Flavor: The Science of Our Most Neglected Sense - Bob Holmes 2017
This is your Brain on Wine
Among the countless restaurants that have come and gone on Upper Street in the trendy Islington district of London, the House of Wolf may have been the strangest. The nondescript, three-story gray brick building with a large mullioned window on the ground floor was once a Victorian music hall. Today, it has morphed into a nightclub called The Dolls House, but a few years earlier, it was what its owners called “a multi-functional, multi-sensory pleasure palace, dedicated to the creative pursuits of dining, drinking, art and entertainment.” The rest of us called it London’s most experimental restaurant. Its kitchen featured a predictably unpredictable parade of avant-garde guest chefs, each holding the stove for just a month or two before yielding to the next visitor.
If you had been lucky enough to stumble on the House of Wolf when it first opened in October 2012, you would have experienced one of the most peculiar meals of your life. As you enter the dining room, you’re greeted by the sight of bread rolls dangling by strings from helium balloons floating at the ceiling. The chef—artist Caroline Hobkinson—instructs you to put in earplugs, then eat your roll off the string without using your hands. As you nibble away—rather like bobbing for apples in midair—you hear the crunch of the crust, magnified by the plugs in your ears. “Can you hear the taste?” Hobkinson asks in the printed menu.
For the next course, you don a blindfold. Your waiter brings you a cracker topped with warm goat cheese redolent of rosemary and roasted red peppers. After your first bite, you remove the blindfold and see that the rosemary and pepper aren’t on the cracker at all—they had merely been wafted before your nose as you ate the cracker and unflavored cheese. “Can you see the taste?” Hobkinson’s menu asks.
No blindfolds or earplugs for the next course. Instead, your waiter sets before you a plate of salmon sashimi and a syringe filled with an amber liquid. Following instructions, you inject the fish with the liquid, which turns out to be ten-year-old Ardbeg, an intensely peaty Scotch whisky. Magically, the aroma of peat smoke from the whisky transforms the flavor from raw fish into smoked salmon. “Can you smell the taste?” the menu asks.
After a palate cleanser—a gin-infused cucumber ice, which you eat alternately with two spoons, one coated with salt crystals and the other with rose water crystals, giving two odd textures to your tongue—comes the main course, a straightforward, classic loin of venison with mushrooms, prunes, and wild cherries. Ah, you think, something normal at last. Well, not quite. Instead of a fork, the waiter brings you a tree branch as long as your arm, with the thick end carved into a forked prong. Like a Stone-Age hunter, you spear the meat with the branch and bring it to your mouth. “Can you feel the taste?” Hobkinson asks.
The final course, dessert, is a “sonic cake pop,” a spherical chocolate brownie on a lollipop stick. It is served with an unusual garnish: a telephone number. You pull out your cell phone, dial the number, and hear instructions to press “1” for bitter or “2” for sweet. Depending on your choice, you’ll hear a low rumble or a high whine—and the sound makes the dessert taste either bitter or sweet. “Can you dial a taste?” Hobkinson asks.
It all sounds a bit over the top—more performance art than meal. And at one level, of course, it is. But as with most art, there’s a deeper message here, and Hobkinson is doing much more than just playing with our preconceptions about the dining experience. Her eccentric banquet also draws on a lot of solid science as it stretches our concept of flavor to include sight, sound, touch, and even thought. In fact, perceptual scientists can make a strong argument that a food’s flavor isn’t really contained in the food at all. Instead, you construct flavor in your mind from the whole range of senses you experience with each bite—and each of the courses in Hobkinson’s meal is carefully designed to illustrate some part of that creative process.
Hobkinson’s behind-the-scenes collaborator in all this is Charles Spence, a psychologist at Oxford University. A well-built man with a receding head of wispy hair, a deeply cleft chin, and a slightly protruding lower lip, Spence has the enthusiastic, self-satisfied air of someone who loves his work. And why wouldn’t he? As one of the world’s leading experts on what he calls “multisensory perception,” Spence is forever playing with his food to better understand why things taste the way they do and how chefs, industrial food companies, and ordinary home cooks can heighten the flavor of the food they prepare. Along the way, Spence has collaborated with some of the best chefs in the world, including England’s Heston Blumenthal and Spain’s Ferran Adrià. Spence is one of the few scientists with the clout to command a VIP table at almost any high-end restaurant in the world.
Like many star scientists, Spence took a backdoor route into the research that made him famous. He was always interested in multisensory perception, but at first he focused on the better-studied senses of sight, sound, and touch. Back then, in the 1990s, hardly anyone worked on such “minor” senses as taste and smell. “It seems very bizarre, but most psychologists have only been interested in the so-called higher senses,” he says. “There isn’t much to read about food and flavor.” But early on, he landed a few grants from food companies such as Unilever to apply the multisensory approach to flavor. Soon he was hooked, for both personal and professional reasons. “Food and drink are among life’s most enjoyable activities, and they are the most multisensory, as well. It’s an obvious place for a psychologist to end up,” he says.
