His daughter had been working as a summer intern in his Baylor laboratory. To give her a taste of practical neuroscience at work, he wanted to frame a research question that a teenager "could wrap her head around."
What happens in the brain, Montague wondered, when people decide between Coca-Cola and Pepsi, two of the most popular — and most similar — soft drinks in the world?
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.
Coca-Cola, in the words of one industry analyst, is "advertising incarnate." The company was the first sponsor of the Olympic Games, gave its cola free to U.S. soldiers during World War II, and is credited with inventing the modern image of Santa Claus.
But against such a formidable competitor, Pepsi was able to transform itself from a bankrupt company in the 1930s into a $69-billion enterprise today, largely through marketing.
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."
The study is a first step, he said, in the effort to answer a more fundamental scientific question: "Why do we believe anything?"
The creation of belief is the essence of marketing.
Brain scanning has opened the possibility of new forms of manipulation, by charting ways for marketing savants to harness neural circuits of reward and desire more effectively.