Neurogastronomy: How the Brain Creates Flavor and Why It Matters

When we talk about gastronomy, people usually mean how the food stimulates our senses to give rise to particular flavors. However, this requires moving the food within our mouths in a coordinated manner. We take this part of eating for granted, but closer inspection shows that it is incredibly complicated. I’d like to convince you it is one of our supreme motor acts. The next time you take a bite, think of what is happening in your mouth. There is a complex sequence of movements involving the muscles of the lips, jaw, and tongue and those involved in swallowing; these movements have to be coordinated with breathing, and all of them must be coordinated while we are focusing our senses on enjoying the flavors that are produced.

We are not the first to be interested in understanding the mouth as a motor organ. On this subject, Jean Anthelme Brillat-Savarin was particularly eloquent:

I have discovered at least three movements [of the tongue] which are unknown to animals, and which I describe as movements of SPICATION, ROTATION, AND VERRITION (from the Latin verro, I sweep). The first takes place when the tip of the tongue protrudes between the lips which squeeze it; the second, when it rolls around in the space between the cheeks and the palate; the third, when it catches, by curving itself now up and now down, the particles of food which have stuck in the semicircular moat between the lips and the gums.

The reader may try these tongue motions in order to verify the savant’s claims.

This theme has been taken up by two modern-day disciples of Brillat-Savarin. A vivid description of the extraordinary range of movements involved in manipulating food has been given by a well-known chef, Jean-Marie Amat, and a leading neuroscientist and gourmet, Jean-Didier Vincent, in their book L’art de parler la bouche pleine (The Art of Talking with Your Mouth Full). In one passage of the book, they write of the culinary adventures of a band of five good friends who gather every month or so to eat and talk—often, as the title indicates, attempting both at the same time. Most of the talk is about food and the human species that consumes it. At one point, the following spirited exchange takes place:

The human mouth, what a marvel, exclaimed the Professor, there’s no animal that matches it!

I don’t detest a cow’s facial appearance, replied the Artist. The way a cow chews its cud defines its way of life, its serenity in dealing with its world.

Of course! responded the Professor. And what about the big cats? They’re matchless in tearing meat apart. And dogs? Without equal in cleaning bones. But no beast anywhere combines as many talents as the human in being aware of its nourishment: chewing, crushing, manipulating, as well as “spication,” rotating, “verrition”; and at the core, the tongue, that admirable muscle, veritable prayer rug of the palate, which by its delicate texture and its neighboring membranes expresses the sublimity of the operations for which they are destined. It is therefore not surprising that the orifice which enables an animal to nourish itself should become in humans at the same time the organ of language, the hallmark of the species and of the meal, which is the foundation of human society.

Be that as it may, you’re talking with your mouth full, interjected the Merchant. Couldn’t you restrain your oratorical ardor?

The splutter is the salt of eloquence, observed the Tobacconist.

Here we find emphasized, in more elegant words and narrative, several points we have been making about the physiology of eating.

First is the amazing range of ways that food is manipulated in the mouth, which reflects the much greater interest that a human takes in the sensory nature of the meal. The varieties of ways the food is moved about within the mouth of course enhance its sensory qualities, especially the retronasal smells, but also cause multiple exposures to the taste buds and to the multiple receptors for mouth-sense in the inner walls of the oral cavity.

Second, the Professor draws attention to the fact that the tongue is critical both to the sensory experience of the meal and to the production of language. It seems, in fact, paradoxical that the tongue should be so closely related to producing both smells and words, because it is difficult to find words for a smell. The explanation appears to lie in the concept of the smell image, as discussed in chapter 24.

Finally, the Professor makes the claim that the meal is at the heart of human society. Just as for Grethe and me, the gathering of a core family or an extended family or clan at mealtime to share the catch of the day was surely a key activity in forming human societies and cultures, and therefore must have played a key role in human evolution. This idea is discussed further in chapter 26.

Building on this eloquent testimonial to the movements within the mouth that produce the flavors of our food, let us take a look at what modern science is telling us about the motor systems that move our mouths. This evidence comes first from study of the central motor pathways in the brain, and second from using modern video techniques to observe the exact sequence of movements while consuming foods and beverages. There are also important clues gleaned from studies by anthropologists of our human ancestors. The following draws on The Evolution of the Human Head, a book by Daniel Lieberman that describes in exquisite detail the movements of the human mouth in eating.

The descending motor system that runs our muscles and glands begins at the highest level in the motor strip of the cerebral cortex. Just as there is a sensory homunculus representing the body surface, with large areas devoted to the lips and tongue, there is a corresponding motor homun-culus with a similar enlargement of the lips and tongue. It represents the larger numbers of cortical microcircuits devoted to receiving the sensory inputs and controlling the fine movements of the lips and tongue when we eat and drink.

From this high level, the output neurons—large pyramidal neurons controlled by the same kinds of feedback excitatory and inhibitory lateral connections found in all cortical areas—send their fibers all the way down through the interior of the brain and into the brain stem to terminate directly or indirectly on the large motor neurons that control the muscles of the lips and tongue (figure 17.1).

This motor system is an essential part of the flavor system. The lips and tongue are constantly moving as we eat and drink. We have noted that the sensation of flavor seems to be coming from the mouth, even though much of the flavor is due to retronasal smell. This is called mouth capture. In addition to the sensory signals, mouth capture is believed to be due also to the high level of activity in the tongue and lips areas of the motor cortex. It all adds to the illusion we live under when we enjoy the flavor of our food and give the mouth all the credit.

