Obviously, this is why my MP3 player failed: A week was too long to wait. If I'm going to associate exercise with a positive payoff, the reward has to be immediate. But beyond that, Schlund tells me, I have to unlearn the rewards I've already associated with not exercising (no pain, more time for other things). Doing this actually requires changing my neural circuitry. And rewiring an adult brain, I am about to discover, is very tricky.

A few days into my meeting with Schlund, I'm sitting at a small desk in a psychiatric ward at Yale, staring at a computer screen with two clickable buttons: CHE and SHE. The computer says "che" (or is it "she"?), and I'm supposed to press the appropriate button. I click CHE. The computer buzzes and tells me to try again. "Che" or "she"? I click SHE. Buzz. Over and over, I get the buzz. I'm thinking this must be a joke, but then I squint, listen hard, and finally hear it. I hit CHE. The computer dings, then two pink kissing fish appear on the screen and do a funky dance with a hermit crab. That's my reward, which clearly gets my dopamine going: I start playing compulsively, completely hooked on picking the right answer so I can see what my next goofy reward will be. After a while, my attention starts wandering... buzz. So I squint, listen hard, and hear it again: "che." A spaghetti-thin man suddenly appears on the computer screen playing a xylophone, until a musical note hits him on the head. Then Bruce Wexler, MD, walks in the room.

Wexler, a leading neuroscientist and the author of Brain and Culture, studies brain plasticity and how it affects our ability to change. I've come to try out this program, which he uses to help patients with schizophrenia improve their audio processing and memory. "You're very good at that," Wexler tells me. Not really, I say, pointing out how many errors I made before figuring it out. But actually, that's the whole idea of the program: Successful change requires abnormally intense, uninterrupted concentration and repetition. Why? Because we're working against evolution: Our brains are designed to conserve energy for really important things, like breathing and coordinated motion, even though sometimes, altering behavior is just as important as breathing. Our brains revert to habits when given the chance because they require less energy than change. That silly exercise with "che" and "she" actually changes the way adults hear because it doesn't let that happen. It forces intense concentration resulting in instant rewards that make you want to repeat the exercise over and over again.

"You want to know why it's hard to change?" Wexler asked when I first walked into his office. "There are a hundred billion neurons in your brain. Each one is connected to thousands of others. Everything you're talking about—behaviors and learning and memory—involves the integrated actions of hundreds of thousands of cells in intricate systems throughout the brain." In adults those systems are essentially hardwired.

When you're a kid, it's a different story: Young brains are constantly forming new connections between neurons, changing the way children process information based on their experiences. That's plasticity, and it's why children soak up language and adapt to new cultures at rates that put adults to shame. "By the time we hit our 20s," Wexler says, "our brains have lost most of their plasticity." But fortunately, they haven't lost all of it.