That makes researchers’ lack of interest rather surprising—especially since, if you think about it in the right way, we all know that multiple senses must contribute to flavor. Imagine, for example, the wonderful taste of ripe strawberries slathered with vanilla-laced whipped cream. Easy, right? But now remember that all you really taste is sweet, plus maybe a little sour from the berries. All the rest is smell, experienced in the nose—yet it seems for all the world to be a taste, experienced in the mouth. Even worse, we often say the berries smell sweet, even though sweet is the one part of the flavor experience that we’re not actually smelling. We’ve grown so used to combining smell and taste into a single flavor, though, that we commonly confuse them. “Maybe it’s so common, in fact, that people never think about why strawberries smell sweet,” says Spence. This kind of sensory magic also explains how the mere scent of peppers or rosemary lent their flavor to the plain goat cheese in Hobkinson’s dinner—as long as the guests’ dominating visual sense wasn’t present to spoil the illusion.
Scientists, of course, are rarely content with this sort of vague hand waving, so Spence—along with several researchers in other labs—brought this sensory cross-wiring into the lab for dissection. More than a decade ago, for example, Richard Stevenson at the University of Sydney, Australia, had volunteers rate the sweetness of pure sucrose, an odorless sugar, both alone and in the presence of a caramel odor, which the researchers verified had no sweet taste on its own. Sure enough, the sugar tasted sweeter when people also smelled the caramel.
A whole host of similar studies show that the effect is widespread: Odors such as vanilla and strawberry also make sugar taste sweeter. Strawberry aroma enhances the sweetness of whipped cream, while peanut butter aroma doesn’t. Chewing gum “tastes” less minty—really a smell and a mouthfeel, not a taste—as it loses its sweetness, and the mintiness returns when the researchers slip in a second dose of sugar.
Sometimes, these experiments point to another noteworthy fact: Smells and tastes often go together differently for different cultures. For example, caramel odor doesn’t enhance sweet tastes for many Asian people, who are likely more used to encountering caramel in savory dishes instead of the sweets that Westerners are used to. The same thing happens with benzaldehyde, the main component of almond aroma. It enhances sweet tastes in Westerners, who usually encounter almond in pastries. But for Japanese, benzaldehyde enhances umami taste, because almond is a common ingredient in savory pickles.
In fact, researchers have found that they can mess with people’s smell/taste perceptions almost at will. Several years ago, John Prescott of the University of Otago, New Zealand, and his colleagues got their hands on some obscure odors that people would have no prior associations with. Then they presented those odors to volunteers together with either a sweet or a sour taste. After familiarizing the volunteers with the combination a few times, they tested the smells and tastes separately. Sure enough, sweet tastes seemed sweeter—and sour ones sourer—when the volunteers smelled whichever odor they had learned to associate with that taste. In short, we learn how to put smells and tastes together to create flavor.
It doesn’t take a great leap of faith to accept that smell plays a big part in our perception of flavor. As Spence likes to point out, that notion has even penetrated that bastion of bureaucratic bean counting known as the International Organization for Standardization. As its name suggests, this is the agency that sets definitions and industrial standards for everything from telephone dialing codes (ISO 3166) to energy-efficient buildings (ISO 16818). If you like that sort of thing, and have the patience to wade through volume after volume of technical specifications, you’ll eventually come across ISO 5492, which defines flavor as “a complex combination of the olfactory, gustatory and trigeminal sensations perceived during tasting.” In lay terms, smell + taste + mouthfeel = flavor.
But that simple equation leaves out some crucial dimensions of flavor—dimensions that Hobkinson draws on in her multisensory feast, and that Spence has built a career around. A decade ago, for example, Spence did some of the pioneering work to show that our sense of hearing also contributes to flavor. In short, a steak’s sizzle is part of its flavor.
Spence didn’t actually use steaks in his experiments—they’re expensive, and it’s difficult to standardize “sizzle” in the laboratory. Instead, he turned to a foodstuff that could have been designed expressly with experimental psychologists in mind: Pringles potato chips. Instead of being sliced from individual, idiosyncratically flawed potatoes, Pringles are formed from a uniform slurry of pulped starch (rice, wheat, corn—and, yes, potato), so every chip in every can is identical to the next one—a perfect, standardized experimental replicate.
Spence and his associate Max Zampini asked twenty volunteers to munch their way through 180 Pringles each, rating the flavor of each chip, while wearing audio headphones that played back the sound of their crunching. As the volunteers soldiered on, a computer modified the playback sounds to make them quieter or louder, and to emphasize certain audio frequencies. The crunch, Spence found, was a key part of the chips’ flavor. When volunteers heard a louder crunch, or even just a louder high-frequency part of the crunch, they rated the chips about 15 percent crunchier and fresher tasting than when they heard quieter sounds. The finding was surprising—and amusing—enough to net Spence and Zampini an Ig Nobel Prize, a tongue-in-cheek research award for research that “first makes you laugh, then makes you think.” It’s a laurel that Spence continues to wear proudly, mentioning it often in his later scientific papers, and even listing it among his “Academic Distinctions” right at the top of his résumé.
