But her middle brain won't listen. Man, it swoons, he looks great in those jeans, his black hair curls onto his forehead so adorably, and when he drags on a cigarette, he's so bad he's good.

His front brain is lecturing, too: She's flirting with every guy in the place, and she can drink even you under the table, it says. His mid-brain is unresponsive, distracted by her legs, her blouse and her come-hither stare.

"What could you be thinking?" their front brains demand.

Their middle brains, each on a quest for reward, pay no heed.

Alas, when it comes to choosing mates, smart neurons can make dumb choices. Sure, if the brain's owner is in her 40s and has been around the block a few times, she might grab her bag and scram. If the guy has reached seasoned middle age, he might think twice about that cleavage-baring temptress. Wisdom — at least a little — does come with experience.

But if the objects of desire are in their 20s, all bets are off. A lot will depend on the influence of Mom and Dad's marriage, the gossip and urgings of friends, and whether life experience has convinced these two brains that what they're looking at is attractive. She just might sidle over to Mr. Wrong and bat her eyes. And he could well give in to temptation.

And so the dance of attraction, infatuation and ultimately love begins.

It's a dance that holds many mysteries, to psychologists as well as to the willing participants. Science is just beginning to parse the inner workings of the brain in love, examining the blissful or ruinous fall from a medley of perspectives: neural systems, chemical messengers and the biology of reward.

The free fall of love's first rush can happen at any age, whether people are 20 or 70, says Elaine Hatfield, psychology professor at the University of Hawai'i and relationship researcher.

What differs is that the older people get, the more memories they harbor of joy and trust, rejection and disappointment. And as people learn from experience, the front brain, with its logic and reason, probably gets a greater say.

"When you are young, passion and hope are so strong that it's almost impossible to stop loving someone," Hatfield says. "After you've been kicked around by life, however, you start to have a dual response to handsome con men: 'Wow!' and 'Arrrrrrgh!' It takes not willpower but painful experience to make us wise."

Somehow, it all comes together, for better or for worse, the sum total of what's found in the mating dance of the ancient reptilian brain, the passion of the limbic brain and the logic of the neocortex.

Complex chemistry

It was only in 2000 that two London scientists selected 70 people, all in the early sizzle of love, and rolled them into the giant cylinder of a functional magnetic resonance imaging scanner, or fMRI. The images they got are thought to be science's first pictures of the brain in love.

The pictures were a revelation, and others have followed, showing that romantic love is a lot like addiction to alcohol or drugs. The brain is playing a trick, necessary for evolution, by associating something that just happened with pleasure and attributing the feeling to that magnificent specimen right before your eyes.

All animals mate: The most primitive system in the brain, one that even reptiles have, knows it needs to reproduce. Turtles do it but then lay their eggs in the sand and head back to sea, never seeing their mate again.

Human brains are considerably more complicated, with additional neural systems that seek romance, others that want comfort and companionship, and others that are just out for a roll in the hay.

Yet the chemistry between two people isn't just a matter of molecules careening around the brain, dictating feelings like some game of neuro-billiards. Attraction also involves personal history. "Our parents have an effect on us," says Helen Fisher, evolutionary anthropologist at Rutgers University who studies human attraction. "So does the school system, television, timing, mystery." Every book ever read, and every movie ever wept through, starts charting a course toward the chosen one.

Is it lust, romance or true love?

"Love," that one small word, stands for a hodgepodge of feelings and drives: lust, romance, passion, attachment, commitment and contentment. Studying this brew is made harder because the pathways aren't totally distinct. Lust and romance, for example, have some overlapping biology, even though they are not the same thing.

Similarly, the dance that leads, if we're lucky, to a stable commitment moves through several key steps.

First comes initial attraction, the spark. If someone's going to pick one person out of the billions of opposite-sex humans out there, it's this step that starts things rolling.

Next comes the wild, dizzying infatuation of romance — a unique magic between two people who can't stop thinking about each other. The brain uses its chemical arsenal to focus our attention on one person, forsaking all others.

"Everyone knows what that feels like. This is one of the great mysteries. It's the love potion No. 9, the click factor, interpersonal chemistry," says Gian Gonzaga, senior research scientist at eHarmony Labs.

The passion lasts for at least a few months, two to four years tops, says relationship researcher Arthur Aron, psychologist at the State University of New York at Stony Brook.

As it fades, something more stable takes over: the steady pair-bonding of what's called companionate love. It's a heartier variety, characterized by tenderness, affection and stability over the long haul. Far less is known about the brains of people celebrating their silver anniversaries or more, but researchers are beginning to recruit such couples to find out.

Figuring out lifelong attraction

When Kelly and Robert Iblings of Calabasas, Calif., had their first face-to-face meeting after a month of corresponding online, all signs of a spark were there. Kelly, 30, recalls thinking "Wow!" Robert, 33, thought Kelly was beautiful. "I love his height," Kelly says of Robert's 6-foot-4 frame. "And those eyes. He's quite handsome. I mean, look at him. He's cute. He's hot." "She's very cute," Robert says. "And I like the way she laughs." Their brains' signals were in sync, and it was good.

The forces of attraction are in many ways mysterious, but scientists know certain things. Studies have shown that women prefer men with symmetrical faces and that men like a certain waist-to-hip ratio in their mates. One study even found that women, when they sniffed men's T-shirts, were attracted to certain kinds of body odors.

That initial spark can flash and fade. Or it can become a flame and then a fire, a rush of exhilaration, yearning, hunger and sense of complete union that scientists know as passionate love.

