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A University of Basel experiment with mice reports a curious fact about memory: The memory of a specific experience is stored in multiple parallel “copies.” These copies are kept for varying lengths of time, , modified to some extent, and sometimes eventually deleted.

Professor Flavio Donato’s research group at the University’s Biozentrum found that memory development begins long before birth. At least three different groups of neurons in the brain’s hippocampus emerge at different stages during embryonic development. A single event is stored in parallel memory copies in all three — in triplicate, if you like.

First to arrive during development, the early-born neurons are responsible for the long-term persistence of a memory. In fact, even though their memory copy is initially too weak for the brain to access, it becomes stronger and stronger as time passes. Also in humans, the brain might have access to such memory only some time after its encoding.

In contrast, the memory copy of the same event created by the late-born neurons is very strong at the beginning but fades over time, so that if one waits long enough, such a copy becomes inaccessible to the brain. In the middle ground, among neurons emerging in between the two extremes during development, a more stable copy could be observed.

University of Basel. “The brain creates “three copies,” for a single memory.” ScieScienceDaily, 15 August 2024.

The hippocampus is the seahorse-shaped inner part of the brain that plays an important role in memory and learning. There are actually two hippocampi, one on each side, but they are generally referred to as a single unit. People who have lost one or both have great difficulty forming or retaining memories.

Why the extra copies of memories?

The researchers think that the type of neuron in which a memory is stored there might relate to how easy it is for memories to change. The memories stored short-term via late-born neurons can, they say, be modified and rewritten: “This means that remembering a situation shortly after it has happened primes the late-born neurons to become active and integrate present information within the original memory.” But when an event is remembered after much time has passed, the memory retrieved from the early-born neurons is hard to change.

At Nautilus, Deena Mousa comments,

In their study, Donato and his colleagues find that which copy is accessed when one is remembering determines, in part, how easily that memory can be modified or used to form new associations. Recalling an event soon after it occurs generally draws on those neurons that emerge late in development, which store the more malleable trace of the memory—which means that as we remember it, we can layer onto that memory trace associations with related events and ideas, and other new information. For example, you may learn to associate your first memory of a room with more recent experiences in that room, such as of a bad smell or painful accident. The different neuron populations allow us to preserve fundamental aspects of a memory over the long term, while also enabling us to adapt and incorporate new or related information we have learned about the world.

Deena Musa, “Our Memories Are Stored in Triplicate,” Nautilus, September 6, 2024

The researchers are not saying that there are only three groups of neurons in embryos that are associated with memory; rather, they have identified three separate groups. Others may remain to be discovered. Earlier this year, New Scientist reported that there were 3300 types of cell in the human brain and “we don’t even know what most of them do.”

Here’s the Editor’s Summary of the researchers’ findings, published at Science:

Memories are not fixed; they evolve after encoding and their content can be updated upon changes in external or internal contingencies. Kveim et al. investigated the mechanisms that govern the reorganization of neuronal ensembles linked to a specific memory and how these dynamic changes affect memory persistence over time. They found that in the hippocampal network, learning resulted in the parallel establishment of two distinct memory traces. These traces were represented in distinct neurogenesis-defined subpopulations of early- and late-born neurons. Even though temporally restricted, the transient recruitment of late-born neurons was necessary for a memory’s long-term permanence, whereas shifts in the recruitment of early- and late-born neurons had a strong impact on the plasticity of a recently acquired memory.

—Peter Stern Vilde A. Kveim et al., Divergent recruitment of developmentally defined neuronal ensembles supports memory dynamics. Science 385, eadk0997(2024).DOI:10.1126/science.adk0997

You may also wish to read: “What neuroscientists now know about how memories are born and die” Where, exactly are our memories? Are modern media destroying them? Could we erase them if we wanted to?