The same principle holds for other foods that feature distinctive sounds. Recently, for example, one group of researchers had volunteers rate the flavor of several coffees as they listened to sounds of a coffee maker in the background. Unknown to the tasters, every cup of coffee was actually identical—yet they rated the coffee 10 percent tastier when they heard sounds of a more “expensive” coffee maker (actually the same recording with annoying high frequencies muted).
Probably the best known of these experiments, in the foodie world, at least, was a test Spence conducted with oysters. He asked eaters to rate the flavor of the oysters while listening through headphones to one of two soundtracks: either sea sounds such as crashing waves and shrieking seagulls, or barnyard sounds such as clucking chickens and mooing cattle. By now you won’t be surprised to learn that the eaters found the oysters tastier—and many people also found them saltier—when accompanied by the sea sounds.
This experiment has found its way straight onto the plate at what many people consider the finest restaurant in the world: The Fat Duck, in the little village of Bray, England, not far west of London. You’ll find it just a few miles past Heathrow Airport, near the manicured grounds of Windsor Castle, where the queen likes to spend her weekends, and the famous playing fields of Eton, home to centuries of upper-class schoolboys.
It’s not easy to get a table at The Fat Duck. Unless you can talk your way into a VIP table, you’ll need to call at the stroke of noon, UK time, on the first Wednesday of the third month before your intended date. If you’re lucky enough to score a reservation, expect to pay £255 (approximately $337, at the time of this writing) each, not including wine or tip. It’s a small fortune, but you’ll also spend four and a half hours savoring one of the most extraordinary meals of your life. Your menu might include an egg-white puff flavored with gin and tonic and frozen in liquid nitrogen, a quail jelly served alongside a bed of moss that emits forest-scented smoke as you eat, and something rather unpromisingly called “snail porridge.”
But perhaps the most famous of chef Heston Blumenthal’s creations is a dish he calls “Sound of the Sea.” Your server sets before you a conch shell with a set of earbuds emerging from its opening. You put the buds in your ears and hear a soundtrack of crashing surf and calling gulls. Soon an edible seaside diorama arrives, with raw fish, seaweed, a seawater foam, and “sand” made from ground-up fish, seaweed, breadcrumbs, and other binders. Just as Spence found with his oyster experiment, the seaside sounds you hear as you eat are not just background frills but an integral part of the flavor experience. You’re tasting not just with your mouth but with your ears as well.
Even abstract sounds can affect the flavors we perceive, as shown by Hobkinson’s “sonic pop” dessert, where low-pitched sounds bring out the chocolate’s bitter notes and high-pitched ones accentuate its sweetness, for reasons Spence cannot yet explain. Words, too, have “flavors,” he’s found—people associate spiky-sounding words like kiki with bitter flavors and rounder-sounding words like bouba with sweet ones. Other researchers have shown that people expect a mythical ice cream called “Frosh” to taste richer and creamier than one named “Frish.”
If squeals, rumbles, and words can alter flavor, the logical next step would be to ask whether music can, too—and the answer appears to be that it does. “Sound is the last sense that people think about when it comes to flavor,” says Spence, “but there’s a huge explosion of work showing that people match flavors to classes of instruments or pieces of music.” Heavy, powerful music such as Carl Orff’s “Carmina Burana” makes tasters notice the heavy flavors in red wine, for example, while “zingy” pop music such as Nouvelle Vague’s “I Just Can’t Get Enough” brings out the brighter flavors in white wine, he notes. A few food writers are already working on cookbooks that pair food and music in “musical recipes”—and Spence himself says he now gives a lot more thought to his choice of background music for dinner parties.
He’s also paying more attention to the crockery he serves his meals on, thanks to some other research he’s done. As usual, this involved some trickery. In this case, Spence’s colleague Betina Piqueras-Fiszman asked fifty volunteers to evaluate three different yogurts, presented one at a time in apparently identical bowls. By now you probably see the trick coming: In reality, the three yogurts were all the same, but Piqueras-Fiszman had invisibly weighted some of the bowls to be heavier than others. Sure enough, the raters judged the yogurt in the heavier bowls as being both richer and more pleasurable than the identical yogurt served in a lightweight bowl.
Even the color of the crockery can make a difference to flavor, Spence finds. In one test, for example, tasters rated a strawberry mousse as being sweeter when served on a white plate than on a black one. Most likely, he thinks, that’s just because the white plate shows off the bright red strawberry color more dramatically, and this ripe-fruit color triggers us to expect sweetness. It’s a simple effect, but hard to escape. As a result, says Spence, “I guess the black plates we used to serve on we no longer use.”
