• Neurogastronomy: How the Brain Creates Flavor and Why It Matters
• Gordon M. Shepherd
• Columbia University Press (2011)

Whether we're talking about America's obesity epidemic, mocking the "foodie" movement on The Simpsons, the USDA's revamped food pyramid or what they're cooking up on Top Chef, food and eating are a national obsession -- especially at this time of year.

But just as fascinating is the hard science behind our intimate relationship with food. Gordon M. Shepherd, professor of neurobiology at the Yale School of Medicine, has spent a lifetime researching the brain mechanisms involved in "olfaction" (our sense of smell) and its impact on flavour perception in the brain. His new book is Neurogastronomy: How the Brain Creates Flavor and Why It Matters, out this month from Columbia University Press. Shepherd's work is anchored in a burgeoning field within neuroscience -- figuring out the mysteries behind our olfactory system, the ways in which smells are represented and processed in the brain.

Shepherd argues for the quintessential importance of olfaction in our everyday experience of food. Without smell, Shepherd says, there is no flavour. Neurogastronomy takes a detailed look at just how smelling in the nose, mouth and brain produces the unique experience of flavour that we associate with eating our favourite or least-favourite foods.

Shepherd explained why sitting down to Thanksgiving dinner triggers our instincts to overeat, how to do the nose-pinch test and the best way to fool an experienced wine taster.

What came first, your interest in gastronomy or your interest in neuroscience? Did one lead to the other?

My first interest is in neuroscience, in particular in the part of the brain that processes the sense of smell. And we all think we know what smell is.... It's when we sniff something in and it has a nice smell. But as I went further along I began to realize that increasingly smell was for sensing the flavour of food. It goes almost unrecognized as we eat our food because we think it all comes from taste in our mouths. The more research that I did on flavour, the more I realized that the sense of smell was the dominant sense in flavour -- and that we are almost totally unaware of it. The science of smell is increasingly important for understanding flavour, and flavour really contributes to whether we are eating healthy or unhealthy diets.

You explain that smell is a dual sense. It's composed of "orthonasal" and "retronasal" smelling. What's the difference between them?

Orthonasal is sniffing in. It happens when we sense perfume, something in the environment, or in the aromas of the food we are about to eat. Retronasal is called "retro" because it comes from the back of the mouth, and it's the way we smell when we have food in our mouths. While we might not be aware of it, vapours that come from our food while we are breathing out stimulate cells in our nose. That is how smell contributes to flavour. We don't realize it because all we usually notice is the mouth-feel, the textural quality of food.

In the book, you discuss the ways in which smelling relates to the experience of flavour, and how it is different from tasting. You say that smell is "synthetic" while taste is "analytic." That's an interesting idea -- could you elaborate on what you mean?

It's really simple. Taste is sweet, salt, sour and bitter -- and "umami," a meaty taste. The amazing thing is that when you have something in your mouth, you can taste whether it's sweet, sour, bitter and so forth. This is analytic, because you analyze taste piece by piece, just as you would picking things up from a table. Each one is picked up separately. Smell is different because smell arises as a whole entity. It's impossible to take the pieces apart. That's why we call it synthetic: it's synthesizing and bringing things together. And these are two aspects of sensation that are very important for the different senses.

One of the simpler concepts in olfaction is that smell molecules bind with receptor cells in our nasal cavity. After that things get a little bit more complicated as far as how information is relayed and processed within the brain.

Smell molecules are represented in the first stage in a part of the brain called the "olfactory bulb." The olfactory bulb processes the information about smell in a way that is very similar to the way our retinas processes an image, or the way auditory parts of the brain represent musical patterns. It's important to understand that smell is not completely unique in the way it stimulates the brain, because it processes information in a pattern, the same way other senses do. Only in olfaction is this sensory pattern represented in the frontal cortex, which is the most developed part of the human brain. All of the other senses have to be relayed forward in order to get to the cortex, so in mammals the sense of smell has privileged access to the front of the brain.