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Vol. 21 No. 7 (July 2000) ©
Tourist in a Taste
The brain is the matermind of flavor but tongues are where it starts - and some are far more sensitive than others.
By Patricia Gadsby
A group of us, some scientists but mostly not, have just had dinner together at the New York Academy of Sciences. We've eaten well broiled salmon with spears of asparagus, a leafy salad, goblets of green-tea ice cream, wine and coffee for those who want them and settled down for the evening's lecture. The speaker, from Yale University, is Linda Bartoshuk, a specialist in human taste. She hands us each a small packet containing what looks like a Communion wafer. It's a piece of filter paper saturated with a compound called propylthiouracil, known in taste circles as PROP. We're instructed to put the papers in our mouths. As my saliva wets it, a nasty bitterness blooms. My neighbor, too, is making a face that says yuck. Then Bartoshuk asks for a show of hands. How many of us tasted something? How many of us didn't? Of course, Bartoshuk knows the punch line: Typically, a quarter of the audience tastes nothing. This routine never fails to impress an audience. Jaws go slack as hands shoot up in answer to both questions. How can something be tasteless to some people and so unpleasantly bitter to others? If we trust our senses to reflect the real world, the answer seems unnerving: We may think we all ate the same dinner tonight but we don't all live in the same taste world.
Actually, researchers began suspecting as much in the 1930s, after a chemist who was making a batch of a compound called phenylthiocarbamide (PTC) let a puff of the crystals fly into the air. A lab colleague, who must have swallowed some of the airborne crystals, noted how bitter they were. The astonished chemist, who tasted nothing himself, became the first to describe "taste blindness" to the bitter compound. Of course, taste blindness to PTC or its chemical cousin PROP might just have been a scientific curio so-called nontasters do respond to other types of bitterness. But, as it turned out, nontasters respond to all types of bitterness less intensely than tasters, and the degree to which people taste PROP can serve as a general indicator of their overall taste capacity.
Judging from family studies, the inability to taste PROP is genetic and most likely due to a recessive gene. That would fit rather nicely with Bartoshuk's finding that there exists a subset of PROP-tasters supersensitive to bitterness. She calls them super-tasters. Looking at the three groups, you see just the sort of patterns you'd expect for a recessive gene. Roughly 25 percent of people tested with PROP don't taste it, consistent with two recessive genes; 50 percent are tasters, consistent with one recessive and one dominant gene; and 25 percent are super-tasters, consistent with two dominant gene copies, one from each parent. The distribution is slightly sex-skewed. More women than men are super-tasters, perhaps because bitter is the skull and crossbones of the natural world, and evolution once favored mothers with superior poison-detecting systems.
What's fascinating, says Bartoshuk, is that tongue anatomy spells out the differences among these groups. Tasters have more taste buds than nontasters, and super-tasters have the most. Because taste buds are surrounded by nerve endings that sense not only taste but pain and touch, super-tasters, perhaps not surprisingly, have a more sensational taste repertoire. Bitter tastes bitterer, salt a bit saltier, sour sharper, and some sweets sweeter. Fat feels fattier, gums thicker, and alcohol and chili burn more fiercely. The inside of a nontaster's mouth makes up "a very small world compared to the super-taster's," says Bartoshuk, a non-PROP-taster herself. But because the super-tasters' sensory realms are so intense, they may avoid strong tastes especially bitter ones like grapefruit, coffee, beer, or broccoli and thus actually shrink their dietary horizons.
Intrigued by her discoveries, I persuaded Bartoshuk to let me visit her lab for a two-day immersion in taste. One experiment in particular had caught my attention. A surgeon injects anesthetic through an ear to knock out a cranial nerve that runs into the tongue. ("Sure you want to do this?" a friend asked, envisaging my face with some kind of lopsided palsy.) "If you're lucky," Bartoshuk said, "you may get a taste phantom," a taste that appears for no apparent reason. I had a flicker of recognition. Every once in a while a metallic taste sneaks up on me, as if iron had somehow leached into my mouth. Here was a chance for me to confront my phantom.