Hobkinson was aiming for something similar in her dinner. The look and feel of forks carved from tree branches would subconsciously evoke associations with wildness, she hoped, thus enhancing the flavor of the venison. In effect, it’s a kind of visual and tactile rhyme intended to emphasize a flavor message, just as a poet’s rhyme emphasizes a verbal message.
That kind of visual rhyme turns up over and over in the world of flavor, and it usually works by modifying our expectations. One study, for example, showed that a food’s color can profoundly affect our perception of the food’s sweetness, but not its saltiness. Presumably that’s because in the natural world, color signals whether a fruit is ripe and sweet or underripe and sour, but we have no similar color clues to saltiness.
One experiment performed more than a decade ago—and now notorious among wine aficionados—showed just how powerfully our visually produced expectations can affect flavor, even for highly trained tasters. In this case, the victims were a set of budding wine professionals, fifty-four undergraduate students in the highly regarded enology program at the University of Bordeaux, France. One day, the students were given three glasses of wine—two red, one white—and asked to describe each wine’s aroma. For enology students, of course, this is a routine task, and the students set about it with their usual thoroughness, discerning familiar aromas such as raspberry, clove, and pepper in the two reds and honey, lemon, and lychee in the white.
Gotcha! What the students didn’t know is that there were, in fact, only two wines in the test, one red and one white. The third glass, the other “red,” contained the same white wine, but researcher Gil Morrot and his colleagues had tinted it red with odorless food coloring. Simply changing the wine’s color had totally altered the students’ experience of the flavor. And remember, these were not naive, beer-swilling philistines, but people who were training for careers in the wine industry. (Their training might even have made them more prone to fall for the trick, because as experienced wine drinkers, they would have had stronger expectations linking color to certain flavors.)
So far, we’ve talked as though these multisensory effects on flavor act by altering our expectations, and there’s no doubt that accounts for part of the effect. “When I smell a certain smell like strawberry, there’s an expectation that what’s coming next is going to be sweet,” says Spence. Similarly, we expect “red” wine to smell like red wine, yogurt in a heavier bowl to be richer and more satisfying, red foods to taste sweeter than green ones. And what we expect to find, we do.
In this sense, everything about a meal’s context—the paintings on the walls, the lighting, the tablecloths, and more—helps to create an expectation of the meal to come, and this expectation probably biases our perception of its flavor. To illustrate this, Spence and his colleagues recently held a public event in London’s trendy Soho district where participants compared the experience of sipping a single Scotch whisky (The Singleton, for you Scotch geeks out there) in three different rooms. In the Nose Room, a green-lit space filled with leafy plants and the fragrance of cut grass, participants found that the whisky had a more pronounced grassy flavor. In the Taste Room, with red lights, rounded furnishings, and fruity aromas, the Scotch tasted sweeter. And in the Finish Room, a dimly lit, wood-paneled chamber redolent of cedar, its woody aftertaste became more prominent. All this, despite every participant knowing it was exactly the same whisky each time, because the cup never left their hand. Even circumstances that have nothing to do with food can bias our flavor perceptions. One study, for example, found that fans attending ice hockey games at Cornell University thought ice cream tasted sweeter after the home team won, and sourer after it lost!
This “everything contributes to flavor” attitude is not a new one. The Italian Futurist movement of the 1930s famously took the notion and ran with it to bizarre extremes. Diners at the movement’s flagship restaurant, La Taverna del Santo Palato (the Holy Palate Tavern) in Turin, dined on olives and fennel hearts with their right hands (sans cutlery) while stroking sandpaper and velvet with their left—all while the headwaiter doused them with perfume. Another course featured a sea of raw egg yolks surrounding a meringue island and airplane-shaped slices of truffle. I’m not entirely sure what expectations the futurist chef was trying to create, but suffice it to say that it didn’t revolutionize Italian cuisine for very long.
But these multisensory influences on flavor aren’t just a question of expectations. Even the faintest whiff of strawberry aroma—so faint it can’t be consciously detected—is enough to boost our perception of sweetness, several studies have found. If you can’t consciously smell the strawberry, you can’t consciously expect a sweeter taste. Instead, Spence thinks, what’s happening is something he calls “sensory integration.” At first pass, that sounds not much different, but as Spence points out, expectations have a cause-and-effect timing pattern, in which you first smell the strawberry and then expect to taste the sweetness. In sensory integration, on the other hand, the two sensations arrive at the same time and reinforce each other.
You’ve experienced this kind of integration if you’ve ever watched someone’s lips to help you hear what they were saying at a noisy cocktail party. Even when neither hearing nor lip-reading alone would be enough to understand the conversation, you can do just fine with both together. The simultaneity of sight and sound are crucial to doing this. “If you had the lip movements presented a half second ahead of the voice, the effect would disappear,” Spence says.
This integration helps in understanding one of the great mysteries of flavor science, which you can experience for yourself right now. Take a bite or a sip of something flavorful—a rich stew, a ripe peach, a full-bodied wine—and take a moment to really savor it. Now, quickly, point to where the flavor is. Unless you’re very unusual, you pointed to your mouth, but you know from chapter 3 that most flavor comes from your sense of smell, which is in the nose. The illusion is so strong that even knowing this reality, as you now do, doesn’t change what you experience. So why does the flavor seem to happen in your mouth?
