Imagine you've got one strong eye and one weak eye, he tells me. If you cover the good eye with a patch, so it gets no stimulus, the weak eye will get stronger. But the second you remove the patch, the strong eye kicks in again and the weak one gets weaker. The same is true of all pathways in the brain. Once established, they stick around and remain strong as long as they're being used. So the first step toward change, Wexler says, is putting a "patch" over the pathway you want to lose (like, say, a chocolate obsession), which means eliminating anything that activates it (having chocolate in the house, going places where you usually buy chocolate). This is why, for many people who try to quit drinking or smoking, it's impossible to have just one glass of wine or cigarette. It's why heroin and coke addicts must avoid places and people connected to their drug days.

For dieters, just walking into your regular grocery store can activate an old familiar food pathway and keep it alive. So successful weight loss is as much about lifestyle change as it is about what you eat: Shop at a new store; buy new brands of food; use a new set of plates; eat in another room, at a different time of day. All these things will help starve an old, unhealthy pathway so you can develop a new, healthy one. "The more drastically you restructure your habits," Wexler says, "the more the established pathway that you're trying to change is weakened."

But disabling the old pathway isn't everything. Searching your brain for an existing healthy pathway—even a tiny weak one—and then strengthening it can make things much easier. So Wexler tells me to find an "I like exercise" pathway. I tell him I don't think I have one. He doesn't buy it. "Wasn't there some activity you loved as a kid?" he asks. I don't think so.

On the train ride home, however, as I stare out the window listening to my new MP3 player, David Bowie's "Changes" comes on and I start laughing. Appropriate, yes. But it was also the song my next-door neighbor and I skated to in my backyard when I was a girl. For my entire young life, I was obsessed with roller-skating. My first kiss was on skates; I roller-skated to high school every day, then rolled down the hall from class to class. I actually convinced my high school to waive my PE requirement and give me credit for my constant skating. Sitting on the train remembering all this, I smile and think, "I just hit my dopamine jackpot."

When I get home, I strap on my 10-year-old Rollerblades and give it a try. I turn on some disco and start rolling. It's sunny; my dog is running next to me. I can practically feel the dopamine coursing through my veins. My exercise problem is solved. Life couldn't be better.

The next day I wake up, walk into my living room, then sit down at my computer thinking, "Oh my God, I have so much to do." A few hours later I think, "I should go Rollerblade now." But I'm busy. I've got a deadline, I exercised yesterday, and besides, it looks like it's going to rain. I'll do it later. But when later comes, I'm tired from working all day, and now it's getting dark. Then I think, "Wait a minute. Why isn't all that dopamine from yesterday driving me to get up and Rollerblade again? Did my brain forget?"

A week later, I call Monika Fleshner, PhD, a neuroimmunophysiologist at the University of Colorado at Boulder who has done extensive research into the physiology of exercise. I explain my situation. I say I found an exercise I like, and I think I've got the dopamine thing solved, but funny thing is: I'm still not doing it.


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