Dr. David Eagleman

These are normally silent.

They don't normally do anything.

but they are ready.

They're like silent sentinels that say, hey, just in case this territory stops getting used, I'm taking over.

Okay, so here's what my student and I realized is that

Because we live on a planet that rotates into darkness every night, the visual system is at a unique disadvantage.

Because when it's dark, you can still hear and smell and touch and taste, but you can't see.

And obviously I'm talking about evolutionary time before the invention of lights, which is the last nanosecond of evolutionary history.

It was really dark at night and you can't see.

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.