Our memories do not just fade away on their own. Our brains are constantly editing our recollections, from the very moment those memories first form.
Toma Vagner for Quanta Magazine
by Dalmeet Singh Chawla
Decades of research have focused on how the brain acquires information, resulting in theories that suggest short-term memories are encoded in the brain as patterns of activity among neurons, while long-term memories reflect a change in the connections between neurons.
What hasn’t received nearly as much attention from memory researchers is how the brain forgets. “The vast majority of the things that are happening to me in my life — the conscious experience I’m having right now — I’m most likely not going to remember when I’m 80,” said Michael Anderson, a memory researcher at the University of Cambridge, who has been studying forgetting since the 1990s. “How is it that the field of neurobiology has actually never taken forgetting seriously?”
Oliver Hardt, who studies memory and forgetting at McGill University in Montreal. If we remembered everything, he said, we would be completely inefficient because our brains would always be swamped with superfluous memories. “I believe that the brain acts as a promiscuous encoding device,” he said, noting that at night many people can recall even the most mundane events of their day in detail, but then they forget them in the following days or weeks.
The reason, he thinks, is that the brain doesn’t know straight away what is important and what isn’t, so it tries to remember as much as possible at first, but gradually forgets most things. “Forgetting serves as a filter,” Hardt said. “It filters out the stuff that the brain deems unimportant.”
Experiments in the past few years are finally beginning to make the nature of that filter clearer.
Traces of Memories
Memory is a complicated subject for many reasons, not the least of which is that all manner of creatures have memories, from very simple organisms like sea slugs and insects up through humans and other animals with complex brains. Differences in how memory works may sometimes go along with those different nervous system architectures.
Moreover, even within a single species, there can be several types of memory, and they may be interrelated but also centered in different parts of the brain. For example, recently acquired memories in mammals often depend on the involvement of the hippocampus, while longer term memory can involve more cortical areas of the brain. The mechanisms may vary among those types of memory, too.
In this magnified slice of rodent brain tissue enhanced with a fluorescent protein, the green glow reveals which cells in the hippocampus seem to be storing the engram, or physical trace, of an experimentally induced memory.
Dheeraj Roy, Tonegawa Lab/MIT
Going along with all that variety is now a growing appreciation that forgetting — the functional loss of memories — may also come in diverse forms. Past theories about forgetting mostly emphasized relatively passive processes in which the loss of memories was a consequence of the physical traces of those memories (what some researchers refer to as “engrams”) naturally breaking down or becoming harder to access; those engrams may typically be interconnections between brain cells that prompt them to fire in a certain way. This forgetting process could involve the spontaneous decay of connections between neurons that encode a memory, the random death of those neurons, the failure of systems that would normally help to consolidate and stabilize new memories, or the loss of context cues or other factors that might make it hard to retrieve a memory.
Now, however, researchers are paying much more attention to mechanisms that actively erase or hide those memory engrams.
Intrinsic Forgetting
One form of active forgetting that scientists formally identified in 2017is called intrinsic forgetting. It involves a certain subset of cells in the brain — which Ronald Davis and Yi Zhong, who wrote the paper that introduced the idea, casually call “forgetting cells” — that degrade the engrams in memory cells.
This idea emerged after Davis, a neuroscientist at the Scripps Research Institute in Jupiter, Florida, and his colleagues reported giving fruit flies mild electric shocks while exposing them to an odor. The flies quickly learned to avoid the smell, associating it with the shock.
In 2016 they found that the inhibition of a specific protein called Rac1 in the hippocampal neurons prolonged the retention of memories from less than 72 hours to at least 120 hours in many cases. Increasing the activity of Rac1 reduced the life of memories to less than 24 hours. Earlier work by Zhong’s group had shown that Rac1 was similarly involved in several forms of forgetting in fruit flies.
As Davis and Zhong argued in their jointly written 2017 review, all those findings suggested that cellular processes mediated by dopamine and Rac1 constantly erode newly formed memories. “From this perspective,” they wrote, “forgetting as mediated by intrinsic forgetting mechanisms may be the default state of the brain; intrinsic forgetting may operate chronically at a low level to slowly remove each newly acquired memory, although its strength may be regulated by internal or external factors.”
New Neurons and Old Memories
Another cellular process that seems to cause its own form of forgetting is neurogenesis, the birth of new neurons in the brain.
The connection of neurogenesis to memory and forgetting is complicated. Previous studies have shown that neurogenesis can be important to the formation of new memories: In tests on lab animals, drugs that inhibit neurogenesis in the hippocampus can interfere with new memory formation, and drugs that enhance neurogenesis seem to help with learning new tasks if they are given before the learning process.
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