Gordon M. Shepherd - Neurogastronomy_ How the Brain Creates Flavor and Why It Matters (2011, Columbi

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Unformatted text preview: $24.95 smell and flavor and their relationship to the neural basis of consciousness. Everyone from casual diners and ardent foodies to wine critics, chefs, scholars, and researchers will delight in Shepherd’s fascinating, scientificgastronomic adventures. ø “NEUROGASTRONOMY is a path-breaking account of flavor from how we perceive it to how it affects society. Gordon M. Shepherd’s explanation of our food preferences is a tour of the intellectual senses and a model of brain science.” —RICHARD WRANGHAM, Harvard University, author of Catching Fire: How Cooking Made Us Human å “NEUROGASTRONOMY is a personal yet magisterial account of the new brain-based approach to flavor perception. Gordon M. Shepherd’s panoramic view of science, culture, and behavior is that of a true pioneer of the chemical senses.” —AVERY GILBERT, author of What the Nose Knows: The Science of Scent in Everyday Life æ Jacket design: Milenda Nan Ok Lee —HERVÉ THIS, author of Molecular Gastronomy COLUMBIA UNIVERSITY PRESS New York cup.columbia.edu PR I NTED I N TH E U .S .A. NEUROGASTRONOMY 7 How the Brain Creates Flavor and Why It Matters LEADING NEUROSCIENTIST Gordon M. Shepherd embarks on a paradigm-shifting trip through the “human brain flavor system,” laying the foundations for a new scientific field: neurogastronomy. Challenging the belief that the sense of smell diminished during human evolution, Shepherd argues that this sense, which constitutes the main component of flavor, is far more powerful and essential than previously believed. SHEPHERD     Columbia GORDON M. SHEPHERD is professor of neurobiology at the Yale School of Medicine and the former editor in chief of the Journal of Neuroscience. He has made fundamental contributions to the study of brain microcircuits, as summarized in his highly regarded edited reference work, The Synaptic Organization of the Brain. His current research focuses on olfaction at the level of microcircuits and how they construct the spatial patterns of smell, which are essential to the perception of flavor. “COOKING? It is first love, then art, then technique. Chefs and food lovers alike can benefit from a better appreciation of the phenomena at play throughout the culinary process, from the field to the fork and beyond. This is why flavor is so important and why Gordon M. Shepherd’s well-named Neurogastronomy is such a welcome addition to the literature.” 4 NEUROGASTRONOMY (CO NT INUED F RO M F RO NT F LA P) ¡F 9 G J © ç D H ƒ 6 Gordon M. Shepherd SHEPHERD begins Neurogastronomy with the mechanics of smell, particularly the way it stimulates the nose from the back of the mouth. As we eat, the brain conceptualizes smells as spatial patterns, and from these and the other senses it constructs the perception of flavor. Shepherd then considers the impact of the flavor system on contemporary social, behavioral, and medical issues. He analyzes flavor’s engagement with the brain regions that control emotion, food preferences, and cravings, and he devotes a section to food’s role in drug addiction and, building on Marcel Proust’s iconic tale of the madeleine, its ability to evoke deep memories. SHEPHERD connects his research to trends in nutrition, dieting, and obesity, especially the challenges that many face in eating healthily. He concludes with human perceptions of (CONTINUED ON BAC K F LAP) NEUROGASTRONOMY C O L U M B I A U N I V E RS I TY PR ESS NEW YOR K NEUROGASTRONOMY How the Brain Creates Flavor and Why It Matters Gordon M. Shepherd Columbia University Press Publishers Since 1893 New York Chichester, West Sussex cup.columbia.edu Copyright © 2012 Columbia University Press All rights reserved Library of Congress Cataloging-in-Publication Data Shepherd, Gordon M., 1933– Neurogastronomy : how the brain creates flavor and why it matters / Gordon M. Shepherd. p. ; cm. Includes bibliographical references and index. ISBN 978-0-231-15910-4 (cloth : alk. paper) ISBN 978-0-231-53031-6 (e-book) I. Title. [DNLM: 1. Olfactory Perception—physiology. 2. Brain—physiology. 3. Taste Perception—physiology. WV 301] LC-classification not assigned 612.8—dc23 2011029170 Columbia University Press books are printed on permanent and durable acid-free paper. This book is printed on paper with recycled content. Printed in the United States of America c 10 9 8 7 6 5 4 3 2 1 References to Internet Web sites (URLs) were accurate at the time of writing. Neither the author nor Columbia University Press is responsible for URLs that may have expired or changed since the manuscript was prepared. for Grethe Contents Preface ix Acknowledgments xi Introduction Retronasal Smell and the New Age of Flavor PART I Noses and Smells 1 The Revolution in Smell and Flavor 11 2 Dogs, Humans, and Retronasal Smell 19 3 How the Mouth Fools the Brain 28 4 The Molecules of Flavor 33 PART II Making Pictures of Smells 5 Smell Receptors for