Of course, for some, the thrill of gambling is so great that it's worth throwing away relationships, forfeiting life savings, or facing big, burly loan sharks named Tiny, just to stay in the game. Those are extreme cases; but even for a casual gambler like Knauf, who logs on to online poker sites in his spare time and drives out to Foxwoods with friends every few months, gambling is mostly about the rush.

So what accounts for the titillation of Texas Hold ’Em and the buzz of blackjack? What is going on in our bodies to make these high-risk exploits so pleasurable, even when the reality of a loss can be devastating?

The answer lies in the brain — in a particular area neuroscientists call the pleasure center. Scientists have a much clearer idea now of how the brain responds to gambling than they did 20 years ago, thanks to the development of “functional” magnetic resonance imaging (fMRI). Unlike previous MRI technologies, which took static pictures of the brain, functional MRIs can record the brain in action. By measuring blood flow to different areas in the brain, fMRIs allow scientists to see, in real time, which areas are triggered during a given activity.

"For a long time, people thought that behavioral addictions didn't exist, or weren't as powerful as being addicted to drugs or alcohol," says Christine Reilly, executive director of the Institute for Research on Pathological Gambling and Related Disorders, based in Medford. But fMRI technology, she says, introduced new physical evidence of just how strong behavioral addictions can be.

According to fMRI images, much of the pleasure of addiction comes not from the fix, but from anticipation of the fix. In fact, the same pathways in the brain are activated when people anticipate a monetary reward as when cocaine addicts anticipate doing cocaine. A few years ago, scientists at Mass General mapped the brains of 12 non-pathological gamblers while they engaged in a betting game (with real money on the line). The results, published in Neuron in 2001, showed a pattern of brain activity in the gambling participants similar to patterns found in previous studies of drug addicts receiving cocaine or morphine. When the stakes increased, so did the brain's response.

The part of the brain triggered by both gambling and illicit drug use is known as the mesolimbic dopamine pathway. This is the same pleasure center involved in other feel-good activities, such as drinking alcohol, having sex, eating a delicious meal, or even receiving a compliment. The mesolimbic dopamine pathway is part of the limbic system, an area of the brain that has existed in vertebrates for 500 million years, and is driven by primal responses: if something makes you feel good, you do it again. That simple reaction has been integral to the propagation of many species, including humans, encouraging essential functions like eating and reproducing.

So you can thank the chemical dopamine, the chief neurotransmitter in the pleasure center, for those age-old feelings of bliss. When you start rubbing that scratch ticket or spinning a roulette wheel, the brain releases dopamine, which swims across the synaptic cleft and binds to receptors. While the dopamine is attached to these receptors, you experience pleasure. Once the dopamine is broken down or reabsorbed, the pleasure subsides.

In the case of gambling, the dopamine fires up even before you know whether Lady Luck is with you or against you. Why shouldn't you feel good in those glorious few seconds of hope, as you silently choose the upholstery for the seats inside the new Jaguar XKR convertible that you'll buy with your winnings, or weigh the benefits of vacationing in the Bahamas versus Amsterdam? If you win, the dopaminergic neurons maintain their gratifying activity, hugging those receptors for another few moments of glee. If you lose, the firing decreases, the fun dies down, and reality hits: no Jag convertible, no Dutch chocolate.

Simply thinking about gambling can also activate the brain's pleasure center. In a study by Marc Potenza, a professor of psychiatry at Yale University, pathological gamblers who were shown videos of people gambling and talking about gambling had altered brain activity in the same parts of the brain that light up when cocaine addicts see images of their precious nose candy. Of course, that response often leads addicts to seek out their vice, just as a hungry person might hit up the local pizzeria in response to beckoning smells.

The pleasure of gambling lasts longest when the outcome is most uncertain. If you're playing a game where the odds are stacked in your favor, you won't get the same rush as playing a game where the odds are only 50/50. This was illustrated with a 2003 primate study conducted by scientists at the University of Fribourg, in Switzerland, and the University of Cambridge, in England. The monkeys were exposed to distinct, easy-to-differentiate images, which were followed, with varying frequency, by a reward: a few drops of fruit juice. Some images were always followed by the reward, while others prompted a reward only some of the time. Those images that prompted rewards exactly half of the time — that is, the situations with the highest level of uncertainty — led to the strongest sustained activation of the monkey's dopamine neurons.

So just how good is that dopamine rush? What would those monkeys give up for a spot at that teasing fruit-juice dispenser, the one that keeps them atwitter in anticipation of whether they'll win or lose? Do pathological gamblers get such a high that it makes their sacrifices worthwhile?

In fact, pathological gamblers don't get a stronger buzz from gambling than anyone else does. Rather, studies indicate they're less capable of feeling good from the everyday uppers that we take for granted. While some people might only need a quick kiss or a warm chocolate-chip cookie to feel good, pathological gamblers need something stronger to maintain a normal baseline of dopamine activity. They turn to gambling to trigger that feeling.

Think of Dan Mahowny, the bank manager played by Philip Seymour Hoffman in the 2003 film Owning Mahowny. The character, based on the true story of Canadian banker Brian Molony, embezzles millions of dollars to support his gambling addiction. When a psychologist asks him to rate on a scale of one-to-100 the pleasure he feels while gambling, Mahowny answers, “100.” When the psychologist asks him to rate his best experience outside of gambling, Mahowny pauses, then responds, "Twenty."

But, as with hard drug use, the high of gambling wears off after a while, and pathological gamblers often need higher and higher stakes to experience the same rush. "It's a hedonic treadmill," says Read Montague, a professor of neuroscience at Baylor College of Medicine.

There's another way that gamblers resemble drug addicts: their lack of impulse control. Gamblers often refer to someone being "on tilt," a trance-like mania following a losing streak, during which the person can't stop anteing-up in a crazed attempt to recoup losses. The fallout from such uncontrollable sprees is often devastating. When Greg Hogan Jr. —  a 19-year-old student at Lehigh University, class president, son of a Baptist minister, and all-around "good boy" — lost more than $7500 in a year of obsessive online poker, he robbed a bank so he could pay off his accumulated debts. Although he knew, even while on tilt, that he had to stop, he couldn't gain control. As he told a New York Times reporter, "The side of me that said, 'Just one more hand,' was the side that always won."

That evil side, the red devil perched on Hogan's shoulder telling him that maybe this hand would be the one to win it all back, can be traced to the prefrontal cortex, the part of the brain responsible for impulse control. Neuro-imaging has also revealed that pathological gamblers have decreased activity in the prefrontal cortex — as do drug addicts.

For casual players like Knauf, though, the devil is easy to brush off. "I think poker's more of a temporary rush, and once in a while you need it," Knauf says. On most nights, though, he's happy to trade one vice for another, and just go out drinking with friends. When it's you, your buddies, and a round of Jack Daniel’s, everybody wins.

Samantha Henig is a self-described flop who lives by the river. She can be reached at samantha.henig@gmail.com.