Château Neuro: how the brain creates flavor
May 1, 2017
By Steven Shapin
LA Review of Books (December 30, 2016)
Gordon Shepherd’s compact Neuroenology is a straightforwardly didactic exercise, tightly focused on wine. It's a companion to his previous work, Neurogastronomy (2012), a well-received study of “how the brain creates flavor,” mostly about food. Lots of wine drinkers, and even wine writers, don’t know some of the facts about wine sensation that Shepherd wants us to learn.
WRITING ABOUT the objective basis of wine taste is like writing about the objective causes of humor. The risk with the latter is that it’s not funny and with the former that it doesn’t taste good. Take away the subjective funniness of a joke and you’re left, at best, with a deep understanding of how the linguistic forms of paradox and the rhythmic contractions of the diaphragm generate and indicate humor; take away the subjective experience of drinking wine and you’re left with a deep understanding of how certain volatile chemicals figure in the production of odor and how odor depends on the “movement of molecules to the nasal epithelium […] dependent on air currents induced by active alteration of the musculature of the mouth and pharynx.” Nevertheless, many scientists have said, for example, that the more you know about optics and pigments, the more beautiful you’ll find Van Gogh’s sunflowers, or that the more you know about acoustics and the friction of horse-ha
ir on cat-gut, the more moving is the Kreutzer Sonata.
There is a common table-thumping humanist response to these sorts of claims. You can, for example, insist on the integrity of the subjective domain: the way things “really are” has nothing to do with the subject’s experience. Someone told a joke either finds it funny or not; the drinker finds the wine smells nice or not. Most of us, most of the time, don’t feel that the funniness of a joke or the pleasant smell of a wine depends on accurate knowledge of their underlying bases or that our experience is impoverished for want of such knowledge. In fact, we might feel the opposite. The joke “I wouldn’t want to belong to any club that would have someone like me as a member” is spoiled by a prologue or epilogue that, for instance, invokes “the script-based semantic theory of humor.” In the opening to Annie Hall, Woody Allen gets away with linking the club-joke to Freud’s study Der Witz und seine Beziehung zum Unbewussten (1905) — because we understand that it’s w
ith the intention of making another kind of joke, a po-faced send-up of pedantry. Analytic talk about the nature of a joke can itself be a joke, even if it usually isn’t. After all, the joke and the joke-analysis are both forms of language, and anything that’s said can be funny in the right context. But that recursive relationship isn’t available with smell and taste. Talk about how and why wines smell nice can never smell nice. At most, such talk might summon up sensory remembrances of wines you had in the past and frame experiences of those you might have in the future.
There’s risk of philistinism on both sides. The analytic philistines can claim that subjective experience is really the objective bases to which it can be reduced and which are said to produce it, but the humanist push-back may be another form of philistinism. Humanists often insist that subjective experience is real enough, that reducing it to its supposed material bases isn’t necessary, and that it cannot be adequately described or explained away by reductive accounts. But it would be quite another thing to claim that nothing connected with knowing the material bases of subjective experience can illuminate or alter that experience. Even though Gordon Shepherd’s compact Neuroenology is a straightforwardly didactic exercise, tightly focused on wine, it’s also an occasion to reconsider some of our culture’s opposed philistinisms. Unlike, say, the current culture wars over consciousness, altruistic behavior, or economic rationality, you can be relatively relaxed abo
ut the matter at hand. After all, it’s just a glass of wine.
Shepherd is a distinguished neuroscientist at Yale, specializing in the analysis of olfaction and gustation, and he has an admirable track record in writing for general readers. His Neurogastronomy (2012), a well-received study of “how the brain creates flavor,” was mostly about food, though with some material on wine. That book has had a substantial institutional effect: together with Shepherd’s previous academic research — much of which Neurogastronomy summed up and made more accessible — it helped catalyze an International Society of Neurogastronomy, some exceptionally well-catered expert conferences, and, for lay readers, a new (sometimes informative, sometimes irritating) mode of table-talk. Where once we’d just say that the habanero chili was “hot,” now we might find ourselves being told by a well-read salsa-eating friend that the 200,000 Scoville units we’re sensing is really a response to capsaicin by polymodal nociceptive neurons and their axons in th
e trigeminal nerve.