Linda Bartoshuk's laboratory is on the second floor of a redbrick building in the Yale University School of Medicine complex. It differs little from any other equipment-crammed lab, except for the food paraphernalia jelly-bean jars, hard candies, bottles and bottles of Tabasco, jalapeño, and other hot sauces. And then there are the magnified pictures of tongues. The photographed tongues are strikingly individual, some lightly patterned and others crammed with bumps.
Bartoshuk is an affable, outgoing, and generous woman in her early sixties. She likes a good laugh. This morning, though, she's all business. After a few minutes' casual conversation, she snaps on her concentration like a light. Studying taste is tricky, time-consuming work. And having to coax human volunteers through batteries of tests as opposed to pointing electrodes at rats' tongues or manipulating cells in a dish doesn't make the task any easier. Although she is not a physician, Bartoshuk also investigates severe taste problems in patients referred to her by their doctors.
First, Bartoshuk performs a checkup a spatial test of my tongue by painting tastes on it one section at a time. Using cotton swabs as brushes, she dips into various concentrations of sodium chloride, paints the salt solutions on my tongue, and asks me to rate their intensity on a scale of zero to 100. Then she brushes on sucrose to test my sense of sweet, citric acid to test sour, and quinine to test bitter. She's aiming for little bumps called papillae on the tongue's front, sides, and back. Those on the front are the so-called fungiforms, because they look like button mushrooms. The foliates, supposedly leaflike, show up as a reddish series of folds on each edge near the back. Way, way back, practically down the throats of some people, lie the circumvallates, which stand like round moated towers across the tongue's surface in an inverted V.
We call the bumps of our papillae taste buds, but they're more like Xs marking taste-bud spots: Our taste buds, for the most part, are nested into them and much too minuscule to see. Special receptor cells poke out of these tiny organs to catch the sweet, salty, sour, and bitter molecules that land in the mouth. When tasty molecules stimulate the receptors, they in turn stimulate nerve endings inside the tongue, and the messages ricochet along nerves to the brain.
"So this is essentially a neurological test for nerves in your mouth," explains Bartoshuk. "I'm putting solutions on areas of the tongue where I know which nerve is innervating the tissue, and I want you to evaluate what you're perceiving."
The nerves to which she's referring are the two main cranial nerves for taste that run from the brain to the tongue's front and back. When she dabs sweet onto the fungiform papillae, she knows she's stimulating the taste nerve at the front: the chorda tympani. When she puts sweet onto the circumvallates, she's testing the taste nerve at the tongue's back: the glossopharyngeal. In addition, by dabbing on pure alcohol or capsaicin, the fiery chemical in chilies, she can test the nerve for touch, called the trigeminal, which sends little, pain-sensitive fibers to the papillae. It's as if she were examining a fuse box, systematically checking the switches and wiring of my taste system.
As Bartoshuk continues to dot my tongue with tastes, I suddenly comprehend the inaccuracy of those textbook tongue maps the ones showing sweet corralled at the tip, salt and sour on the sides, and bitter on the back. We clearly taste them all over. "The tongue maps are wrong," Bartoshuk says flatly.
Judging from the way I rate tastes, especially bitter tastes, I'm a relatively "strong responder," says Bartoshuk. "You're not going to have a nontaster tongue, that's for sure. The question is just how many fungiform papillae you have." A typical nontaster's tongue has few fungiforms, a super-taster's tongue packs loads of them, and a medium-taster's tongue falls somewhere in between.
To make the papillae more visible, Bartoshuk stains my tongue's surface a ghoulish blue with vegetable dye (it delineates the fungiforms, which stay pink, from other, bluish tongue tissue). Then, because the tongue muscle tends to fidget, I'm asked to hold mine between two little plates of clear plastic. A video camera is trained on the setup, and suddenly an image of my tongue, magnified 10 times, lurches onto the video screen.