Let us discuss four types of movement within the mouth that are activated and controlled by the descending motor system involved in producing flavor: chewing, swishing, swallowing, and breathing. What does evolution tell us about their roles in producing human flavors?

The evolutionary record leaves few traces of the things that really interest us about how we became human, but they do give us fascinating glimpses relevant to flavor. This is because the only parts of our human ancestors that have survived are the bones, and of these the bones of the head are among the hardest, and of those the teeth are the hardest of all. The patterns of wear on the teeth tell anthropologists much about what people ate; this is usually used to infer what the diet was. Our interest is in how chewing produces flavor.

Mammalian teeth come in several varieties. In front are the incisors, sharp and bladelike, and the canines, pointed like daggers. We use both to grip, cut, and tear meat and vegetables into morsels. This action requires considerable force, exerted by the jaw muscles. The size and placement of the muscles contribute greatly to the form of the human face.

The food morsels are then moved by the tongue to the premolars and molars, where chewing begins. The tongue directs the traffic between the food mass in the mouth and the moving teeth, feeding small bits between the teeth for grinding into smaller bits and finally into a mush (now called a bolus). This grinding (now called mastication) crushes the food cells, releasing the volatile molecules of the morsels of meat or fruit or vegetables. Like all mammals, we chew mostly on one side in order to concentrate more force. This requires the tongue to keep the chewed mass on the one side. Most foods require many chewing repetitions. The comedian Bob Newhart used to insist that his mother told him to use as many chews as he had teeth in his mouth. Another rule of thumb is that the most flavor is obtained (and the bolus is in its most digestible form) when the individual types of food in the bolus are no longer distinguishable by the touch receptors on the tongue.

Reducing the time and effort for chewing was one of the key steps in human evolution. Our ape cousins spend half the day chewing tough meat or vegetable matter. As discussed in chapter 4, there is considerable evidence that cooking was one of the most important steps in human evolution, because it tenderized not only meats but also fruits and vegetables, increasing their nutritive value while reducing the chewing time.

Consuming fluids is absolutely necessary for all mammalian life, including primates and humans. Fluids may enter the mouth in various ways: for most animals by lapping up from a pool or a dish, and for humans from various types of containers or through straws. The lips enable the transfer from the source to the mouth. The fluid may spend very little time in the mouth, for example, when one is thirsty. Or the fluid may spend quite a while in the mouth, as in the drinking of wine. In this case, the tongue comes into play in a different way, swishing the fluid about slowly in order to present it to the taste buds on the tongue in as many different ways as possible, while allowing prolonged sensing and evaluation of the retronasal smells that emanate from the fluid. It is probably not an overstatement to suggest that the expertise of a wine connoisseur is highly dependent on the particular tongue movements that have been learned in presenting the wine in an optimal manner for sensory evaluation.

As the bolus reaches its choice consistency, the tongue moves it toward the back of the mouth, where there is a final evaluation through the touch receptors of whether it is ready to be swallowed. Then an exquisitely coordinated movement takes place. The soft palate is stiffened and raised to seal off the part of the nasopharynx that mediates retronasal smell. The tongue moves the bolus into the pharynx at the back of the mouth and then rises to force the bolus down into the pharynx. At the same time, muscles relax in the lower pharynx to allow the bolus to pass into the esophagus, and other muscles contract to close the vocal folds and raise the epiglottis so that the bolus cannot go the wrong way down the trachea. Finally, peristaltic movements move the bolus down the esophagus to pass into the stomach. Food physiologists are studying these events in ever greater detail. For example, Andrew Taylor and his colleagues at the University of Norttingham in England have analyzed flavor release during chewing and swallowing of different amounts of food with differing fat content. They find relatively more flavor release from larger amounts of low-fat food. How much we stuff into our mouths is thus another variable in the complex perception of flavor.

Breathing produces flavor in two ways: during mastication and after swallowing. During mastication, the bolus is in the mouth, and as long as it stays there we can continue breathing, sampling the volatiles emitted from the bolus with each expiration, together with the stimulation of the other senses (taste and touch). Videofluorography is being used to document the exquisite coordination required for breathing during mastication, pharyngeal bolus aggregation, and swallowing. You can document this for yourself by observing each step the next time you are eating your dinner. During the steps of swallowing, the bolus leaves the tongue, so there is no taste, and the airway is blocked off, so there is no retrograde smell. During the swallow we hold our breath to prevent food from going down our windpipe. However, as soon as the swallow is over, an exhalation takes place. It is usually completely automatic, even if we have not inhaled before. We may continue to sample the flavor with continued breathing. In that case, what is being sensed is the thin layer of food remaining on the pharyngeal walls.

Similar steps apply to fluids, as reported by Taylor and his colleagues in a recent study of aroma release during drinking wine. We breathe while swishing a sip of wine in our mouths, the flavor sensation coming from the volatiles when we exhale.

In summary, these many coordinated muscle movements mean that flavor is an “active” sense. Other senses may be stimulated in a passive way, merely by exposing us to a sight, a sound, or a smell. However, flavor always must result from movements involved in taking the food into our mouths, masticating it, and swallowing it. All the movements originate in motor systems distributed widely in the brain, further emphasizing how much of the human brain is involved in the seemingly simple acts of enjoying what we are eating and drinking.