Sensory neuroscientists actually spend a fair bit of time obsessing over questions like this, and over the years they’ve arrived at a plausible answer. One of the brain’s important jobs is to edit the raw stream of sensations, choose the relevant ones, and package them into concepts that we can think about. The timing of events is a valuable clue to that packaging: If two sensations occur together, they probably belong together. Ventriloquists exploit this tendency in their performances, by carefully timing their dummy’s lip movements to match the sounds of their speech. If the match is good enough, the audience’s minds will bind sight and sound together, leaving the strong illusion that the sound is coming from the dummy, not the human.
The same thing happens when you eat a mouthful of food. The bite delivers a whole suite of sensations—tastes, smells, texture, temperature, perhaps some crunch—all at once. The mind packages them together into a single experience, and assigns that experience to the mouth, where the most prominent physical stimuli occur. No one notices that some of the sensations—notably the food’s aroma and the sound of its crunch—actually come from somewhere else.
With their penchant for taking things apart to see how they work in detail, neuroscientists have explored this further, of course. One particularly squirm-inducing experiment involved threading volunteers with a series of plastic tubes so that researchers could puff smells either through the nostrils or up the back of the throat, at the same time that they squirted scentless, odorless water into the mouth. The subjects identified the front-of-nose scents as smells coming from the outside world, but called the back-of-nose smells “tastes,” and perceived them in the mouth.
We’ve just seen how the brain binds together sensations that come in at the same time, treating them as a single, unified flavor that can be greater than the sum of its parts. But as it turns out, not every set of simultaneous sensations gets bound into a unified perception. “In order to combine as a flavor, they need to be viewed as similar, as things that go together,” says Johan Lundström, a sensory researcher at the Monell Chemical Senses Center.
As an example, Lundström, who’s Swedish, points to an unpleasant experience common in kitchens back home. In Sweden, as in much of Europe, milk is often sold in cardboard cartons that cannot be resealed tightly after they’re opened. When he happens to store an opened carton in the fridge with a leftover half of an onion, the milk picks up oniony odors—and that sensory dissonance makes it impossible for him to drink the rest, even when he knows it’s fresh. “You cannot make yourself drink the milk,” he says. “Your system is screaming at you that there’s something wrong here.”
That is healthy caution. One of the reasons we smell and taste in the first place is to make sure we don’t eat something we shouldn’t, so most people react with dislike to new or peculiar flavors, especially if they come as a surprise. (Adventurous foodies who know what they’re getting into have other coping mechanisms that can override this aversion, says Lundström.)
This “go togetherness” turns out to be an important part of sensory integration. Lundström recounts an as-yet unpublished experiment that some of his colleagues at Monell did to test whether the brain is better at integrating flavors that go together—and whether we can teach our brains new flavor combinations.
Paul Breslin, Pam Dalton, and their collaborators made up special chewing gum that allowed them to give tasters tiny, measured doses of an aroma (rose scent) and either a bitter or a sweet taste. First, they figured out the minimum dose that people could detect of each scent or taste on its own, then dialed back the amount even more so that they ended up with what should have been flavorless gum. Then they paired taste and aroma in either familiar (rose—sweet) or unfamiliar (rose—bitter) combination gums. Sure enough, they found that people could taste the flavor in the gum with the familiar rose—sweet combination, indicating that they could integrate these two components, just like lip-reading at a noisy party. The people still couldn’t taste anything in the rose—bitter gum, though, which shows their brains didn’t know how to combine these discordant flavor stimuli into one integrated perception.
But then Breslin and Dalton took their experiment one step further: They made a new rose—bitter gum, but this time with enough of each flavoring that chewers could taste it. Volunteers chewed this peculiar gum daily for a month, then returned to the lab to see if their experience had changed their ability to taste the original, low-dose gum. And indeed it had—proof that after a month of practice, their brains had learned to integrate rose and bitter just as they had once automatically combined rose and sweet. “You can definitely train the system that these go together in a relatively short time,” says Lundström.
All this evidence—from the gimmickry of Hobkinson’s strange dinner to Lundström’s oniony milk to Spence’s Pringles and barnyard oysters—suggests that flavor is not what we usually think it is. Gordon Shepherd puts it best: “A common misconception is that the foods contain the flavors,” he says. “Foods do contain the flavor molecules, but the flavors of those molecules are actually created by our brains.”
The notion that flavor lives in the mind, not the food—or even the mouth or nose—is startling enough. But it looks like we can even go a step further: We construct our notion of flavor almost from scratch, building it from the ground up as we experience the world. Sure, a few preferences do appear to be hardwired, such as a liking for sweet tastes and an avoidance of bitter ones. But even those can be overridden by experience, as anyone who drinks gin and tonics can attest. And once we get to more complex flavors, it’s clear that most of our perceptions and preferences are based on experience. To really understand how our brains create our experience of flavor, we need to dig into the details of what happens in the brain as we taste. First, a little background.