"You have an area of your tongue where the fungiform papillae are missing," Bartoshuk says. "Now, no big deal. It suggests some trigeminal damage. The most likely cause would be a mild dental injury. Did you ever have a shot of novocaine that nailed a nerve?"
Not that I recall.
She counts the fungiform papillae on the monitor. "Hard to know whether you're a high medium-taster or a low super-taster. You're on the borderline, one of those people for whom we need a gene test, which is an area of research that's getting very exciting. You see here, look at the density of the fungiforms," she says, pointing to a region on the front-right of my tongue, where the fungiforms crowd together much more tightly than in other places. "There it's high, more characteristic of a super-taster tongue."
Then she points to widely spaced spots on the screen. "On my tongue I'm a non-PROP-taster the fungiforms would be like polka dots, here and here and here. But notice over here," she says indicating a barren patch on my tongue's left half. "We don't see very many. There should be more fungiform papillae over here."
Next Bartoshuk ponders my fungiformless spot. "Something has clearly happened to your trigeminal nerve," she says. In humans the upkeep of the fungiform papillae isn't done by the front taste nerve; it's done by the trigeminal nerve. "This is the nerve that dentists try to deaden when they're doing a lot of drilling work. And it's very easy to accidentally penetrate it with the anesthetic injection," she adds. "It could have happened when you were a kid, a long time ago."
The nerves going to the mouth are vulnerable to many kinds of insults: dental work, ear surgery, knocks to the head, whiplash, common viral infections, and the like. For anatomic reasons, the taste nerve serving the front of the tongue, the chorda tympani, takes the brunt of viral hits. As the nerve travels between the brain and the tongue, it passes right under the eardrum, the tympanic membrane. So during an ear infection, or an upper respiratory infection affecting the ears, a virus sometimes infiltrates this taste nerve and knocks it temporarily or even permanently out of commission.
Lucky for us, our sense of taste has a system of compensation. Normally the nerves of the tongue inhibit one another. The taste nerve in the front, for instance, inhibits the taste nerve at the back, and vice versa. Injure the front nerve, and there's a release of inhibition on the signals from the back nerve, which makes up for the deficit.
"So when taste in the front goes down, taste in the back goes up," says Bartoshuk. Our system for maintaining taste works so well, she adds, that "many people who have local taste losses aren't aware of them until we test them." But sometimes there's a cost. A loss in one nerve can lead to exaggerated responses in the disinhibited nerve, or even conjure up sensations that seem to have no real-world cause.
These "taste phantoms," says Bartoshuk, "are sensations in the absence of stimulation." They come in all four of the classic tastes, as we might expect, as well as the sour-mineral taste we interpret as metallic. The most common "release-of-inhibition" phantoms turn up at the back of the tongue when the taste nerve up front sustains some type of damage. My metallic visitor tends to make an appearance when I have a stuffy cold or flu, probably because the invading virus has made it into the chorda tympani.
It's possible to induce phantoms like this experimentally by mimicking injury to the chorda tympani with a carefully aimed, nerve-deadening anesthetic injection. The technique came about by accident, Bartoshuk recalls. She and John Kveton, an otolaryngological surgeon at Yale University School of Medicine, were walking away from a lecture together. "John made a comment about a patient who'd had an anesthetized chorda from an eardrum injection. And I said, �You can do that?' " From a taste researcher's point of view, that injection represents a vast improvement on the traditional lingual block or dental injection. Unlike the block, the eardrum injection doesn't deaden touch. And it numbs only one taste nerve the one serving the tongue's front, and only on the injected side. A collaboration was born.
