Scientists tell us the human brain is divided into three areas. Each represents a different evolutionary stage: a baby’s palm, held inside a woman’s hand, engulfed in a giant’s fist.What they tend not to say is this: each portion has a very different sense of fear.

The largest section, the neocortex, is also the most recent. It’s where “thought” occurs; where you balance your checkbook and learn Spanish.

Inside the neocortex is the limbic system. It’s where maternal and paternal emotions come from. When you watch a child cross the street, the limbic system’s working overtime.

Inside it lies the center of your brain: the R-complex, the direct attachment to the spinal cord. It’s called the lizard brain, because snakes and reptiles have one, too: little more than a tight bundle of nerve endings. It’s the source of hair-trigger responses: when you hear a loud noise, the R-complex tells you to jump.

When most people think of fear, they think of R-complex fears: the scream on the roller coaster, the pulse one second after the near-accident on the freeway, the gripped armrest in turbulence at 30,000 feet, the sudden shadow in a horror movie.

But some fears are more refined.

The limbic system’s fear is unselfish: it’s for the lost puppy in the road, or the child not yet home although it’s very very late.

The neocortical fear is civilized: the urge to leave a party where you might be embarassed, the sickness before a college entrance exam, or the tightness in the throat at meeting future in-laws.

The advantage of the brain’s three-part design is that the sections can communicate with one another. The unselfish limbic system calms the R-complex, “Don’t act this way in front of the children.” The R-complex might tell the neocortex (if it actually could use words, that is) that there was no real threat from failing an exam–compared, that is, to falling off a mountain or being shot at. The neocortex is the one that tells the other two, “It’s only a movie.”

In most cases, the three can make each other see reason.

But there is another kind of fear. And it is far, far worse.

It usually happens around four a.m., when the circadian rhythms are at ebb tide, the pulse is slowed and the body temperature is lowest. It’s when you are as near death as you ever come while still alive.You suddenly realize you are preternaturally awake. Your skin is cool and moist, and the pillow beneath your head is damp. As you begin to think of something, it’s as if your brain is somehow too tender; as if it’s been scraped and peeled. You can hear the house asleep in the moonlight. And everything is wrong.

You feel a presence near you. You do not turn. This person lying beside you–who has slept beside you for years–is somehow a stranger. The room, the house is someone else’s, or it should be. And everything–the form of the shirt draped over the chair, the vertical oblong of the slowly opening door–terrifies you.

It is yours, but it shouldn’t be. For you suddenly aren’t sure you understand the innumerable decisions and minute circumstances that lead you here and now, in this bed beside this person. What if I hadn’t been on that subway that afternoon? What if I had seen whats-her-name instead?

And at that moment, the three brains can’t help each other. The logical neocortex wants to scream, “What if everything I know is wrong?” while the limbic system thinks of the stranger beside you, and can only shudder in response, as the lizard brain sends an electric jolt that says LEAVE RUN NOW.

Then suddenly, with no provocation, her eyes open. Her face is blank, expressionless. Her eyes, wide and glistening, seem focused behind you. There is no recognition of you in them.

She blinks once. Then she becomes herself. She smiles, touches your shoulder, and whispers it’s all right.

She, who was someone–or something–else the moment before. Like you were, as well. EndBlock