Dr. David Eagleman

And so, you know, you'd go into the corner of a cave and curl up and go to sleep.

But the key is that the visual system was in danger of getting taken over during this long extended period of darkness.

So what we hypothesize is that dreams are the brain's way of defending the visual cortex against takeover from the other senses.

And when you look at the circuitry,

very specific circuitry, starts in the midbrain, goes to an area called the lateral geniculate nucleus, and plugs straight into the primary visual cortex.

And that's it.

Every 90 minutes, you have this volley of activity that just slams into the primary visual cortex.

It doesn't go anywhere else in the brain.

And so every 90 minutes, you've got this automated way of making activity happen there.

And because we are visual creatures, we see that as a dream.

We see a whole story.

And because the brain is a storyteller, we impose plot, meaning, and we have emotion that goes with that.

But the key is this is the brain's way of defending territory in the dark.

And so what we did then was,

is we examined very carefully 25 species of primates and looked at their brain plasticity.

And you can measure this with different proxies, like, you know, when they start to walk and when they get to reproduction age and so on.

And, you know, some creatures like the gray mouse lemur, which is a type of monkey,

You know, they are born, let's just say, pre-programmed.

You know, they pop out.

They're really quick to stop, you know, to wean and reach juvenile age and reproduce and so on.