The brain stores at least 3 copies of every memory

Memories evolve throughout our lifetimes, changing as we learn and experience new things and as we recall a memory repeatedly. And then, memories degrade as we age.

Previously, scientists thought that this malleability was the result of changes in the brain cells that originally encoded the memory, and they believed these cells stored just one copy of every memory in the brain. However, new research suggests that might not be true.

The scientists found that, in rodents, the brain stores at least three copies of a given memory, encoding it in multiple places in the organ.

These copies are encoded by different groups of neurons in the hippocampus, a brain region critical for learning and memory. The copies vary in terms of when they're created, how long they last and how modifiable they are through time.

Related: How accurate are our first childhood memories?

In the new study, published Aug. 16 in the journal Science, the scientists showed that, as mice encode new memories, they first create so-called early-born neurons. These neurons are responsible for storing a long-term copy of the memory that is initially weak but becomes stronger over time.

Next comes middle-ground neurons, which are more stable from the outset, followed by late-born neurons that from the beginning encode very strong copies of a memory. However, that strength fades over time.