As I was going to sleep last night, I was thinking about not remembering many specific memories from childhood.  Does the brain overwrite older memories to make way for new ones?

I then remembered an NPR story from a year or more back about some brain researchers who had determined in rats that while a rat sleeps, its brain plays back its short term memories backwards and places in the part of the brain the short term memories (or an abbreviated form of them) into the place that stores long term memories.  Lack of sleep disrupted this process and lead to poor recollection of the now long term memories.

It got me asking how do brains determine which memories to keep?

Repetition certainly helps, otherwise why else would I remember the layout of the SciFi club at UMass?  Many of us have heard the idea that stress helps to cement memories.

Certainly there are evolutionary justifications for both.  Repetition could allows us to create more connections to that part of the brain that stores a particular location or fact.  The more connections the better.

Likewise, it would make sense that a memory could get tagged as stressful due to fight/flight responses.  The brain could then use this tag to decide that such a memory was more deserving of storage in long term memory.

So a proper research topic would be to determine what types or memories are more likely to be stored for long term retrieval.