Thanks to our memory, we can learn from the past and react appropriately to new situations. Our memory is therefore based on a certain stability and dynamism. However, the exact neural mechanisms behind this have remained a mystery. But thanks to recent research in the hippocampus of mice, we are now beginning to gain deeper insights. According to this research, the brain creates three copies for a single memory.
What we already knew: When we remember, i.e., retrieve certain memory contents, the same neuronal patterns are activated that were active during encoding. In particular, the focus was on discrete populations of neurons that emerge at different points in embryonic development-and neurons of the same age have similar properties. The researchers subjected mice to a conditioning paradigm in which the animals learned to associate an initially neutral event with a fearful stimulus. The researchers used calcium imaging to measure cell signals and optogenetic methods to manipulate cell activity.
It has been found that a single experience is stored in at least three different groups of neurons in parallel. The three memory traces differ mainly in when they are relevant when the memory is recalled and how easily they can be changed. Thus, “young” neurons support recall shortly after the memory is acquired. This memory trace is initially very strong and fades over time.
Applied to humans, this means that when we think about an experience shortly after it happened, these neurons become active and integrate new information into the original memory.
If we remember it only after a longer period of time, the “older” neurons retrieve their memory copy, which, however, can hardly be changed. In a third group of neurons “born” between the young and old neurons, the copy created is consistently stable. Thus, both the persistence and the modifiability of memory can be attributed to the dynamics of neuronal ensembles in the hippocampus – dynamics that are ultimately based on the differential recruitment of different cell populations.