Brain creates parallel copies of the same memory, new study reveals

A new study published today in Science A study finds that the memory of a specific experience is stored in multiple parallel “copies.” These are retained for varying lengths of time, modified to some degree, and sometimes deleted over time, report researchers from the University of Basel.

The ability to transform experiences into memories allows us to learn from the past and use what we have learned as a model for responding appropriately to new situations. Therefore, as the world around us changes, this model of memory cannot be reduced to a simple frozen archive of the good old days.

Rather, it must be dynamic, evolving over time and adapting to new circumstances to better help us predict the future and choose the best course of action. How the brain could regulate the dynamics of a memory was a mystery until multiple copies of it were discovered.

The research group of Professor Flavio Donato at the Biozentrum of the University of Basel uses mouse models to study how memories are stored in the brain and how they change over the course of life. His team has now discovered that in the hippocampus, a region of the brain responsible for learning from experience, a single event is stored in parallel memory copies between at least three different groups of neurons, which appear at different stages of embryonic development.

Memory copies come and go and change over time

The first to appear during development, early neurons are responsible for the long-term persistence of a memory. In fact, even if their memory copy is initially too weak for the brain to access, it becomes increasingly strong over time. Moreover, in humans, the brain may not have access to this memory until some time after it is encoded.

In contrast, the memory copy of the same event created by late neurons is very strong at first but weakens over time, so that if one waits long enough, such a copy becomes inaccessible to the brain. Halfway through, among the neurons emerging between the two extremes during development, a more stable copy might be observed.

Surprisingly, the copy used may also be related to how easily a memory can be modified or used to create a new one. Memories stored for a short period of time after they are acquired by late neurons can be modified and rewritten. This means that remembering a situation shortly after it happened prompts late neurons to become active and integrate the information present in the original memory.

In contrast, remembering the same event after a long period of time prompts neurons born earlier to reactivate themselves to retrieve their copy, but the associated memory can no longer be easily modified. “The dynamic way in which memories are stored in the brain is evidence of the brain’s plasticity, which underlies its enormous memory capacity,” explains first author Vilde Kveim.

Flexible memories enable appropriate behavior

Donato’s research team thus demonstrated that the activation of specific memory copies and their timing could have important consequences for how we remember, modify and use our memories.

“The challenge the brain faces when it comes to memory is daunting. On the one hand, it has to remember what happened in the past to help us make sense of the world we live in. On the other, it has to adapt to the changes happening around us, and our memories have to adapt as well to help us make good choices about our future,” Donato says.

Persistence in dynamics is a delicate balancing act, one that we may now have an entry point to fully understand. Researchers hope that one day, understanding what causes memories to be encoded and modified in the brain could help soften memories that are pathologically intrusive in our daily lives, or bring back those we thought were lost forever.