These are big ideas. We have a lot of explaining to do, but we wanted you to see the big ideas up front. If you lifted just two words from the first sentence—feedback loops—you could mesmerize a medical school class for a year. The body is an immense feedback loop made up of trillions of tiny loops. Every cell talks to every other and listens to the answer it receives. That’s the simple essence of feedback, a term taken from electronics. The thermostat in your living room senses the temperature and turns the furnace on if the room gets too cold. As the temperature rises, the thermostat takes in that information and responds by turning the furnace off.
The same back- and- forth operates through switches in the body that also regulate temperature. That’s nothing fascinating, so far. But when you think a thought, your brain sends information to the heart, and if the message is one of excitement, fear, sexual arousal, or many other states, it can make the heart beat faster. The brain will send a countermessage telling the heart to slow down again, but if this feedback loop breaks down, the heart can keep racing like a car with no brakes. Patients who take steroids are replacing the natural steroids made by the endocrine system. The longer you take artifi cial steroids, the more the natural ones ebb, and as a result the adrenal glands shrink.
The adrenals are responsible for sending the message that slows down a racing heart. So if a patient stops taking a steroid drug all at once rather than tapering off, the body may be left with no brakes. The adrenal gland hasn’t had time to regrow. In that event, somebody could sneak up behind you, yell "Boo!" and send your heart racing out of control. The result? A heart attack. With such possibilities, suddenly feedback loops start to become fascinating. To make them mesmerizing, there are extraordinary ways to use the brain’s feedback. Any ordinary person hooked up to a biofeedback machine can quickly learn to control bodily mechanisms that usually run on automatic. You can lower your blood pressure, for example, or change your heart rate. You can induce the alpha- wave state associated with meditation and artistic creativity.
Not that a biofeedback machine is necessary. Try the following exercise: Look at the palm of your hand. Feel it as you look. Now imagine that it is getting warmer. Keep looking and focus on it getting warmer; see the color becoming redder. If you maintain focus on this intention, your palm will in fact grow warm and red. Tibetan Buddhist monks use this simple biofeedback loop (an advanced meditation technique known as tumo) to warm their entire bodies. 

This technique is so effective that monks who use it can sit in freezing ice caves meditating overnight while wearing nothing more than their thin silk saffron robes. Now the simple feedback loop has become totally engrossing, because what we can induce merely by intending it may have no limit. The same Buddhist monks reach states of compassion, for example, that depend on physical changes in the prefrontal cortex of the brain. Their brains didn’t do this on their own; they were following orders from the mind. Thus we cross a frontier. When a feedback loop is maintaining normal heart rhythm, the mechanism is involuntary—it is using you. But if you change your heart rate intentionally (for example, by imagining a certain someone who excites you romantically), you are using it instead. 

Let’s take this concept to the place where life can be miserable or happy. Consider stroke victims. Medical science has made huge advances in patient survival after even massive strokes, some of which can be attributed to better medications and to the upsurge of trauma units, since strokes are ideally dealt with as soon as possible. Quick treatment is saving countless lives, compared to the past. 

But survival isn’t the same as recovery. No drugs show comparable success in allowing victims to recover from paralysis, the most common effect of a stroke. As with the discouraged children, with stroke patients everything seems to depend on feedback. In the past they mostly sat in a chair with medical attention, and their course of least resistance was to use the side of the body that was unaffected by their stroke. Now rehabilitation actively takes the course of most resistance. If a patient’s left hand is paralyzed, for example, the therapist will have her use only that hand to pick up a coffee cup or comb her hair. 

At first these tasks are physically impossible. Even barely raising a paralyzed hand causes pain and frustration. But if the patient repeats the intention to use the bad hand, over and over, new feedback loops develop. The brain adapts, and slowly there is new function. We now see remarkable recoveries in patients who walk, talk, and use their limbs normally with intensive rehab. Even twenty years ago these functions would have languished or shown only minor improvements. And all we have done so far is to explore the implications of two words.