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Flavor and the Brain

The sensation of flavor has until recently been one of nature’s more arcane secrets. But in recent years progress has been made in parsing out its complex and often counterintuitive nature, writes Gordon M. Shepherd in Neurogastronomy: How the Brain Creates Flavor and Why It Matters.

The book, just out from Columbia University Press, is Shepherd’s accessible explanation of the studies that he and many colleagues have made to understand the perception of flavor.

Shepherd, a physician and a professor of neurobiology at Yale University School of Medicine, has been a pioneer in the scientific study of smell. The volume he edited in 1974, The Synaptic Organization of the Brain, is a standard reference work in the field. It is in its fifth edition from Oxford University Press. In the 1960s, Shepherd’s studies of the olfactory bulb, a structure in the forebrain of vertebrates that receives neural signals from cells in the nasal cavity, were among the first to tease out the properties and nature of brain micro-circuitry. He was also at the forefront of designing computer models of those complex phenomena, particularly as they related to the faculty of smell.

The “brain flavor system,” to use his term, is revealing surprises relating to fundamental aspects of the combined perception of taste and smell. “Most people are unaware,” Shepherd writes, that flavors “are due mostly to the sense of smell and that they arise largely from smells we detect when we are breathing out with food in our mouths.” Simple tests make apparent the importance of this “retronasal” component—try eating with your nasal cavity blocked off, or recognizing flavors introduced at the back of your mouth.

Taste—the tongue’s registering of salty, sweet, and so forth—is a simple-enough process; but much more goes into perceptions of flavor. In normal eating, chewing releases smell molecules from food and those are carried in air to the nasal chambers to stimulate smell receptors. Those send signals to the brain, which conceptualizes smells as spatial patterns. The brain combines those with perceptions of texture and other qualities of food and the action of eating it to construct perceptions of flavor.

As all that is happening, writes Shepherd, “astonishingly, the sense of flavor produced is a mirage; it appears to come from the mouth, where the food is located, but the smell part, of course, arises from the smell pathway. No wonder it has taken so long to realize what an amazing sense retronasal smell is.”

The scientist describes research performed since the 1980s that has shown that odors set up patterns of activity in the brain—“smell images”—that are the basis for perception of flavor and that are like the images that form the basis for other sensations such as touch and sight. He writes: “These smell images are hidden factors that determine most of the pleasure we get from eating, and they share the blame for the problems we incur when eating foods that are not good for us.”

“Neurogastronomy” is a term Shepherd has coined in his efforts to bring together findings from the many fields that bear on the scientific, cultural, and behavioral dimensions of flavor. The role of taste and flavor is inscribed in human history, he writes: “Royal empires have been built, unexplored lands have been traversed, great religions and philosophies have been forever changed by the spice trade.”

Cultures sought a variety of flavorings because the human brain possesses great plasticity when it comes to smell—humans all taste the same tastes, but they develop highly divergent attachments to the innumerable flavors that build on those tastes.

Unfortunately, from plasticity can come many problems—war among spice-hungry nations is one, mass self-extermination by poor eating is another. Neuroscientists could play a greater role in combatting obesity. But even among his colleagues, obesity’s entanglement in “the human brain flavor system” remains largely ignored, the researcher said by phone, last week, from the annual meeting of the Society for Neuroscience, in Washington DC. “There is very little discussion of why kids—why we all—eat what we eat,” he said. “And that really comes down to understanding how the brain creates flavor.”

The hold-up, he said, is the lack of discussion among researchers in the many fields related to flavor perception: “The physiologists talk to the anatomists, and the anatomists talk to the pharmacologists. There’s some cross talk. But there’s a whole world out there of food physiologists who study what happens to the food as we chew it and swallow it, and that’s practically unknown to the people in neuroscience. In the same way, the insights from anthropology, about what makes us uniquely human, are almost unknown to neuroscience. And yet, the physiology of flavor is not only important for us now—cooking must have been critical for the evolution of humans.”

Taking stock of the full range of such studies, and their connections, “provides the opportunity to deal with obesity in a way that is similar to the way we deal with cravings for drugs,” he says. Research in several medical disciplines into the links among flavor, emotions, and memory, for example, “has converged onto the same parts of the brain that the craving for drugs has focused on, and that is really an important insight,” he says.

Shepherd laments the many contributors to poor modern diets—fast-food companies conspicuous among them. He says they commandeer taste vulnerabilities and make virtual addicts of consumers. Research has shown, he notes, that early exposure to flavors, including those that fast-food scientists tinker with to suit their companies’ ends, influences humans’ food-consumption patterns for life. In one sense, he says, he conceives of his book as a neurobiologist’s addendum to Eric Schlosser’s best-selling book, Fast Food Nation: The Dark Side of the All-American Meal (Houghton Mifflin, 2001).

Far from trying to scare his readers off flavor enjoyment, Shepherd preaches informed flavor management. He begins his book by relishing his and his wife’s own nutritious home-cooked meals, and he isn’t about to begrudge any Americans their upcoming Thanksgiving dinners. Of course, he views those from a scientist’s perspective: “You have a lot of different dishes, and since they’re different, your taste flavor system doesn’t get saturated with one.” But he grants the indulgence a traditional wise-eating dispensation: “You’re allowed one day of sin.”

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