Engram cells are an ensemble of neurons where memories are physically stored.
When you activate the ensembles, the memory that is stored is recalled.
Forgetting occurs when the engram cells can not be reactivated. The memories are still there, but the engrams cannot be activated, thus the memories cannot be recalled.
Forgetting sometimes involves changing the neural circuits from an accessible (activating) to an inaccessible (no reactivation). And when you suddenly remember, the opposite happens, the inaccessible circuits become accessible. This is normal.
In dementia, it is believed that these circuits are taken over and cannot be converted to accessible, thus the memory cannot be recalled.
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