The brain stores memories through a neuronal ensemble, termed engram cells.

Cecile G. Tamura

What happens to memories as days, weeks and years go by has long been a fundamental question in neuroscience and psychology.

For decades, researchers have attempted to identify the brain regions in which memory is formed and to follow its changes across time.

The theory of systems consolidation of memory (SCM) suggests that changes in circuitry and brain networks are required for the maintenance of memory with time.

Various mechanisms by which such changes may take place have been hypothesized.

Recently, several studies have provided insight into the brain networks driving SCM through the characterization of memory engram cells, their biochemical and physiological changes and the circuits in which they operate.

Crucial to this process of linking engram cells is the ability of neurons to forge new circuit connections, via processes known as "synaptic plasticity" and "dendritic spine formation."

Importantly, experiments show that the memory initially stored across an engram complex can be retrieved by its reactivation but may also persist "silently" even when memories cannot be naturally recalled, for instance in mouse models used to study memory disorders such as early-stage Alzheimer's disease.

The hippocampus, a structure located deep within the brain, has long been seen as a hub for memory. The hippocampus helps “glue” parts of the memory together (the “where” with the “when”) by ensuring that neurons fire together. This is often referred to as “neural synchronisation”. When the neurons that code for the “where” synchronise with the neurons that code for the “when”, these details become associated through a phenomenon known as “Hebbian learning”.

But the hippocampus is simply too small to store every little detail of memory. This has lead researchers to theorise that the hippocampus calls upon the neocortex – a region which processes complex sensory details such as sound and sight – to help fill in the details of a memory.

The neocortex does this by doing the exact opposite of what the hippocampus does – it ensures that neurons do not fire together.

This is often referred to as “neural desynchronisation”. Imagine asking an audience of 100 people for their names. If they synchronise their response (that is, they all scream out at the same time), you’re probably not going to understand anything.

But if they desynchronise their response (that is, they take turns speaking their names), you’re probably going to gather a lot more information from them. The same is true for neocortical neurons – if they synchronise, they struggle to get their message across, but if they desynchronise, the information comes across easily.