Introduction

Task 1: Time for a small exercise. Take a few minutes to relax right now. Lean back in your chair, clear your mind of your endless cares and stressors, breathe deeply, hold it a second, then exhale slowly. Do it again a few times. Feel those muscles relax, feel the tension drain from your face, feel your heart begin to beat more slowly. Now, quick, think the following thought:

You know, someday that heart is going to stop beating.

Don’t stop there. Really think about that fact, think about how every heartbeat is counting down toward that final one. Think about how the flow of blood will come to a standstill, how your brain will shut down for lack of oxygen. Think of your toes and fingers turning blue.

Now think about what thinking about this feels like. If you’re like me, your heart is now racing, and there’s this weird, icy panic clutching from your stomach down to your crotch, and your throat is sort of constricted in a way that makes you want to whimper or puke.

And thus you will have learned the first of two points that are critical to this part of the book: sometimes, all you need to do is think a thought and you change the functioning of virtually every cell in your body.

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Task 2: Next, consider the following three things:

Now, what do they have in common?

You grope for an answer. They, er, all have something to do with biology. Well, yes, that’s a pretty safe guess. They all have something to do with behavior or emotions or emotional behavior, or something like that. Getting warmer.

Give up? Each has been proffered in a court of law by defense attorneys as an explanation for a violent crime.* And thus we learn the second of the two critical points in this third of the book: sometimes, all you need to do is change events in the body—change the levels of some hormone, some nutrient, some immune factor—and you will change how your brain thinks and emotes.

This intertwining of our brains and bodies, their mutual capacity to regulate each other, is now a central concept of modern biology. Few dualists are out there anymore, for whom our mindness floats pristinely above all that nuts-and-bolts biology of cells and organelles and molecules. We are the product of those cells et al., and our brains are as fundamentally biological organs as are, say, our bladders—some muscle cell from the wall of your bladder, and some fancy turbocharged neuron from your cortex, have a lot more in common with each other than they have different. The brain is just another organ, albeit a fancy one, and its functioning is inseparable from its existence within the body. As the neurologist Antonio Damasio has put it, “The mind is embodied…not just embrained.”

The ways in which the brain alters function throughout the body are numerous. There’s the obvious way in which your brain, via the “voluntary nervous system,” sends commands to your skeletal muscles. And suddenly your body shakes hands or signs a check or does the Bunny Hop.

Then there’s the involuntary nervous system (also known as the autonomic nervous system), by which your brain regulates your body in ways over which you don’t normally have much conscious control. And thus you blush or feel aroused or have an array of “autonomic responses” when you contemplate your inevitable mortality. Those cables of neurons from your brain that form the autonomic nervous system are turning out to project to all sorts of unexpected places. Some course from your spine to your bones, those most static and phlegmatic of outposts in the body, and influence what turns out to be the manic process of bone remodeling. Others project individually to each of millions of tiny, vestigial hair follicles in our limbs—and thus we are capable of getting gooseflesh. Others project to our immune organs, a basis of an entirely new scientific discipline, psychoneuroimmunology, the study of the ways in which the brain can regulate our immune defenses.

And then there’s the ability of the brain to regulate the body through the secretion hormones. The brain turns out to be an endocrine gland, the master gland (a title that Reader’s Digest–style science circa 1950 used to award to the pituitary), releasing scads of hormones that in turn direct the function of endocrine glands throughout the rest of the body.

And consider the other half of this brain/body circle of interactions, how events in your body can influence the brain. This has been known for a long time: thousands of years ago, someone carried out what was probably the first experiment in psychoneuroendocrinology, castrating a bull, thereby demonstrating that something or other emanating from the testes has something to do with male surliness. A long-standing belief was that parts of the brain were relatively unaffected by events in the body. This privileged domain, in this view, was the cortex, the most “cerebral” part of the brain. Sure, the brain regions that specialized in emotion were pickled in hormones, but the cortex was viewed as this gleaming, stainless-steel, affectless realm, half-calculator and half–objective philosopher king. This dichotomizing between emotion and cognition (and between the brain regions responsible for each) turns out to be completely false, a misconception termed “Descarte’s error” by Damasio (in a superb book by the same name).

Instead, events in the body influence every aspect of brain function. The type and levels of sex hormones in your bloodstream will alter whether your brain is better at picking up the local details or the global features of a pattern. Chemical messengers released by your immune system will increase your risk of depression. Stress hormones will alter the functioning of a key outpost of your brain, the frontal cortex, and the prudence of the decisions that you make. In the aftermath of a trauma, your blood pressure, along with other autonomic measures, influences the likelihood of your succumbing to post-traumatic stress disorder. And even something as mundane as your blood sugar levels will alter how readily you remember some factoid.

The varied ways in which the brain and body interact, and the ways in which the brain is ultimately a mere biological organ, are the subjects of the essays in this part of the book. Essay seven, “Why Are Dreams Dreamlike?” introduces that critical part of the brain, the frontal cortex, and considers how in every one of us it is completely dysfunctional, shut down, and out of business, many times each day. Essay nine, “The Pleasure (and Pain) of ‘Maybe,’ ” continues the theme of the frontal cortex, examining what it has to do with discipline and gratification postponement.

Essay eight, “Anatomy of a Bad Mood,” and essay ten, “Stress and Your Shrinking Brain,” focus on how the body influences the brain. The first examines how your autonomic nervous system can make you do stupid things in your relationships, can make your brain decide that its feelings are deeply, sincerely hurt about something when it just isn’t so. Essay ten takes a more tragic focus, examining how a class of hormones released during stress just might be damaging an area of the brain in people with certain types of post-traumatic stress disorder. This finding may turn out to be of ghastly relevance to the huge numbers of people seared by the events of 9/11.

Essay eleven, “Bugs in the Brain,” examines a truly bizarre example of how the brain can be influenced by the world outside it. In this case, the brain’s function is not being influenced by some hormone or nutrient in the bloodstream. Instead, some facets of its function are being controlled by parasites that get into the brain. Creepy stuff.

Essay twelve, “Nursery Crimes,” examines a brain that is profoundly, grotesquely diseased and produces disease in the vulnerable body of someone else.

*For all but San Franciscans, the junk-food explanation is probably the most obscure. In late November 1978, Dan White, a disappointed office seeker and political malcontent, murdered George Moscone and Harvey Milk, respectively the mayor and the city supervisor/gay political icon of San Francisco. Among the defenses offered at his trial was the notion that a drastic change in blood sugar due to White’s junk-food consumption (i.e., the notorious “Twinkie Defense”) impaired his volitional control over his behavior. Though convicted of the murders, White was generally viewed as having received an appallingly short sentence.