefore, the brain has no choice but to automatically, indiscriminately, and comprehensively record what just transpired—a promiscuous encoding device steadily logging the events of our lives.
This necessary strategy reliably locates us in space and time and ensures that we rarely fail to commit something important to memory. But it comes at a significant cost. After a short time, most of the remembered events have turned into outdated signposts to the past that are no longer relevant to explaining the present. Even more problematic, most of them detail trite everyday occurrences that are little more than mnemonic dead weight.
Indiscriminate collection of memories can only work in the long term if it is accompanied by automated forgetting as a corrective counter-process. In other words, indiscriminate retention requires systematic forgetting—remembering having taken the daily heart medication this morning will prevent a potentially fatal double dose hours later, but why retain that specific memory for days, weeks, or years? Eventually, this will lead to uncontrolled mind-wandering causing near constant confusion. We can get glimpses of this in daily life—waiting in our car at a red light we catch sight of a pedestrian who reminds us of someone we knew in high school and remember that once in art class she painted the strikingly lifelike portrait of an actress we deeply admired, and didn’t she play the lead in that movie with... until angry honking takes us back to the fact that the light has long turned to green. If we retained all our memories, ordered thinking and effective behavior would be overwhelmed by a permanent barrage of intruding memories. Clive Wearing lost the connection to the present moment because he forgot too fast, but remembering too much can have the same effect.
That memory will inevitably fail without forgetting is an insight as old as the scientific study of memory itself, which began in 1885 with the publication of Über das Gedächtnis (“On Memory”), in which Herrmann Ebbinghaus provided sobering evidence with the famed forgetting curve that even well-learned material has the tendency to largely disappear from memory rather rapidly. Yet, five years later, the American philosopher William James arrived at a more positive outlook. Reviewing those and other related findings, James concluded in his textbook on Psychology that “if we remembered everything, we should on most occasions be as ill off as if we remembered nothing.”
What James meant has been best illustrated by the Argentine writer Jorge Luis Borges, who explored living with “perfect memory” in a short story. Having provided a foreword for a Spanish translation of William James’ Pragmatism, Borges clearly was intimately acquainted with his philosophical work, but we can only speculate whether James’ remarks on forgetting were what drew him to this subject.
Borges tells us the fantastical tale of Ireneo Funes, who lived at the end of the 19th century (curiously enough, just around the time James published his textbook) in Fray Bentos, a town in the Western part of Uruguay. Ireneo was a young man of modest education and rough reputation, but with some unusual intellectual qualities. He was known in town as the chronometric Funes because he could always tell exactly when—to the minute—he had seen someone last, even if it was just for a few seconds when passing each other on the street, even if this had been months ago. But a fall off a horse grotesquely enhanced this talent, crippling body and mind. From then on he was largely confined to his bed, facing the world through the window in his room with a mind that could no longer forget anything.
This had some advantages. For example, thanks to his immaculate memory, he learned a foreign language to fluency simply by reading a book written in it once. He readily retained the entire text and, much like modern large language models, used it to produce likely phrases, which might have been mostly performative rather than genuine comprehension. But this meticulous memory was also the reason why it would take Ireneo one entire day to recall what happened on a day in the past because every event had been preserved in all its details, including his emotions and thoughts. William James raised exactly the same issue to illustrate what minds face without forgetting, and it may have been this that had inspired Borges.
Truly problematic, however, was that perfect recall turned identity into a matter of perspective: looking out his window on the street, he would see a dog from the front in the morning, and then, in the afternoon, the same dog would pass by, and he would see him from the side—but, for Ireneo, these were two different dogs. His unfailing high-fidelity memory for all the precise visual detail of the dog as seen in the morning did not match what he saw in the afternoon. Self-similarity became the exception, not the rule, that we depend on to identify objects and people despite their constant changes in appearance.
As essential oils concentrate what defines a scent, forgetting distills the gist of memories by removing what is irrelevant. This leads to what psychologists call semantic memory, general knowledge that grew out of specific experiences. It allows us to immediately recognize novel objects as what they are; for example, we readily identify a plant we have never seen before as a flower. For humans, these abstractions form the basis of communication. If someone tells us that they climbed a tree to pick cherries but unfortunately slipped and fell, we will immediately understand what happened and can even picture it vividly in our mind if we wish. We will not imagine exactly the tree that was climbed, but that does not make any difference as the essence of the event was transmitted. This is also the case for the person sharing this episode, whose memory will not have preserved the experience in high-fidelity but rather in a compressed form that allows re-imagining a sufficiently similar version of it, not an accurate reproduction.
Similarly, forgetting helps with extracting rules from concrete life experiences. When a child touches a hot plate, she will soon not only be wary of that particular oven in that particular kitchen, but also of surfaces of appliances where food is heated, from coffee makers in offices to airplane galleys, but not play ovens in a toy store. Forgetting the details of the specific event in which something was experienced makes it independent of this context, and the knowledge contained can be applied to various future situations.
A memory stripped of its original context provides the raw material for thinking in analogies. We can use it to understand a new problem by likening it to a familiar one, as we are applying memories from which the specific details that would otherwise obscure the similarities between the two situations have been removed. It is also what allows us to immediately know what to do when a road sign instructs us to use the zipper method as two lanes merge—no explanation is necessary. This way, forgetting can shape our memories into powerful tools for thought.
