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Memories are stored as chemical changes at the connection points (synapses) between neurons in the brain. There are more than 100 trillion synapses in the human brain, and each memory is stored as slight changes to thousands or millions of synapses. In this sense, memories have a “distributed” representation: each memory involves thousands of synapses, and each synapse is involved in possibly thousands of memories.

If a memory survives until at least the next day, then its storage has already resulted in possibly permanent change to thousands of synapses in the brain. The question is for how long will that memory be retrievable? And is it still there even if it is not retrievable?

Episodic Memory

The memory for past events is among the least understood processes in neuroscience. It is only known for certain to exist in humans, and the experiments required to study its neural mechanisms are problematic for logistical and ethical reasons.

One fascinating study published in Nature looked at the activity of individual neurons in the episodic memory region (the hippocampus) […]