Once we've got a good psychological theory, then it makes sense to ask how the brain does it. . . . Neuro-scientific evidence only makes sense if you already have a theory of structure and function, well worked out at the psychological level of analysis. . . . Otherwise, it's all just pixels.

– John Kihlstrom

THE CASE WAS MYSTIFYING. FOR YEARS, THE PATIENT’S RELATIONSHIP with his father was normal. But now, he didn’t even recognize him! What do you think was wrong?

Maybe the patient had amnesia and couldn’t remember his father. But that wasn’t it. When he talked to his father on the phone, everything was normal: He recognized his father’s voice, remembered their relationship, and they conversed as always. Problems arose only when he saw his father in person.

Maybe a part of the patient’s brain that detects faces was damaged and he couldn’t recognize anybody. But that wasn’t it, either. He easily recognized other people: neighbors, casual friends, and the like. Yet his father was unrecognizable to him.

As it turns out, all the individual parts of the patient’s brain were working properly. Yet something was broken: a connection between parts (Hirstein & Ramachandran, 1997).

Everyone’s brain contains one region that detects faces and another that generates emotions. In most brains, they are interconnected. When a loved one comes into view, both brain regions are activated; the interconnection combines their activity, and the result is a “warm glow of recognition.” You see and feel as if you’re seeing your loved one.

In the patient’s brain, the connection had become severed. Once this happened, he no longer experienced the “warm glow”; upon seeing his father, he didn’t feel as if he was looking at his father. His brain damage—the broken connection—then caused his mind to play a trick on him. Without the feeling that usually occurred when he looked at his father, the patient concluded that the man was an imposter.

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Capgras syndrome, the patient’s disorder, is rare. But the lesson it teaches is broadly important. When it comes to the brain, connections are key.

The brain is like the Internet. The Internet’s power comes from connections among vast numbers of computers. The brain’s power derives from connections among vast numbers of brain cells. Without the Internet connections, you couldn’t email friends, watch YouTube videos, or play interactive games. Without the brain connections, you couldn’t sing a song, read this book, or recognize your father.

WHAT IS THE BRAIN LIKE? It seems so mysterious: a collection of biological cells packed under the skull that, somehow, gives you extraordinary powers—to create works of art; to feel the warmth of the sun; to imagine yourself traveling in a spaceship; and, even more remarkably, to think about the fact that you are creating art, feeling the sun’s warmth, and imagining yourself in a spaceship.

This chapter begins with some general principles that help to explain the brain’s workings. Next, we’ll “zoom in” on the brain by reviewing its overall organization and then its individual cells. Finally, you’ll learn about two communications systems that run throughout the body: the nervous system and the endocrine system.