Psychologists generally think of the brain much like a layer cake. The bottom layer is made up of raw sensations—tastes, smells, touch, and so on. On top of that is a layer of synthetic perceptions, where raw sensations are assembled into objects: a series of shapes, colors, and shadings become a face, for example. Crowning the cake are one or more “cognitive” layers—exactly how many is a matter of debate—where higher-order thought takes place. Here, for example, we attach a name to the face and develop expectations of how that person will behave, how important they are to us, and so forth. For flavors, these cognitive layers are responsible for identifying and naming flavors, deciding whether they’re good or bad, and choosing whether to eat something or not.
In this standard picture, all the information flows upward, with lower levels serving as data for higher processes. If there’s no reverse flow, we’d expect the lower levels, the sensations and perceptions, to be “clean”—that is, driven purely by the sensory inputs themselves, and unaffected by any preexisting cognitive or emotional baggage. But we’ve already seen that’s not exactly true, since experience can modify the way we bind sensations together. So what’s going on here?
That’s the question Edmund Rolls, a neuroscientist then at Oxford University, set out to answer. Rolls is one of the grand old men of sensory neuroscience, and many research paths in the subject led through his lab at one time or another. Rolls got to thinking about the particularly pungent spoiled-milk product that we know as cheese. Most people from Western nations like the stuff, while many Asians find it disgusting. (The tables are turned, of course, when it comes to Asian delicacies like aged duck eggs or the slimy, rotten-soybean preparation the Japanese call natto.) We know that cultural experience affects our liking for these foods. But, Rolls wondered, could these cognitive-level concepts reach back down and modify the raw perceptions, too?
To find out, Rolls and his student Ivan de Araujo devised yet another bit of psychology lab trickery. They prepared a synthetic “cheese flavor” and gave it to volunteers to smell. Half the volunteers read a label describing the odor as “Cheddar cheese,” while the other half saw it labeled as “body odor.” If you’ve been following along to this point, you won’t be surprised to learn that the first group liked the smell better than the second group.
But then Rolls and de Araujo dug one step deeper, using brain scans to peer into the subjects’ brains. There, they did find a surprise: the two groups’ brains lit up differently all the way down to the second layer of the cake, the regions responsible for basic perceptions, even though nothing had changed except the words that described the odor. In other words, higher-level thought processes—and it’s hard to find a level much higher than language—can change not just how we think about flavor perceptions, they can change the perceptions themselves. Thought itself, in other words, is one of our flavor senses. The brain constructs flavor by piecing together inputs from virtually every one of our sensory channels, plus inputs from thought, language, and a host of other high-level processes like mood, emotion, and expectation. That makes flavor a remarkably complex and changeable concept. It’s a wonder we can talk about it coherently at all.
Actually, maybe we can’t. Perhaps our flavor perceptions are so individual, so idiosyncratic, so circumstance dependent that we’re fooling ourselves when we think we’re saying anything objective about flavor. That’s certainly the impression you get when you look more closely at wine. Wine should be a perfect test bed for exploring the reliability of our flavor perceptions. No other foodstuff is so thoroughly, obsessively described and quantified. Detailed tasting notes are available—usually from not just one, but several trained, professional tasters—for almost any wine available commercially. Not only that, but those tasters often assign numeric scores to every wine, too, allowing for numeric comparisons of quality. Wine, if you have the right mind-set, is where the world of food meets Big Data.
Bob Hodgson has the right mind-set. An oceanographer by training (now retired), he’s also owned a winery in northern California for forty years. Like any other professional winemaker, he enters his wines in competitions such as the California State Fair, where trained judges taste their way through hundreds of wines and hand out coveted gold medals to the best—medals that can make or break a wine’s salability on store shelves. Sometimes Hodgson’s wines won gold medals, sometimes they didn’t. But unlike most winemakers, he didn’t just shrug at the injustice and carry on. With his scientific turn of mind, Hodgson started to wonder why the very same wine could garner a high score last week and a low one this week. Could you really trust the judges’ scores, he wondered? Hodgson must be a persuasive guy, because somehow, he managed to convince the California State Fair to let him find out.
Judges at a big competition like the California State Fair taste about 150 wines every day, organized into 4 to 6 “flights” of 30 wines each. The wines within a flight are presented in identical glasses marked with identifying codes, so that no judge knows the identity of any wine he or she is tasting. Each judge individually—no discussion at this stage of the judging—gives each wine a numeric score on a 20-point scale. (Actually, the fair uses a 100-point scale like the ones you sometimes see on the shelves at your local wine shop. But any wine that’s halfway drinkable scores at least 80 points, so for all practical purposes it’s a 20-point scale.)