There are three main reasons you might want to read Shepherd’s new book. The first is that you’re a wine-lover — the geeky sort who just happens to be fascinated by the physiology and physics involved, in much the same way as Downton Abbey–lovers might be interested in “behind-the-scenes” accounts of production practices. Or you might reckon (like the objectivist position on the painting or the violin sonata) that your appreciation of wine would be enhanced if you knew more about the chemistry of aromatic substances and what happens in your sensory apparatus when you drink a glass or two. The third reason to take up this compact book doesn’t depend on caring about wine at all; it’s about thinking through what it might mean to add the prefix “neuro” to cultural practices like enology or gastronomy — but we’ll come to that later. (First I need to take you through some of Neuroenology’s “technical bits”: try to persist.)
Shepherd’s first lessons reprise materials already available in Neurogastronomy, and they remind us of how little most people evidently care about, or even notice, the distinct sensory modes of smell and taste. In fact, there’s a long historical tradition of disregarding, and disrespecting, both senses. They have been widely identified as “low” ways of experiencing the world — to do with pleasure and the animal activities of feeding the body. Philosophers haven’t seen much reason to take seriously the senses that engage the world through contact, and still less those involved with taking things into your body. The contact senses are regarded as blunt instruments: after all, how much can you really say about the taste of an apple or the smell of a rose? Apples taste like apples and roses smell like roses. And isn’t having knowledge properly identified with the ability to talk about things, to parse and clearly represent their characteristics in language?
It’s hard to find words for how things smell or taste and to use language reliably to convey such experiences to others. Proper knowledge has traditionally been modeled on vision (“I see”; “you are blind to the facts”), and, less commonly, on hearing (“I hear your point”; “you are deaf to the facts”), but hardly at all on the contact senses of gustation and olfaction (“your argument doesn’t smell right”), and only a little bit on touch — compare the force of “I see” to “I feel.” The notion of “tacit knowledge” had a bit of a revival in academic understandings of science and technology some time ago, with haptics assuming a substantial place in cutting-edge work in artificial intelligence and robotics.
It’s not just philosophers who have been, so to speak, tone-deaf to smell and taste. Laypeople, and even many wine writers, talk loosely about the “taste” of a wine when the sensory experience to which they refer actually integrates not just gustation (with its palette of sweet, sour, salty, bitter, and — now — umami) and olfaction (with its immensely large range of recognizable odors), but also other senses — the tactile sense and those which detect chemical constituents and temperature. Vision is also involved in the sensory experiences associated with wine: you visually inspect the wine to see whether it looks good, whether it’s red, white, or rosé, and whether it’s the right color for its variety and age, but what you see also prepares you for, and frames, a smell and taste experience.
Sensory scientists use a humbling little demonstration to deflate wine-lovers who have great confidence in their discriminations: the drinks are served in black glasses and subjects are asked whether the wines are red or white. You may think that’s easy, but it’s not — neither for the amateur nor for the expert. You take a sniff of the wine (and therefore smell it); then you take some into your mouth, and, because that’s where the wine now is, you say you taste it. The sensory scientists might also ask you to pinch your nostrils closed, then have someone put a fruit-flavored candy in your mouth, asking you to report on what you “taste.” You will probably “taste” very little. Then they ask you to unpinch your nose and reflect again. You’ll be surprised that the candy now “tastes” of something quite particular or that it does so with much greater intensity. People are often surprised that they can be made to distinguish taste and smell, even though they usua
That’s one reason why Shepherd says that “smell is the major component of flavor” and why other popular science writers specify that “as much as 80% of what we think of as flavor comes from olfaction.” But olfaction involves a flow of volatile substances in twodirections. The “orthonasal” smell of a wine is what you experience at the first sniff, before you sip. But there is also a reverse pathway to the olfactory epithelium, when the wine is in your mouth and then when you swallow. This is the “retronasal” odor. These two vectors of olfaction make for different sensory profiles. For example, the retronasal odor differs from the orthonasal partly because volatile substances in the mouth are warmed and mixed with saliva, whose enzymes break down some wine constituents, changing their odors. If you wanted to use sensory scientists’ terms of art, you would say that “flavor” is the brain’s integration of all the sensory modes involved.