Since forgetting is fundamental to adaptive behavior, it cannot be left to chance—there must be dedicated mechanisms in place that bring it about. These were discovered about twenty years ago, first in the fruit fly Drosophila melanogaster. These animals can be trained to fear an odor, such that when given the choice between a smell that was present when they received an electric shock and a neutral smell, they will fly towards the latter. But after some time, they forget, and no longer express this preference. Ron Davis’ group at the University of Florida discovered that specialized neurons in the fly brain are responsible, as their activity erases the memory that links the odor to experiencing electric shock.
Around the same time, we found that similar principles also govern forgetting in the mammalian brain. We trained rats to remember the location of objects and found that they would forget this in about seven to ten days. It was known that learning and memory depend on changing the strength of connections between neurons. Speculating that the memory loss in our rats reflects reversing this change, we prevented it in the hippocampus, the brain area critical for spatial memory—and the same region whose destruction left Clive Wearing unable to remember where he had been moments before. This preserved memory, and our rats remembered the location of the objects even after fourteen days. Since then, mechanisms that target such memories for removal have been found in other animals as well, like the microscopic nematode C. elegans and mice. Clearly, brains are wired for forgetting.
In animals, too, it has been discovered that active forgetting supports adaptive behavior. Like the child that learns not to touch hot plates, rats can learn to generalize from specific experiences. After receiving a mild electric shock in a chamber (causing a sensation similar to static electricity), rats will fear this locale, but not novel ones—they seem to know where the unpleasant event took place. But wait two weeks, and they will fear any box that vaguely resembles the one where they experienced the shock. In other words, like the child learning to avoid hot plates in general, they start to fear boxes in general. However, when we stopped active forgetting in the hippocampus, there was no generalization of fear after two weeks, as the rats were only fearful of the place where they experienced the shock, but not of other boxes, even similar ones. It seems this form of generalization occurs because memory for exactly where the original experience occurred was stripped away, which left behind only the gist: when someone suddenly puts you into some box, there is good reason to be cautious.
Another form of adaptive behavior is cognitive flexibility. Anybody who has had to navigate road traffic in countries that drive on the opposite side of the road will know how fundamentally important cognitive flexibility can be for survival. In animals, this capacity is often studied with reversal learning. This involves, for example, an initial cue that signals safety and then another cue that signals danger. Then, after some time, these are switched, such that safety is now linked to the cue that had indicated danger, while danger is associated with the one that had announced safety. The animal has to learn to change its behavior, avoiding what it once sought out and approaching what it once avoided. Usually, animals are quite good at this.
But when active forgetting was genetically impaired in fruit flies, reversal learning failed. Unlike their unmodified counterparts, they continued to avoid what had previously been unpleasant but no longer was. They simply could not let go of their behavior—the fruit flies needed forgetting to revise their knowledge about which of the two odors to approach after the dangerous odor had been switched. Without forgetting, they were stuck in a past that was no longer relevant to present conditions. Similar effects have also been documented in rats.___Remembering and forgetting are so intertwined that neither can exist without the other.___
Clearly, forgetting helps to adapt to changing circumstances, so it is no surprise that humans have long been interested in suitable performance-enhancing drugs to better deal with the dynamic nature of life that can quickly turn memory assets into liabilities. In ancient Greece, for example, the souls of the deceased needed to drink from the waters of Lethe, the river of oblivion that flows through Hades, the mythological underworld. It would erase all memory of their former lives, and they could reincarnate without mourning what they had lost. Tabula rasa for a fresh new start is also why the poor souls in Diyu, the Chinese version of hell, are offered a magic potion before reincarnation. It comes in the form of a bowl of Mi Hun Tang, a flavorful soup that the Goddess of Forgetfulness, Meng Po, brews out of carefully selected herbs, the tears shed in life, and waters from a nearby stream. While in Diyu, the souls experience all kinds of torture and malice, and Mi Hun Tang wipes out these traumatic memories and dissolves all knowledge of the former life—the next cycle of existence commences unburdened of the weight of the past.
Remedies that further forgetting to promote liberation and renewal raise difficult ethical questions. Total obliteration of the past certainly works for souls about to be reborn, but losing memories of aspects of our past while on this side of the underworld can affect our personal identity, the knowledge about who we are. This is why forgetting enhancers are best when they target specific memory components, and Homer seems to have been aware of this when he tells us in his Odyssey about nēpenthés phármakon, the not-(nē)-sorrow-(pénthos)-drug-(phármakon) offered to Telemachus and Menelaus, who grieve the loss of their comrades and loved ones. It leaves memory for the deceased untouched, but induces amnesia for the painful emotions. Modern interventions to accelerate forgetting work similarly, reducing the emotional burden of traumatic memories without touching memory for what happened. Even long-held memories of past events can become malleable again under specific circumstances, and if at that moment beta blockers are given—the same drugs used to treat hypertension—it can significantly dampen the emotional part of the memory of the event. This life episode can still be remembered in the future, but the emotional impact is less than before.
Clive Wearing and Ireneo Funes both fell victim to broken forgetting. Clive forgets too fast, and his world is reduced to brief periods of sustained consciousness as he wakes up minute by minute on a different isolated island of awareness on the vast uncharted sea his life has become—without a past to anchor him, or a future to orient him. But Funes fared no better—no longer able to forget, he is swallowed by an infinite rabbit hole of memories upon memories upon memories that he cannot organize into knowledge, making ordered thought and understanding impossible.
Forgetting shapes memory, and what we call memory is in no small part the remains of dedicated forgetting processes built into the brain. Remembering and forgetting are so intertwined that neither can exist without the other. The brain promiscuously encodes ongoing experiences because it cannot know in advance what might be important, but as most of it does not matter in the end, it also must forget systematically, so we will be spared the life of Funes. Our well-developed capacity to forget is not a flaw of human memory, but its basic design.
