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Here's what happened: After sliding the volunteers into an MRI machine, he gave them two buttons—one for the right hand, one for the left—then said, "You'll have to make some decisions. If you're correct, you earn money. If you're wrong, no money." Then he fired up the machine, which rattled and clanged as it began scanning their brains. Inside the machine, on a computer screen above the volunteers' heads, a circle appeared and vanished. Next, the word CHOOSE flashed, which meant they had to pick a button, right or left. The game made no sense. There was no correct response: All they could do was click a button randomly, then the computer said WRONG and the circle appeared again. So they picked the other button and the computer flashed, CORRECT. YOU'VE EARNED 50 CENTS.

Once the volunteers knew which button to press in response to the circle, they repeated the process over and over. Circle. Correct button. Reward. Circle. Correct button. Reward. This is where it got interesting for Schlund, because he wants to know what happens in the brain when you learn a new behavior based on rewards, which parts light up, how big that activation is, and how it changes over time as the behavior becomes habitual.

On the first click, when they were guessing, the volunteers' brains lit up a little in the frontal lobe—an area associated with self-control, decision making, and behavior change. After the second click, when they got the reward for answering correctly, suddenly their brains kicked into high gear, and with each repetition, their frontal lobes lit up more and more, which meant their brain activity continued to increase as they learned the new behavior. But—and this is the good news—within about 50 repetitions, Schlund says, the reverse will start happening—the frontal lobe lights up less and less until the brain is exerting minimum effort, which means the new task has officially become a habit.

When Schlund tells me this, I ask if it means I only have to force myself to exercise 50 times and then it will be a habit. "I wish I could say yes," he answers. "But we really have no idea. What I can tell you is, there are many variables." The biggest one is stress. It turns out that the hormones released by the body in response to stress are our worst enemy when it comes to changing: They actually inhibit the frontal lobe, which makes the brain revert to behaviors that don't require conscious decisions (eating our familiar foods, drinking, smoking). Not only do stress hormones impair the areas of our brains that need to be active to change, they also stimulate our emotional centers, which send out signals telling us to decrease the stress. And what decreases stress? Food (because it triggers the release of natural opiates), alcohol, and cigarettes.

So successful change depends in part on stress management. But, Schlund says, it also depends on finding the right rewards. "If people got paid to exercise," he tells me, "everyone would do it. And this country would be much better off."

I ask if he'll pay me to exercise. He folds his hands on the Formica table between us, looks me in the eye, and says, "If you want to convince your brain you should exercise, you have to treat yourself the way you'd treat your dog." It's hardly the answer I'm looking for, but at this point, I'm open to anything.

"Imagine she's wetting on the floor every day," he says. "Are you going to say, 'Hey dog, if you don't wet on the floor for a week, I'll buy you a rawhide bone'? That would be like your boss saying, 'If you work five years, then you'll get your check.' It's too far off."

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