Kveton has a steady, deft touch and a calm, unflappable manner to match. His specialty is operating on acoustic neuromas, a type of tumor that grows next to the brain, eventually involving the taste nerve. I feel in good hands as he injects my right ear the following morning. The needle pushes into the skin inside the ear canal, close to where the nerve runs under the eardrum, and then the injected lidocaine washes over the nerve. After a brief recovery period, Bartoshuk tests the right tip of my tongue with a salt solution. I can feel a little drag from the cotton-topped stick on my tongue, a bit of coolness from the moisture, but there's no taste none whatsoever. The nerve's completely out, so Bartoshuk gets busy with her tests. "Sip some water," she instructs. "This will often precipitate a phantom if you're going to get one." It doesn't take long.
My phantom drifts in gradually, as if it needs time to assemble all its parts. It comes in as sour at the back of my mouth, on the left, and then slowly the characteristic metal notes fill out. I'm oddly pleased, considering that the ferrous taste usually spoils food and drink. There's pleasure in recognizing something familiar, of course, and pleasure in the control that comes with understanding. But the pleasure also comes from sheer wonder.
"Notice the metallic is contralateral opposite to the side of the anesthesia?" says Bartoshuk. "That's incredibly important because it has to be the brain that's doing that. The left and right halves of the tongue are innervated separately. The first time signals from the two halves interact is in the brain. So when we do something on one side and the other side is affected, we conclude it happened in the brain."
My phantom wafts in and out, finally fading for good as the anesthesia wears off. In the meantime, Bartoshuk has something else up her sleeve. She shows me how the brain uses touch to "place" taste in the mouth. She brushes salt around the tip of my tongue, going from my right no-taste side to my left tasting side. And as expected, I taste nothing until salt hits the left. But when she reverses direction, and swabs salt from left to right, something counterintuitive happens. The salt I can taste for real on the left swooshes over to the right side which, in fact, can't taste a thing. My brain produces a continuing illusion of taste on the right, because it's continuing to get touch signals there. As Bartoshuk puts it: "The taste sensation follows the touch path." Feeling is believing.
Much of what we commonly refer to as taste, of course, consists of smell, the aroma of food in our mouths. It's not sweetness that makes a banana banana-y; it's the scent. When we're eating a banana, however, we're not aware of smelling its scent up the back chimney of our noses. It feels as though we're sensing it inside the cavernous laboratory of our mouths, as part and parcel of an overall flavor. Again the brain appears at work here, referring food-aroma to the mouth by the sensations of touch and taste.
Valerie Duffy, a taste researcher at the University of Connecticut who collaborates with Bartoshuk, has come up with a simple demonstration of how taste is crucial for anchoring food-smells in the mouth. With my tongue anesthetized on the right, I'm to take a spoonful of banana yogurt, move it around my mouth, and tell Bartoshuk if banana-y flavor the aroma-taste combo is coming more strongly from any part of my mouth. And sure enough, I can't perceive banana very well on the side of my tongue with no taste but on my tasting side, I sense banana loud and clear. "Yeah, you got it," says Bartoshuk. And with that we call it a day.
Taste-illusion experiments like these show how we taste with our brains. But when we eat, the brain isn't just melding taste, smell, and touch to give us the intricately flavored and textured experience of food. It recalls the pleasures, and displeasures, associated with the foods we're eating. "Sweetness is a deep biological pleasure," says Bartoshuk. It signals safe calories in nature, and the pleasure's probably hardwired. But most of the pleasures of eating are conditioned, learned by experience. "Does a gourmet enjoy his expensive chocolate mousse more than I enjoy my Hershey's bar? I don't think so. I like my Hershey's bar a lot," Bartoshuk says with a big, infectious laugh.
So when it comes to pleasure, it probably doesn't matter much whether we are
super-tasters, medium-tasters, or nontasters. Our brains will work to calibrate
pleasure to our particular sets of chemical senses, because it wants to keep
those calories coming in so we'll survive. Our noggins will make sure we enjoy
our food to the max our max. It really is chacun á son goût.
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