With the collaboration of the contest organizers—but unknown to the judges—Hodgson arranged that for one flight per day (usually the second), three of the thirty wines would actually be identical samples, poured from a single bottle of wine but given different code numbers. If judges’ scores are a true reflection of a wine’s quality, then you’d expect these triplicate samples ought to receive identical scores—or at least somewhat similar scores, allowing for a little bit of imprecision in the judges’ ratings.
The results were shocking. “We did everything we could to make the task easy for the judge: same flight, same bottle. And nobody rated them all the same,” says Hodgson. Only about 10 percent of the judges scored the three samples similarly enough that they awarded the same medal to each. Another 10 percent gave wildly different scores, giving one glass a gold and another a bronze or even no medal at all, and the rest fell somewhere in between. And that wasn’t just because some judges are better than others: judges who were consistent in one year were no more likely to be consistent the next year.
Hodgson wasn’t done. Next, he compared the results of wines that had been entered not just at the California State Fair but in other major wine competitions as well to see whether wines that aced one competition did well in others, too. As you can probably guess by now, they didn’t. Wines would often win gold in one competition, nothing in another—and not a single wine out of more than twenty-four hundred picked up gold every time. The competitions might as well have handed out gold medals at random, Hodgson calculated.
So what’s going on? The answer is that people’s perception of a wine changes from moment to moment depending on the circumstances. The wine will taste blander if it follows a robust, fruity wine than if the previous wine was subtle; a particular aroma might have triggered a fond memory (and a higher score) for one glass but not the next one; the judge might have gotten tired as the flight progressed; they might have been distracted by a ray of sunlight or a twinge from an arthritic knee. All of that adds noise to the judge’s rating—so much noise, Hodgson thinks, that it obliterates any real differences in quality. Maybe, in fact, it’s just not humanly possible to judge wines objectively, especially in the crowded, rushed, overwhelming setting of a state fair.
Hodgson sees this variability at work when he drinks his own wine, too. “Since I have a winery, and I’m cheap, I drink my own wine all the time,” he says. That’s no hardship, because he generally thinks he makes excellent stuff. But even so, he’s not always in the mood. “Sometimes I think, Jesus, I don’t like this wine. But I know not to get upset, because tomorrow is a different day.”
All this points to an uncomfortable conclusion: If trained judges and experienced winemakers don’t consistently prefer one wine over another, then maybe there’s no real basis for calling some wines great and others merely good. And that may be how it really is, though it’s hard to find many wine people who will agree. “I would like to think that Mouton Rothschild is a better wine than Gallo Hearty Burgundy,” says Hodgson. “You and I may agree that one is better—but we may not agree.” Other studies, he notes, have found that ordinary wine drinkers, those of us without special training, tend to prefer cheaper wines over more expensive ones—but only if no one tells us what the price is. If you know the price, on the other hand, that high-level knowledge has a powerful influence on how you perceive the wine’s flavor. Almost everyone tends to think a more expensive bottle of wine tastes better than cheap stuff—even when all that’s changed is the price tag. That sounds like self-delusion—but there’s more to it than that, as one team of researchers learned a few years ago.
A brain scan is not the ideal setting for savoring wine. For one thing, you have to hold your head perfectly still, which precludes all the sniffing, swirling, and other ceremony that usually accompanies a sip of wine. Instead, you get a tiny dollop—a single milliliter, about a quarter teaspoon—of wine dripped straight into your mouth through a polyethylene tube as you lie in the scanner. But at least researchers can see exactly what the wine’s doing to your brain. For the experiment we’re interested in here, the scannees got sips of what they thought were five different wines of varying price, but in fact four of the wines were paired duplicates: a five-buck plonk was also presented as a $45 bottle, and an exquisite $90 Napa cabernet also appeared under the guise of an everyday $10 wine. Sure enough, the tasters liked the wines better when they appeared with a higher price tag. But the brain scans showed that they weren’t just saying so—the “higher-priced” wines activated the brain’s reward circuitry more than the same wines presented at a lower price. In other words, a higher price tag genuinely led to greater pleasure! As one observer noted wryly, this means that if you’re hosting a dinner party, you can maximize your guests’ pleasure by serving them a cheap wine (which most drinkers prefer in a blind tasting) and telling them it’s expensive.
As Rolls and other neuroscientists trace the flow of flavor through the brain, their attention comes back again and again to one particular spot, right behind the eyes at the front of the brain. Neuroanatomists have a daunting catalog of tongue-twisting names for parts of the brain, most of which only an expert would need to know. But this little region, known as the orbitofrontal cortex, or OFC, deserves to be more widely known to anyone with an interest in flavor. The OFC, researchers are learning, is one of the key areas where the brain knits together the independent threads of taste, smell, texture, sight, and sound—together with our expectations—into the common cloth of a flavor perception. It’s not stretching the facts to call the orbitofrontal cortex the birthplace of flavor.