The distinctions between olfaction and gustation, and even between orthonasal and retronasal olfaction, are only a start. There are many more scientific facts to be understood about, for example, how wine moves around in the buccal cavity and then on to the pharynx and esophagus; how these muscle- and gravity-induced movements contribute to sensory experience; how swallowing is controlled by the sCPG (the swallowing central pattern generator); how swallowed wine leaves behind in the mouth and pharynx both a sticky “matrix” and “volatiles” which can be released when post-swallow respiration resumes; how the first expiration of breath after swallowing has the highest concentration of volatiles, which some tasters call “the aroma burst” and which they consider “the strongest contributor to the taste of wine”; how the nerves of the tongue and nasal epithelium are arrayed and what paths they take to the brain; how and where the various sensory modes are integrated into
the experience of flavor; and how some aroma molecules come to elicit olfactory responses.
Here’s a taste of the style (on how the “swallow” proceeds): the back of the tongue starts to rise to continue to hold the wine in. Increasing amounts start to spread over the back of the tongue and slide down into the valleculae, where exhaling air takes them up and increasingly stimulates retronasal smell. Taste buds, we’re instructed, are of three main sorts — fungiform, foliate, and circumvallate — and they’re not just on the tongue: there are taste buds on the tonsils and as far down the alimentary tract as the epiglottis. Some wines taste “astringent” because they have tannins which “bind with salivary proline-rich proteins and precipitate them,” leading to increased friction between mouth surfaces, and a sense of dryness or roughness, due to the way that molecules in the wine combine with molecules in the saliva and in the surface of the mucus membranes to form conglomerates.
The integration of sensory systems to produce “conscious flavor perception” occurs in the cerebral cortex. It involves complex interrelations among distinct brain areas, though most of what is now known about flavor and the brain comes from neuron recordings in monkeys and from seeing in humans what brain areas seem to be functionally active when different senses are in play. Neuroenology isn’t written with a notable lightness of touch, and the periodic summary boxes and chapter-end reminders of “what we have learned” may make you warm to it as to a school textbook. Most readers of this article probably didn’t know any of these facts about how wine is sensed — even if they like wine, buy the better bottles, and think Sideways was a terrific movie. I drink a fair amount of the stuff, and I’ve written academic and non-academic pieces about wine from time to time. Yet I knew almost nothing about the orthonasal-retronasal distinction until maybe 10 years ago, although
scarcely any sensory scientists understood this until the early 1980s. And I knew nothing at all about the chemistry of wine aromatics until even more recently. I’ve always liked wine very much; I fancied that I “knew about” it; I have a decent-sized “cellar”; and, very occasionally, I’ve been able to identify wines at “blind tastings” — in which you have no prior knowledge of what sort of thing you’re drinking. Nevertheless, some “science lessons learned” do have the ability to change sensory experience, even for people who know much more about wine than I do. Lots of wine drinkers, and even wine writers, don’t know some of the facts about wine sensation that Shepherd wants us to learn. It’s not that such people “don’t really know wine”; rather, they know wine like I know my wife — I can absolutely recognize her when I see her, and I can see the family resemblances between her and her relatives, but I don’t know her genome and I can’t descr
ibe her accurately enough to allow others who haven’t seen her to form a very detailed image of what she looks like.
So does any of this newly acquired “objective” knowledge about sensory modes bear at all on the nature and quality of subjective experience? Yes, it may, and no anti-reductionist humanist should feel obliged to deny that. Nevertheless, some claims for the aesthetic significance of scientific knowledge seem dubious. For example, Shepherd writes about the importance of the mucus membranes in the mouth, assuring us that “being aware of the structure of the mucus membranes, their various receptors, and the sensations they produce will enrich the wine-tasting experience.” But other neuroscientific stories seem more plausibly experience-changing. Scientists’ accounts of the retronasal pathway, for example, have the capacity to alter the attention paid to different types of olfactory experience. Our senses engage with a field of potential experience: we can attend to some features of that field and not to others, making some sensible aspects part of our focal awareness, and
backgrounding or bracketing others. Having a “private” conversation in a public room, we focus on our partner’s talk and not on the booming, buzzing “background” sound washing over us. Then we overhear someone mentioning our name and we realize that the background noise has been waiting to be turned into signal through a change in attentiveness.
