Yuji Ikegaya is a leading reseacher of brain, particularly in hippocampus. This is a talk between Shigesato Itoi, the famous ad copy writer, with him.

The topic is about sleeping.

While you are awake, about 30% of neurons are active at some point in time.

While you are in REM sleep, only 30% of neurons are active, but in non REM speep, almost 100% of neurons are active.

While you are in REM sleep, you would have a dream.

Our brain does two simple things.

1. Garner information
2. Memorize information

In computer, it is the same thing. But our brain can't do that at the same time. First, garner information, then memorize it. Garnering is done when you are conscious, awake or in REM sleep, while memorizing is done in non REM sleep.

At times, I see he has great expertise/experiences in computer science. He have tried a crazy test. With a 6-legs-computer, he broke one leg, then observed how it can evolve its program to make it work. According to him, in the process, the computer made similar trials as human do in a dream.

I am a specialist of hippocampus especially in CA3. CA3 garners information while you are awake, and does a very interesting activity in non REM sleep. It is a compression of information.

Then, he explains about it refering to an experimentation with a mouse.

the following image is from the talk page

He let a mouse walk in a place. Neuron A becomes active only at place A. Neuron B only at place B, and so on. So he can get where the mouse is without checking it as long as he is watching the monitor.

Then, he let the mouse take a sleep, then sees it is having a dream in the same order. Suppose the mouse moved A->B->C->D, it is having a dream A->B->C->D. So what happens in non REM sleep?

It is, again, A->B->C->D, but, at a tremendous speed. About 100x faster. Besides, they are compressed.

And then, the compressed information is sent back to cerebrum and is memorized permanently.

It is interesting that our brain has two types of memory, temporal and permanent. And to be memorized permanently, a temporal memory has to be processed by CA3.

Locations are different or only forms to be memorized are different?

I read that a permanent memory in our brain is never deleted. So what about temporal memory? If they are not deleted, our brain is supposed to have duplicated information.

I also read that, while having a dream, our brain randomly put memories together. The process must be required to determine the exact place to store information permanently.