David Linden

Dr. David Linden, a neuroscientist at Johns Hopkins, has written a book called "The Compass of Pleasure." (Sun photo by Barbara Haddock Taylor / April 12, 2011)

Not all addictions live up to their advance press.

In the past decade, it's become common to casually and humorously describe a favorite activity in the parlance of chemical dependency. People speak of being "addicted" to chocolate or high-fat foods, playing video games, buying expensive designer shoes, watching weekly episodes of "American Idol" to sleeping on high-thread-count sheets. But according to "The Compass of Pleasure," a new book by Johns Hopkins neuroscientist David Linden that is being released today by The Viking Press, just two of those pursuits -- eating fatty foods and shopping -- can become genuine addictions for some people. Watching television, playing World of Warcraft and swaddling yourself in Egyptian cotton probably cannot.

Who knew that feeling good could be so complicated?

"Addiction is defined by the changes that certain activities can make in the brain," Linden says.

"Basically, some activities have been shown to short-circuit the medial forebrain pleasure circuit. The process is the same, whether someone is taking crack cocaine or gambling or having risky sex or shopping. These morphological changes bring about a gradual transition from liking to wanting, and the result is compulsive behavior."

The vast majority of people who pursue habit-forming activities will never become compulsive. Our genes play a major role in determining who will become addicted, how badly and to what substance. It also generally takes repeated exposure for a harmful habit to develop.

For example, Linden points out that for even a drug as highly addictive as heroin, two of every three people who inject the narcotic directly into their veins don't become junkies.

In addition, some pursuits that may technically qualify as addictive carry positive benefits for individuals and society that vastly outweigh the potential harm, such as running a marathon, or an extended bout of contemplative prayer.

"Pleasure is our compass, no matter what we do," Linden says.

"But a philosophical question arises from these findings. If we catch a pleasure buzz from our noblest instincts, does that make them less noble?"

Conversely, just because a substance doesn't create a physical dependency doesn't mean it's safe. LSD might not activate the medial forebrain, but someone who drops acid and jumps off the roof because he thinks he can fly is likely to get hurt.

"There's a whole bunch of risk-taking behaviors that aren't addictive," Linden says. "We're motivated by reasons other than seeking pleasure."

Addiction, he says, is just one form of learning. The changes that occur inside the brain when someone studies calculus are nearly identical to the changes that occurs when he smokes crack.

As Linden explains it, when human beings bliss out by watching a sunset, getting a back rub or drinking a glass of red wine, the ventral tegmental area in our brains is releasing a neurotransmitter called dopamine that is getting picked up by a nearby bundle of neurons called the nucleus accumbens.

Under normal circumstances, pleasure flows and ebbs; dopamine gets released and then reabsorbed for later use. But some activities hijack the pleasure circuit, either by increasing the amount of dopamine flooding into our systems or by blocking the portals through which the chemical messenger goes back into storage.

The result is a jolt of pleasure so intense that most people will do anything to feel it again.

"Addiction is a super-potent experience," Linden says. And, just like Pavlov's dog, "we learn to associate it with sensory cues," he says, "because these associations allow us to predict how to behave so we can repeat the experience."

When we make a connection between, say, our sweet tooth and a chocolate store located in the Inner Harbor, our brains get rewired. After repeated exposure, structures on our neurons called "dendrites" grow new spines. (The same thing happens when we learn our multiplication tables or memorize a new route to work -- both experiences that humans experience as pleasurable, though to a lesser degree and by a more indirect pathway.)

In the future, every time our car turns onto Pratt Street, we may feel a sudden urge from out of the blue to stop at the chocolatier we know is just down the block.