Michael Baxandall’s marvelous accounts of what he called “the period eye” in Quattrocento painting told us how late medieval people looked at paintings — the eye attending to the areas of azure and gold in the Virgin’s clothing, guided there because of the known preciousness and expense of these pigments, just as the Quattrocento period eye attended to certain shapes because of the widely distributed mercantile skill in gauging the internal volumes of barrels from their visible surfaces. Knowing this, you can look at paintings in this way too. The French sociologist Antoine Hennion — also a wine lover — has proposed a “sociology of attention” in which features of the sensory field are framed, parsed, and differently stressed, and in which subjects momentarily make themselves passive with respect to the sensed object. (“Ah, yes, now I notice that.”) So the framing impulses that can change or enhance sense experience need not derive from sensory science, and
in these cases they do not, but sensory framing may come from scientific accounts of the structures and processes of sense perception. Neuroenology relates several stories that do have the capacity to change — to reframe, to reconstitute — our sensory experience. It’s an authentic debt that some pleasures might owe to some scientific accounts.
One robust story told by sensory scientists directs attention not just to the cortical integration of sensory modes but also to the cortical integration of sense with memory, with affect, and with preexisting knowledge. The brain knits together gustation; olfaction; the tactile, chemical, and temperature senses; and it also feeds into the overall sensory experience what you feel and what you know. Wine tasters realize just how powerful prior knowledge and expectation may be to the character of sensory experience, and that’s a reason why experts arrange blind tastings, often in unnatural laboratory-like conditions — decontextualized contexts, as it were. Dyeing a white wine red, presenting a Bordeaux in a Burgundy-type bottle, or, indeed, telling a taster that a glass of “Two-Buck Chuck” is actually a classed-growth claret are well-known ways to make people “get things wrong” — and that is one reason why wine experts often arrange to exclude from tastings not just kn
owledge of what the wine is, but also as many contextual influences as they can manage. From the point of view of both professional tasters and sensory scientists, naturally occurring wine-drinking scenes are full of potential distortions and biases. Enjoying a wine with good friends on a pleasant day makes you like the wine more: that’s wrong (the experts say); you should bracket these sorts of things and attend only to what’s “in the glass.”
Of course, from another point of view, expectation is a natural way of framing, sorting, selectively attending to, and, finally, having sensory experience. The roles played by these sorts of foreknowledges and expectations are normal; they are arguably functional; and there is something wrong with them only when the game-to-be-played is the special-purpose one of having sensory experience in an artificially produced world-without-cues. Montaigne, as ever, is a sound commentator on knowledge-as-it-actually-is as opposed to what some theoreticians think it should ideally be: “Things,” he wrote, “are sensed through the understanding, understood through the senses.”
There are lessons taught by neuroscientists that are neutral with respect to the sorts of experiences you already have or might have, and others that implicitly warn us against certain claims to sensory experience. An example of the first sort is an account of what actually happens when — to the bemusement of non-aficionados — a wine lover noisily swishes the wine around in her mouth. The taster is not — necessarily — a pompous show-off; what she is “really doing” is ensuring that the volatile chemicals are fully distributed to sensing organs over all the surfaces of the buccal cavity. Then there are neuroscientific accounts of what areas of the brain “light up” in functional magnetic resonance imaging (fMRI) when laboratory animals sniff different volatile substances. Neuroscientists also tell us that when you — but not, in this case, laboratory animals — are told that one of two wines you’re drinking costs more, even when the wines are in fact the same, a dif
ferent area of the cortex lights up for the “expensive” wine, and does so more brightly. Yet both of these findings bear as much relationship to the experience of aroma as knowing the location of the fuel pump does to the experience of driving a car: the pump and the brain area relating to odor have got to be somewhere, but knowing where they are doesn’t add to, subtract from, or change the experiences of driving or drinking.
An instance of potential neuroscientific debunking is the claim that your sensory constitution is only capable of discerning at most three distinct real components in an aromatic mixture. “Expert” commentary purporting to pick out in a specific wine a long list of odors — brambles, apricot pits, tomato skins, wet stones, roasted lilacs, and on and on — is, accordingly, likely to be bogus. Tomato skins, wet stones, and the like may or may not count as legitimate descriptors, but sensory scientists are skeptical that the discriminating capacities of human senses are equal to the powers implied by assigning to a wine an extended list of distinct odors. Yet the same scientists suggest — albeit here using rat models — that when odoriferous mixtures become familiar, “cortical neurons treat them as unique objects, different from their components,” which is, again, like saying that I can recognize my wife apart from listing and aggregating her serial attributes. On that basi
s, Shepherd concludes that the more experience you have tasting wines, the more likely it is that you will be able to identify them for what they are, as opposed to analyzing and naming their constituent odors. This is, however, something wine-people already knew.
