The brain stores a memory in three copies
Several copies of an experience are created in the brain, as a look into the mouse brain reveals. These memory traces are retained for different lengths of time, are fixed or can be changed.
Thanks to our memories, we can learn from the past and react appropriately to new situations. Our memory is therefore based on both a certain stability and an appropriate dynamic. However, the exact neuronal mechanisms behind this have been a mystery until now. A team led by Flavio Donato from the Biozentrum of the University of Basel has now gained a deeper insight into the processes in the hippocampus of mice and shown that the brain creates three copies for a single memory: The brain creates three copies for a single memory. The research group published their findings in "Science".
What was already known: When we remember, i.e. recall a certain memory content, the same neuronal patterns are activated that were also active during encoding. Donato's group focussed on specific groups of hippocampal neurons. These are discrete populations that "see the light of day" at different times in embryonic development - and neurons of the same age have similar characteristics. The researchers then subjected mice to a conditioning paradigm: the animals learnt to associate an originally neutral event with a frightening stimulus. Meanwhile, the experts measured the signals from the cells using calcium imaging and manipulated cell activity using optogenetic methods, among other things.
They discovered that a single experience is stored in parallel in at least three different groups of neurones. The three copies or memory traces differ primarily in terms of the time at which they are relevant for memory retrieval and how easily they can be changed. "Young" neurons therefore support retrieval shortly after the memory has been acquired. This memory trace is initially very strong and fades over time. Applied to humans, this means that when we think about an experience again shortly after it happened, these neurons become active and integrate new information into the original memory.
If, on the other hand, we only remember it after a long time, the "older" neurones recall their copy of the memory, which can hardly be changed. In a third group of nerve cells, which were "born" between the young and old neurons, the copy created is consistently stable. This means that both the persistence and the changeability of memory can be attributed to the dynamics of the neuronal ensembles in the hippocampus - a dynamic that is ultimately based on the different recruitment of different cell populations.
Spectrum of Science
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