Smell Molecules 47 6 Forming a Sensory Image 59 7 Images of Smell: An “Aha” Moment 66 8 A Smell Is Like a Face 76 9 Pointillist Images of Smell 85 10 Enhancing the Image 92 11 Creating, Learning, and Remembering Smell 99 vii 1 CONTENTS PART III Creating Flavor 18 12 Smell and Flavor 109 13 Taste and Flavor 117 14 Mouth-Sense and Flavor 128 15 Seeing and Flavor 135 16 Hearing and Flavor 143 17 The Muscles of Flavor 147 Putting It Together: The Human Brain Flavor System 155 PART IV Why It Matters 19 Flavor and Emotions 165 Flavor and Memory: Reinterpreting Proust 174 21 Flavor and Obesity 184 Decisions and the Neuroeconomics of Flavor and Nutrition 192 23 Plasticity in the Human Brain Flavor System 200 24 Smell, Flavor, and Language 207 25 Smell, Flavor, and Consciousness 216 26 Smell and Flavor in Human Evolution 224 27 Why Flavor Matters 233 20 22 Bibliography 243 Index 257 viii Preface Eating is our most common behavior. We normally do it every day throughout our lives. Scientists over the past 20 years have elucidated how our urge to eat is largely controlled by hormones, that turn on when we are hungry and off when we are full. But this hormonal control doesn’t explain why we like certain foods and not others; why we may crave too much of what we like or too little of what we don’t like. To address these questions, a new science of eating is emerging that focuses on food flavors. A common misconception is that the foods contain the flavors. Foods do contain the flavor molecules, but the flavors of those molecules are actually created by our brains. If we are to eat healthfully and avoid the many chronic diseases that are affected by poor diet and nutrition, it is important that we learn how the brain creates the flavors that we experience—in short, we are embarking on a new scientific endeavor that I have called neurogastronomy. I have been led to this new field by working on how the brain creates images of smells. These findings and other studies from laboratories around the world are radically changing the common view of the sense of smell, from being one of the weakest of our senses, to being, through its role in flavor, one of the most important in our daily lives. This work is leading to a new concept of a unique human brain flavor system, perhaps the most extensive behavioral system in the brain, creating perceptions, emotions, memories, consciousness, language, and decisions, all centered on flavor. By combining brain studies with food studies, and ix PREFACE drawing on the wisdom about flavor exchanged within families every time they eat together, neurogastronomy holds the promise of putting healthy eating on a new scientific basis. In this book, I draw on these studies to explain this new field and show that it holds benefits for everyone. x Acknowledgments I have many to thank for helping to make this book happen. Jean Black at Yale gave me early encouragement to write a book on smell. After I had given up the project as being too difficult, my colleague Stuart Firestein revived both me and the manuscript and put me together with Patrick Fitzgerald at Columbia University Press; they both insisted that I could do it. I am grateful to the editors at Nature magazine for inviting me to write an “Insight” article on smell. Christian Margot persuaded me to keep the focus of that article on retronasal smell and flavor, and that focus led directly to this book. The theme of smell reflects my many colleagues who have worked with me over the years on how the smell pathway constructs activity patterns of smell molecules that are the basis of smell perception. In addition to being indebted to Stuart, I am grateful to Charles Greer for support at Yale, as well as former students Lewis Haberly, John Kauer, Tom Getchell, William Stewart, Kensaku Mori, Doron Lancet, Patricia Pedersen, Frank Zufall, Trese Leinders-Zufall, Wei Chen, Minghong Ma, Xavier Grosmaitre, and David Willhite, who feature in these pages, and visitors Dennis Lincoln, Burton Slotnick, and Matthias Laska. I am grateful to Terry Acree for inviting me to be on the faculty of a workshop on flavor, through which I met Harold McGee, who has become a wonderful friend and source of knowledge and inspiration. My wife, Grethe, and my daughter, Lisbeth, are Francophiles, and this has opened many doors. Beginning with a sabbatical in Paris in 1986, I xi ACKNOWLEDGMENTS have been privileged to interact many times with Jean-Didier Vincent, one of France’s leading scientists and philosophers in the worlds of wine and culture. He and several close friends gave Grethe and me an unforgettable evening of ortolan. Through Jean-Didier I took part in a national radio program in France on wine in 2000, another stimulus along the way to this book. Through these activities I met Jean-Claude Berrouet, chief wine taster of the house of Petrus, who provided me with an unforgettable personal tasting of 10 different Petrus wines of different ages, an opportunity to test the idea of images of wines. Pierre-Marie Lledo has been