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Smell: the undervalued sense

Our sense of scent is more important than we think, says neurobiologist Gordon Shepherd

Gordon Shepherd, professor at Yale School of Medicine and author of “Neurogastronomy: How the Brain Creates Flavor and Why It Matters.”

When the marketing firm McCann Worldgroup surveyed thousands of young people last year about what they value most, more than half of the 16- to 22-year-olds said they would rather give up their sense of smell than their phones or laptops. Researchers presented this as an example of a particularly modern youthful attachment to technology, but it is also a sign of the persistent human disregard for the sense of smell. Plato associated smell with base urges; Aristotle wrote that “man smells poorly.” Kant dismissed both taste and smell as inferior to the other senses.

In fact, however, smelling engages huge regions of the brain — not just memory and emotion, but also our systems for language and higher cognitive processing. Breathing in an odor creates a complex pattern in the brain, comparable to the one we use to recognize faces. In contradiction to Aristotle, we’re not just good at this; our powerful brains make us uniquely skilled. And when it comes to what we eat — both our flavor perception and the decisions we make — the sense of smell is firmly in charge.


That motivated neurobiologist Gordon Shepherd, a smell expert at the Yale School of Medicine, to write a book about the connection between smell, flavor, and the human brain. Shepherd’s own research began in the 1960s, and one of his first major breakthroughs was the discovery of the “odor image,” the raw pattern of a smell that gets passed up to the brain for processing. His new book, “Neurogastronomy: How the Brain Creates Flavor and Why It Matters” (Columbia University Press, 2011), is his first for a general audience, and at the heart of it is an argument that smell is far more important to humans than we recognize.

Grade-school science students learn that smell affects flavor when they perform the pinch test: squeezing their noses closed, putting a bit of candy on their tongues to experience a simple sweetness, and then opening their noses to release a much fuller flavor. What they don’t learn is the true extent of what’s happening when they do that. As Shepherd writes, “retronasal smell,” in which we send little puffs of air out and backward into our nasal passages as we eat, almost instantaneously engages “activity at the highest cognitive levels of our brain.”


Only if we understand that activity better, Shepherd argues, can we truly understand nutrition, obesity, and aging. Smell affects what we crave, what we become addicted to, what we find pleasure in, and even the will to live. He connects declining powers of smell in the elderly to “failure to thrive,” and says that flavor may point to public policy solutions to poor eating habits. Shepherd spoke to Ideas by phone from his home in Connecticut.

IDEAS: It seems like common sense that humans have worse senses of smell than other mammals. But you argue that although our physical smelling apparatus is relatively puny, our large brains more than compensate. What’s the difference between the way we smell and the way our pets do?

SHEPHERD: Smell is something that’s important in all animals. But in humans the part of smell that produces flavor is uniquely important. For example, if you look at the way your dog eats food that’s put before it, you don’t see your dog swishing the food in its mouth. It’s just literally wolfing the stuff down. So there’s nothing comparable in the way that you look at most animals eating to the way that we literally savor the food that we eat. The human brain flavor system is uniquely important for us. And I think it engages a much larger extent of the brain than people realize . . . .We have big brains and we’re trying to analyze this picture, this unconscious picture in our mind of something that is attractive.


IDEAS: It seems like in popular culture, people often think of smell as something almost mystical, rather than a “hard” sense. Is that because of its connection with memory and emotion?

SHEPHERD: Yes, that’s the way we tend to think of it . . . .But complex music and complex visual images are just as mystical. We tend to think that if we see something square or light or dark, it’s easy to describe it in words. But if you try to describe a composition of modern art, or — I quote a conductor trying to describe music — the word they use is “mystery.”

Of course it’s difficult to describe smells and flavors in words, but that’s why I think our evidence that smells are patterns, images like faces, helps us understand that we’re dealing with a problem that all senses have: describing complicated images with language.

IDEAS: Is that why smell gets so little respect?

SHEPHERD: That’s been one of the motivations for writing the book, to rescue it from exactly that attitude. That has a lot to do with the fact that disorders of vision that lead to blindness and disorders of hearing that lead to deafness are really serious. Each of them has its own institute in the National Institutes [of Health], and nobody would choose if they had to lose a sense to lose one of those senses. Whereas smell, it seems, is just something that is important in perfumes and aromas of food and would be easy to lose without compromising ourselves . . . .But I hope that I can make the point in the book that the human brain flavor system is a tremendously pervasive one in the brain, and it plays a role that has been very important in forming human behavior.


IDEAS: You write in the book that “mouth tastes” are hard-wired, while the flavors we get from retronasal smells are learned. How do we learn to smell?

SHEPHERD: If you think about all of the different cuisines in the world, almost everybody agrees on sweet, salt, sour, bitter, and, increasingly, umami, which is a kind of a meat taste. That’s common to all cultures . . . .Whereas the flavors of the food vary all over of what one can call “flavor space” or “odor space.” That means that we’re brought up to favor particular smells.

IDEAS: And you think we use that fact that to inform our nutritional choices?

SHEPHERD: If you go to the diet books and so forth, they’re all about fats and carbohydrates and proteins, but very little about actual flavors. So one of my hopes with the book is to show that an increasing amount of research is giving insight into how the brain creates flavors and how best to begin to deal with this in a public policy way.


IDEAS: What’s your wildest hope for where this field of research might eventually lead?

SHEPHERD: My wildest hope would be that the human brain flavor system would be recognized as being an essential player in the recommendations of the organization that puts out the nutrition standards for the United States. I would hope that that would be an integral part of the food pyramid, and that this would then be used to develop much more flavorful food for school lunches, for example . . . .We need to understand how the brain does this to be sure that our kids are growing up with healthy brains, not brains that have become addicted to the wrong foods.

Ruth Graham is a writer in New Hampshire.