Key to this state of seeing a person as a soul mate instead of a one-night stand is the limbic system, nestled deep within the brain between the neocortex (the region responsible for reason and intellect) and the reptilian brain (responsible for primitive instincts). Altered levels of dopamine, norepinephrine and serotonin — neurotransmitters also associated with arousal — wield their influence.

Addicted to love

But passionate love is something far stronger than that first sizzle of chemistry. "It's a drive to win life's greatest prize, the right mating partner," Fisher says. It is also, she adds, an addiction.

People in the early throes of passionate love, she says, can think of little else. They describe sleeplessness, loss of appetite, feelings of euphoria, and they're willing to take exceptional risks for the loved one.

Brain areas governing reward, craving, obsession, recklessness and habit all play their part in the trickery.

In an experiment published as a chapter in a 2006 book, "Evolutionary Cognitive Neuroscience," Fisher found 17 people who were in relationships for an average of seven months. She knew they were in love from their answers to what researchers call the Passionate Love Scale. They all said they'd feel deep despair if their lover left, and they yearned to know all there was to know about the loved one.

She put these lovesick, enraptured people in an fMRI to see what areas of their brains got active when they saw a photograph of their beloved ones.

"We found some remarkable things," she said. "We saw activity in the ventral tegmental area and other regions of the brain's reward system associated with motivation, elation and focused attention." It's the same part of the brain that presumably is active when a smoker reaches for a cigarette or when gamblers think they're going to win the lottery. No wonder it's as hard to say no to the feeling of romantic arousal as it would be to say no to a windfall in the millions. The brain has seen what it wants, and it's going to get it.

Lucy Brown, professor of neuroscience at the Albert Einstein College of Medicine, has also taken fMRI images of people in the early days of a new love. In a study reported in the July 2005, Journal of Neurophysiology, she too found key activity in the ventral tegmental area. "That's the area that's also active when a cocaine addict gets an IV injection of cocaine," Brown says. "It's not a craving. It's a high." You see someone, you click, and you're euphoric. And in response, your ventral tegmental area uses chemical messengers such as dopamine, serotonin and oxytocin to send signals racing to a part of the brain called the nucleus accumbens with the good news, telling it to start craving.

"The other person becomes a goal in your life," Brown says. He or she becomes a goal you might die without and would pack up and move across the country for. That one person begins to stand out as the one and only.

It's all in their heads

Biologically, the cravings and pleasure unleashed are as strong as any drug. Surely such a goal is worth taking risks for, and other alterations in the brain help ensure that the lovelorn will do just that. Certain regions, scientists have found, are being deactivated, such as within the amygdala, associated with fear. "That's why you can do such insane things when you're in love," Fisher says. "You would never otherwise dream of driving across the country in 13 hours, but for love, you would." Sooner or later, excited brain messages reach the caudate nucleus, a dopamine-rich area where unconscious habits and skills, such as the ability to ride a bike, are stored.

The attraction signal turns the love object into a habit, and then an obsession. According to a 1999 study in the journal Psychological Medicine, people newly in love have serotonin levels 40 percent lower than normal people do — just like people with obsessive-compulsive disorders.

Experiments in other mammals add to the human chemical findings. Female prairie voles, for example, develop a distinct preference for a specific male after mating, and the preference is associated with a 50 percent increase in dopamine in the nucleus accumbens.

But when the monogamous vole is injected with a dopamine antagonist, blocking the activity of the chemical, she'll readily dump her partner for another.

Chemistry does matter

Kelly and Robert Iblings, now married for nine months, are fascinated by all this talk of nucleus accumbens, addiction and primitive mating instincts. Sure, they admit, they found each other attractive. But they were also making use of their front brains' sharp thinking skills. They were remembering painful past lessons and looking for signs of compatibility.

They had each survived an earlier, failed engagement, and they knew what they were looking for this time around. They were listening to their front brains as they told them to look for compatibility, stability, shared values and commitment.

From their first e-mail exchanges through eHarmony, an Internet dating service, the Iblings each felt they had found a unique mate. She liked to travel. So did he. They both love books and learning, have similar religious beliefs and come from loving, intact families. She no sooner sent an e-mail telling him about an exhibit she saw on a business trip to New York than he sent a message back telling her he knew of the exhibit because he had bought a book on it the day before.

Coincidence, or soul mate?

The front brain certainly gets involved as it ponders all of life's experiences and past mistakes, researchers say — but not just the front brain. The nucleus accumbens, virtual swamp of dopamine that it is, is also holder of memories. Its quest for reward is influenced by childhood experiences, friends, previous failed engagements or the jerk who cheated on you. The sum of those experiences make some people attracted to a prince or a frog, a princess or a shrew.

And, as it happens, practical matters such as whether a couple both like pina coladas and getting caught in the rain do matter in igniting passionate love.

A research project headed by eHarmony Labs' Gonzaga interviewed 1,200 dating and newlywed couples. The results, reported in the July issue of The Journal of Personality and Social Psychology, found that those who reported similar interests and feelings were more satisfied. "Those who reported chemistry said they felt at ease, relaxed, connected. They knew they had some things in common," he says. "Chemistry is more than just being hot or handsome." Clearly, in the matters of love, the stars were aligned for the Iblings. When they met, they were ready for each other. But they were also attracted to each other. The chemistry was there. Most relationship researchers think it has to be.

They had what it took to kick-start the relationship with an undeniable urgency, allowing two people to give up the candy store of other choices and commit to each other.

Odds are that in two to four years, this urgency will fade — and the couple will, if all goes well, settle in for the long haul with companionate love.

It is the state that many desire, yet it is the least studied.