a gracious host on many visits to Paris to pursue common interests in experiments on olfaction in the laboratory and experiments on tasting wines outside. For my training as a neuroscientist and for laying the foundation for this book, I was fortunate to begin with studies at Oxford University under Charles Phillips and Thomas Powell. Tom was in the Department of Anatomy chaired by Wilfrid le Gros Clark, who was also a leading anthropologist. Through this connection I began my interest in the anthropology of the human sense of smell. In taking up this interest more recently, I was greatly aided by Richard Wrangham and Dan Lieberman at Harvard, starting with a seminar to the Department of Anthropology and continuing with extended discussions with Dan on the evolution of the human head, including the retronasal passageway. After Oxford, Wilfrid Rall at the National Institutes of Health has been an extraordinary friend and mentor, together with colleagues Thomas Reese and Milton Brightman. Later, Frank Sharp and Ed Evarts at the National Institutes of Health led our way to discovering the brain activity patterns representing smell molecules. My education in the rich world of flavor and the brain has benefited from many colleagues, especially Linda Bartoshuk, Valerie Duffy, Terry Acree, Christian Margot, Avery Gilbert, Bruce Halpern, Marcia Pelchat, Richard Doty, Anthony Sclafani, and Ed Rolls. I have had valuable advice in putting together this book from Valerie Duffy, Justus Verhagen, Dana Small, Daeyeol Lee, and Gary Beauchamp, who have read chapters and offered criticism in their areas of expertise. Harold McGee and Donald Wilson have provided valuable feedback on the entire manuscript. Any residual errors are mine. xii ACKNOWLEDGMENTS For expert guidance in seeing the manuscript through production, I’m first grateful to Wendolyn Hill for transforming my scientific slides into the simple and elegant illustrations for this book. Bridget FlanneryConnor has provided expert oversight, Pamela Nelson a sure hand in guiding the production, my editor Patrick Fitzgerald his always unwavering support from beginning to end, and Stuart Firestein with his always wise counsel. Grethe and my family have borne with me throughout, for which I can only say, “Tak! Mange tak!” The research on which this book is based has been carried out with support from the Chemical Senses Program of the National Institute for Deafness and Other Communication Disorders, under the National Institutes of Health in Bethesda, Maryland. My main grant has been funded continuously for 42 years. I have also received support from the National Science Foundation, the National Aeronautics and Space Administration, the National Institute on Aging, the National Institute for Drug Abuse, and the Naval Medical Research Laboratory. All of this work has been funded through periodic competitive reviews. I am deeply grateful to the study section committees for their evaluations, and the councils of the institutes for the funds. Our projects have been for animal studies of microcircuits in the smell pathway; the fact that they revealed insights that led to new ways to think about human flavor is one of the great rewards in science, of having the freedom to follow basic research wherever it may lead. xiii NEUROGASTRONOMY INTRODUCTION Retronasal Smell and the New Age of Flavor The origins of this book are the cooked meal around the family dinner table that my wife, Grethe, and I have shared at the end of each day beginning when we were students together. Her interests run to food (as a gourmet cook), books (as a reference librarian and an avid reader), flowers and gardens (at our homes in the United States and Denmark), travel, friends, opera, and keeping tabs on our growing family. My life has been in the laboratory, studying the part of the brain responsible for the sense of smell. Through the years—from England to Washington, D.C., Stockholm to New Haven, Philadelphia to Paris, our home in Connecticut to our summer house in Denmark— through student life and family life, that shared meal has been a constant bond. A sip of sherry or wine or fruit juice, with a nibble, to prepare the digestion for a traditional meal, with meat, vegetables and salad, a glass of wine, dessert or fruit, and tea (coffee is for the morning), that varies every day. Always with Grethe’s elegant touches drawn from her native Denmark and the places we’ve lived and visited, and from burgeoning piles of recipes clipped from newspapers and magazines from around the world. We treasure the time to enjoy the flavors of the food while reflecting on the events of the day. This daily routine would have gone on without reflection except that in 1986, National Geographic prepared its historic article “The Intimate Sense of Smell,” the first comprehensive public overview of this neglected sense. As an olfactory scientist, I was interviewed by the author. You may have seen photographs from this article, the most notorious being of a 1 INTRODUCTION row of young