Much neuroscience writing about experience or cognition advances claims about what experience or cognition really are as opposed to what we ordinarily suppose them to be. A version of this type of assertion is splashed prominently (in italics) on the first page of Neuroenology: “the taste is not in the wine; the taste is created by the brain of the wine taster,” and, two pages later, “taste itself is an illusion.” Attributing taste to something in the glass is said to be wrong, and those who believe such a thing are delusional, even if their wrong thinking can potentially be corrected by neuroscientific expertise. Shepherd generally aims to inform and not to browbeat, so it may be that these remarks are meant mainly as paradoxical rhetorical flourishes. But, taken neat, the claim that taste is an “illusion” is either trivially true or absurdly incorrect. The trivial sense of the claim would make all sensory experiences of the world illusory, including reading a sen
tence of a neuroscience book. After all, the retinal image of the printed sentence is inverted and neural structures work to turn it right-side-up, while the memory-holding and memory-retrieving structures of the brain add to the specific raw sensory experience past instances, similarities, differences, and a set of expectations and learned pattern-recognizing abilities that make black-marks-on-paper into a meaningful English sentence. If that’s illusory, then illusion deserves better press.
So far as taste and smell are concerned, the trivial truth is that sensory experience doesn’t belong to entities that have none, and a glass of wine is not a sensing subject. What else could gustation or olfaction be but responses to substances by the human or animal sensing subject? And where else could this processing reside but in the various sensing organs, the nerves that transmit their impulses, and the integrating areas of the brain? The distinction between the “primary qualities” of a thing (size, shape, arrangement, and states of motion of its ultimate particles), and the “secondary qualities” we attribute to it by virtue of our sensory apparatus (redness, sweetness, fragrance, warmth, etc.), goes back to 17th-century philosophers, including Galileo and John Locke. It’s not a radically new finding by modern neuroscientists, and no one now denies it.
Shepherd’s “neurogastronomy” and “neuroenology” books are benign, informative, and fascinating examples of an increasingly influential scientific genre which can, at its worst, be aggressive, over-reaching, and nonsensical, and which the Italian psychologists Paolo Legrenzi and Carlo Umiltà have called “neuromania.” This is the tendency to go beyond identifying the neural bases for beliefs and sensations to the claim that beliefs and sensations really are their neural bases. The first claim is unexceptionable: of course, sensations are the result of interactions between our neural structures and things in the world and elsewhere in our bodies. In this sense, neuroscience has begotten a set of pleonasms — using more words than necessary to convey a specific meaning — and these pleonasms have metastasized through contemporary culture. Insofar as our mental life is neurally based — and who now doubts that? – neuro-whatever might just be a potentially useful
way of reminding us of this fact: “neuroaesthetics” is aesthetics; “neuroethics” is ethics; “neuromarketing” is marketing; “neuroeconomics” is economics — even if traditional practitioners of aesthetics, ethics, and the like have not routinely had much to say about which areas of the brain “light up” when we see a beautiful painting, do a good deed, or buy a new car, and provided that we appreciate that what “goes on in the brain” includes what people know, remember, feel, and feel to be worth their attention. Similarly, “neurogastronomy” is a pleonastic form of gastronomy and “neuroenology” is a pleonasm for the experience of drinking and enjoying wine, while we might nevertheless expect that the idea of “neuroneuroscience” would be vigorously resisted by neuroscientists. Neuromania, Legrenzi and Umiltà argue, is partly a pathological infatuation with striking visual representations of the brain and brain activity, and partly an inade
quately thought-out power-grab by those who speak in the name of the brain and its reality-defining powers — authorizing them to say, for example, that taste is an illusion because it happens “in the brain.” Château Latour smells very nice; Château Neuro smells of nothing at all.
Steven Shapin is Franklin L. Ford Research Professor of the History of Science at Harvard University.
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