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Chemical Reaction: Learning to Smell

Steve Herman

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“Scents are like fashion—the beauty is not intrinsic in the odor. It’s in the eye of the beholder, or, in this case, the nose of the beholder.”

—Richard Doherty

A soccer ball rolls across a field. Trees in the background sway in the mounting breeze that propels the low lying clouds overhead. You see it all, and are not befuddled by the millions of bits of sensory information entering your eye because processing miracles are functioning deep in your brain. The rolling soccer ball presents a constantly changing pattern, but remains clearly a soccer ball to us. The leaves in the trees keep changing shape in the breeze, and the color varies constantly as shadows play upon them, but they remain clearly the same leaves of the same colors in the same trees.

Objects recognizable yet visually impacted by movement or altered lighting are illustrated wonderfully in the paintings of Claude Monet. He often took a single subject—the Houses of Parliament, Rouen Cathedral, the ponds of Giverny—and painted them repeatedly in different light to show how details changed. A canvas had to be replaced every few minutes as the sun and clouds impacted the artist’s subjects. In this way, Monet was able to isolate and capture what our brain automatically merges into a single, unchanging object.

There are analogies in sound, as well. In a noisy room, you pick out the words of your friend. You hear a dog bark outside. It is not a muddle of sound because your brain has filtered out the background noise and given you the information you need. In both hearing and seeing, the brain’s ability to filter out noises in order to provide us the information we need to understand the world is a critical tool for our survival. But what about smell? How do we pick out a flower in a swirl of other odors?

The role of the brain in vision and hearing has been extensively investigated, and an invaluable new book, Learning to Smell: Olfactory Perception from Neurobiology to Behavior, written by neurobiologist Donald A. Wilson and psychologist Richard J. Stevenson, does the same for olfaction—illuminating how odors are processed in the brain. Just as understanding how the lens in the eye works does not truly explain vision, the understanding of the role of the hair-like cilia in the nose does not provide the whole story of olfaction. The current emphasis on how molecules react at the receptor sites has done little to inform our understanding of odor recognition and discrimination, and even less to explain aromas’ emotional power.

The basic anatomy of olfaction is well understood. A molecule enters the nasal cavity, reaches the olfactory cilia, and a signal is sent to the olfactory bulb and then into the brain. Actually, the cilia are part of the brain. The signal goes to the piriform cortex and then to the orbitofrontal cortex, where it can converge with gustatory and other sensory input—thus the possibility of strong interactions between the senses.

Learning to Smell presents a new theory of olfactory perception, and the authors provide an alternative to the receptor-centered view of olfaction—maintaining that olfaction is not a simple physiochemical process but is strongly tied to memory. Where the traditional approach concentrates on identifying how the particular features of a chemical stimulate the olfactory system, the primary emphasis in the book is how past knowledge and current experience inform our olfactory recognition.

Hundreds of molecules contribute to the smell of coffee, but we inhale and think “coffee.” Our brain puts it together and compares it to other patterns stored in our olfactive memory. Wilson considers it essential to separate the physical stimulus from the psychological effect to understand how we respond to odors.

Although Learning to Smell is an academic book not at all concerned with the personal care industry, we can easily tease out from its pages information that will illuminate how fragrances are perceived, why consumers react to them the way they do, how marketers communicate odor ideas and how perfumers are trained. The key to understanding is the “odor object” and how it interacts with our brain.

We smell Chanel No 5 and think “Chanel No 5,” not, “Wow, a whole lot of aroma chemicals.” Our brain has a pattern that it matches based on memory and past experience, and we have a verbal name to place on it. If a perfumer made something revolutionary, we might think “that smells strange,” the marketing department can’t place it in a genealogy chart, a consumer won’t have some comforting sense of recognition and no one can explain the smell over the phone. Commercially, an unfamiliar scent is a hard sell. Thus, 99% of “new” fragrances smell remarkably like “old” fragrances, a fact that hardly generates excitement among consumers. It is hard to innovate when familiarity is essential to acceptance.

As they train and develop their skills, perfumers smell hundreds of chemicals, and develop odor recognition for most of them, but they also have the advantage of talking about them to other perfumers. This verbal connection forms a much stronger image of the odor. The same is true for anyone involved in the fragrance business, from sales and marketing professionals to the sales associates at the counters in a department store. They see genealogies, read odor descriptions, discuss new fragrances with customers and, all along, are strengthening their odor memories. Those connections, evident in fragrance professionals, provide fertile ground for exploration of how the sense of smell actually develops.

A large part of Learning to Smell is devoted to comparing olfaction research to research on the other senses, particularly vision. The authors combine an understanding of learning, memory and pattern recognition into a comprehensive picture of how our brain interprets an odor. Vision and olfaction both have two modes of sensory discrimination. One is hard-wired for detection of biologically significant stimuli; the second is experience dependent and changes as an individual matures.

Less academic writers have also instinctively recognized the interconnections of the senses. Virginia Heffernan, writing for The New York Times Magazine, mentions “seeing sound, reading smells.” She reports on spending hours on perfume blogs “that describe smells by calling on images, textures, music.” How can fragrances be sold online without some connection between words and smells? What would the fragrance business be like without the learning, experience and verbal cues that can be attached to scented products?

It is clear that the mind, not the nose, drives the response of every odorous product, from popcorn to Downy to Chanel No 5.

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