(As is almost always the case with the brain the reality may be more complex. Another nearby brain region called the frontal operculum could also be a candidate for Flavor Central. In one recent study, researchers monitored brain activity while giving volunteers the odor or taste of orange juice either separately or together. The frontal operculum, but not the OFC, lit up far more strongly to the combined flavor stimulus than you’d predict from its response to smell or taste alone, suggesting that the frontal operculum may be another key area where flavor is constructed.)
If the OFC is where flavors are born, then it may also be the place to turn if you want to know what a flavor looks like in the brain. And, in fact, that’s just what Rolls and his colleagues have done. By recording the electrical activity of individual nerve cells, or neurons, within the OFC of rats, they’ve found that each neuron there responds to a different set of inputs. One might light up in response to a sweet taste, a pepperlike aroma, and the mouthfeel of capsaicin, the molecule that makes chili peppers hot; another might turn on to sweet taste, a vanilla aroma, and the mouthfeel of fat. You could call the first cell a “chili pepper flavor” neuron and the second an “ice cream flavor” neuron.
This mapping of particular flavors onto individual neurons helps explain why the first bite of, say, ice cream tastes so much better than the twentieth bite, and why we can eat our fill of stew yet still have room for pie. In essence, Rolls says, a particular flavor neuron gets tired after responding to its flavor over and over, a fatigue he calls “sensory-specific satiety.” He’s actually measured exactly this in monkey brains, showing how repeated doses of a particular flavor combination provoke smaller and smaller responses from its specific flavor neuron.
These flavor neurons in the OFC are also where learning enters the flavor picture. Remember Paul Breslin’s rose—bitter chewing gum, which people eventually learned to treat as a coherent flavor? It turns out that Rolls had tried a very similar experiment in rats, while looking closely at individual neurons in the OFC. And indeed, when he switched up his odor/taste pairings, he saw those neurons gradually switch their responses to reflect the new associations. “You can watch the neurons learn,” says Rolls.
While the neurons do relearn new odor/taste pairings, though, they aren’t speedy about it. Rolls found he had to expose the rats to the new pairing about fifty times before they switched. In contrast, when he did the same experiment, except pairing tastes with visual signals instead of odors, the neurons began to relearn the very first time they saw the new pairing.
Why the difference? Well, says Rolls, it probably has to do with flavor’s role in protecting us from eating the wrong things. “You don’t want to realign your whole flavor system too rapidly.” In the real world, smells tend to be rather reliably paired with the same tastes day after day, while the appearance of things can change quickly. Our brains, it seems, reflect this reality by being unusually conservative about taste/smell pairings, but looser with visual information.
Not that vision isn’t crucial to our perception of flavor. Humans, after all, are a largely visual species, so it’s not surprising that vision creeps into most of our experiences, says Lundström. A simple experiment shows how central vision is, he says. Try it: Imagine the fragrance of a ripe strawberry. Really focus on it. Now, didn’t you also call to mind a mental picture of the fruit itself? “It’s impossible to do that without actually visualizing a strawberry,” says Lundström. “I think that vision is the key component when it comes to memorizing an odor, and you have a strong input from vision when it comes to odor quality.”
To study this effect further, Lundström turned to a technique called transcranial magnetic stimulation—essentially an electromagnetic helmet that can be programmed to stimulate particular regions of the brain and make them work better. TMS of the visual center of the brain, for example, makes people about 10 percent better at discriminating among subtle shades of gray.
Lundström was after something more, though. If vision is linked to flavor processing, he wondered, could you stimulate the brain’s visual system and improve flavor perception in the bargain? If so, that would tie vision even more firmly into the bundle of flavor senses.
So Lundström and his colleagues set up a study to test the idea. They offered volunteers three smell samples to sniff: two of one aroma and one of a different, but somewhat similar aroma—strawberry and raspberry, say, or pineapple and orange. The people had to identify which of the three was the different sample, a task they could do correctly about three-quarters of the time. Lundström had his subjects do the test three times: once without TMS, once with TMS stimulating their visual center, and once with a fake TMS helmet that emitted an impressive, official-sounding hum but caused no actual changes in the brain.
Sure enough, people treated with the real TMS, but not the pretend one, proved to be about 10 percent better at picking which odor was not like the others. In other words, helping people see better helped them to smell more accurately, too. And the effect wasn’t just a general heightening of the senses. TMS of the visual center did nothing to help people tell which of three odor samples was more intense than the others, he found. That makes sense, says Lundström—visualizing the source of an odor is important in identifying it, but irrelevant in deciding how intense it is.
We’ve seen that the orbitofrontal cortex is the birthplace of flavor. It’s also the crossroads for several other key parts of consciousness. All five senses pass through the OFC on their way into the brain, and the OFC also gets input from the brain regions responsible for emotions, reward, and motivation, as well as higher-order thought. The orbitofrontal cortex has been called the sensory packaging center of the brain, the place where all our world experience comes together. That suggests that flavor is not just a filigree on our lives, a little bit of aesthetic fluff. It’s a key part of our interaction with the world.