Deconstructing the anatomy of choice, researchers also are probing the pliable neural circuits of reasoning and problem-solving — the last of the brain's regions to evolve, the last to mature during childhood and the most susceptible to outside influences.

They have begun to obtain the first direct glimpses of how marketing can affect the structures of the brain.

Consider something as simple as a choice of soft drink.

At Baylor College of Medicine, neuroscientist Read Montague remembered telling his 17-year-old daughter: Let's give the brain the Pepsi Challenge.

Since 1999, consumers have been offered 545 new brands of carbonated beverages. Despite differences in taste, color, caffeine and fizz, they are all based on a single sensory theme: sugar and water.

What happens in the brain, Montague wondered, when people decide between Coca-Cola and Pepsi.

With funding from the Kane Family Foundation and the National Institute on Drug Abuse, they designed an experiment that became a test of the relative importance of the label on a cola can and the contents of the container.

In all, 67 people took the 47-minute test inside Baylor's fMRI machine.

Each swallowed sips of cola from a tube in a series of carefully calculated variations on the classic taste test. Each sip was preceded by a picture of a distinctively labeled red or blue cola can. Montague and his colleagues varied the order of the sodas, the labels and the timing of the sequence.

The volunteers had no preference when the drinks were offered unlabeled, the researchers discovered. But they overwhelmingly preferred Coke whenever that brand was displayed — no matter what cola was actually delivered through the sip tubes.

When the researchers analyzed the brain scans, they discovered that the Coke label appeared to activate a memory region called the hippocampus, along with structures in the midbrain known to compute the likelihood of rewards.

A brain region linked to the sense of self — the ventral putamen and the medial prefrontal cortex — also lighted up.

The Pepsi label prompted no such response.

"What is it about these two almost chemically identical drinks that causes such different behavior?" asked Baylor neuroscientist Damon Tomlin. "The answer, of course, is marketing."

Although Pepsi's marketing campaign has been successful, it apparently has not reached as deeply as Coke's.

Montague elaborated: "We can show that the idea of Coca-Cola activates structures in your midbrain that literally drive your behavior. That is how ideas gain control over instinct."