men stripped to the waist with upraised arms while a row of pleasant middle-aged women in white lab coats have their noses buried in the boys’ armpits, testing the effectiveness of the latest underarm deodorant. The article started by noting the common idea that smell seemed to have faded in importance for humans when our ancestors started walking upright, relying on the visual sense: “But to Gordon Shepherd, a Yale University neuroscientist, people vastly underrate their sense of smell. ‘We think our lives are dominated by our visual sense,’ he said, ‘but the closer you get to dinner, the more you realize how much your real pleasure in life is tied to smell. It taps into all our emotions. It sets the patterns of behavior, makes life pleasant and disgusting, as well as nutritious.’ ” When the author read back to me this quotation that he was planning to use, I protested that I must have phrased it more elegantly, but he insisted that it was what I had said so that is the way it stayed. Anyway, my main point was to oppose the common wisdom that the sense of smell has become weak and insignificant in humans (unless you are a perfumer) by remembering what the aromas and flavors of those daily dinnertimes had meant to me. Later, it began to dawn on me that I must be pretty dense to be working all my life on the physiology of the sense of smell without trying to figure out how it applied to enjoying my evening meal. It was time for me as a neuroscientist to think about how those smells from the food in my mouth were able to reach the sensory cells deep inside my nose, and how those smells were merged with other sensations to produce flavor. Searching for those answers sent me on an odyssey that has been fascinating every step of the way. It has introduced me to many investigators working in areas unknown to one another, to the mainstream of neuroscientists, and to the general public. I learned about food scientists, who study how food is chewed in the mouth and how it is swallowed. Physiologists study how the smells are carried to the sensory cells in the nose by our breathing in, as when inhaling the aroma of a cup of coffee, and by breathing out, as when we chew and swallow our food. Psychologists study how smell is combined with taste and the other senses to produce what is called taste but is really flavor, one of the most complex of human sensations. Cognitive neuroscientists use brain imaging to show how flavor arises from activity at the highest cognitive 2 RETRONASAL SMELL AND THE NEW AGE OF FLAVOR levels of our brains. Neuropharmacologists study how the regions of the brain that are activated by cravings for food involve some of the same regions that are activated by cravings for tobacco, alcohol, and drugs of abuse. Biochemists identify the hormones that circulate in the blood, connecting our bodies with our brains to signal to start eating when we are hungry and stop when we are full. Anthropologists speculate that cooked food was a major driving force in human evolution. Molecular biologists have discovered that the sensory receptors for smell form the largest gene family in the genome, and they are studying how the molecules give rise to our perceptions of different smells. And we are good smellers: behavioral psychologists find that monkeys as well as humans have much more sensitive senses of smell than previously recognized. All these investigators, most of them without knowing one another’s fields, are building the new science of flavor. In addition, food critics have been pushing and pulling us into a new age of food appreciation. They include Harold McGee, whose book On Food and Cooking: The Science and Lore of the Kitchen has educated thousands and entertained them with the ways in which foods give rise to flavors. An important step on this path was taken by Nicholas Kurti and Hervé This, who became fascinated with the “physical and chemical aspects of cooking” and began to hold workshops on their new passion. They helped a new field to emerge, defined in This’s book Molecular Gastronomy: Exploring the Science of Flavor as the science dealing “with culinary transformations and the sensory phenomena associated with eating.” Among all this work, several things stood out to me: 1. When we sense the flavor of the food in our mouths, it is not by sniffing in, which we usually associate with smelling something like an aroma, but by breathing out, when we send little puffs of smell from our food and drink out the back of our mouths and backward up through our nasal passages as we chew and swallow. This back door approach is called retronasal smell (retro = backward); we can also call it mouth-smell. It contrasts with orthonasal smell (ortho = forward), which is what we call the common sniffing-smell. 2. As delivered by the retronasal route, smell dominates flavor. We often characterize our food in terms of how it “tastes,” but the sense of taste as properly defined consists of sensitivity only to sweet...
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