Science Watch

The warm, nutty fragrance of baking cookies; the sharp sting of bleach; the clean, green perfume of spring’s first lilac blooms — these scents may seem straightforward, but there’s more to odor than meets the nose.

Smell is an ancient sense. All living things, from single-celled bacteria to the most accomplished bloodhound, can detect chemicals in their environment. Odors are molecules, after all, and olfaction is just the vertebrate version of chemical sensing.

Despite its ubiquity and deep roots, the importance of olfaction is easy to ignore. According to psychologist Johan Lundstrom, PhD, a faculty member at the Monell Chemical Senses Center in Philadelphia, there are two big reasons why. The first is a lack of words. We can create rich descriptions for objects by expressing their colors, shapes, sizes and textures. Sounds come complete with volume, pitch and tone. Yet it’s almost impossible to describe a scent without comparing it to another familiar aroma. “We don’t have a good language for odors,” he says.

Second, we can blame the brain. For all the other senses, sensory memos are delivered straight to the thalamus, “the big switchboard of the brain,” he says, and from there out to the primary sensory cortices. But olfactory input winds its way through other brain regions, including the centers for memory and emotion, before reaching the thalamus. “In neuroscience, we say a little bit nonchalantly that nothing reaches consciousness unless it has passed the thalamus,” he says. “For olfaction, you have all this basic processing before you have conscious awareness of the odor.”

That basic processing isn’t the whole story, though. An assortment of internal and external factors influences how we perceive a particular scent. And as more researchers turn their attention to this often-overlooked sense, the more interesting the olfactory picture becomes.

A cheese by any other name

At a fundamental level, quirks of physiology can affect your sense of smell. Some people are “smell blind” to certain chemicals. Take asparagus, for example. Many people notice an unpleasant sulfur-scented tinge to their urine after eating just a few stalks. But not everyone. Recently, several of Lundstrom’s Monell colleagues reported in Chemical Senses, (Vol. 36, No. 1) that some lucky folks with a certain single-letter change to their DNA are unable to smell that peculiar perfume.

Hunger state can also affect smell perception. Researchers at the University of Portsmouth in the United Kingdom just reported in Chemical Senses that people are generally more sensitive to odors when they’re hungry; but, surprisingly, they’re slightly better at detecting food-specific odors after a full meal. The study also found that people who are overweight are much more sensitive to food odors than are thinner people.

Context is critical, too. To most people the smell of cow manure is disgusting. But for people who grew up on farms, manure can evoke strong feelings of nostalgia. And while most Americans wrinkle their noses at the scent of seaweed, most Japanese (who grew up with seaweed on the menu) find its aroma alluring. “Our previous experience has a very strong impact on how we experience odors,” Lundstrom says.

Expectations also play a role. Try this, suggests Lundstrom: Hide some aged Parmesan in a cup and tell a friend someone vomited inside. They’ll recoil at the smell. But tell them it’s fantastic cheese, and they’ll swoon. Clearly, there’s some top-down brain processing at work. “You can go from extremely positive to extremely negative just by changing the label,” he says.

That phenomenon has implications beyond practical jokes. Pamela Dalton, PhD, MPH, also a faculty member at Monell, recently found that one’s expectations about an odor actually affect physical health. She presented a synthetic odor to asthmatics, who often report sensitivity to strong aromas. She told half the volunteers that the odor could reduce asthma symptoms, while the rest believed the chemical scent might make their symptoms worse.

In reality, the volunteers smelled a rose-like fragrance known to be harmless even at high concentrations. Yet the people who believed the scent was potentially dangerous reported they felt more asthma symptoms after sniffing it. That Dalton expected. What surprised her was that it wasn’t all in their heads. Those volunteers who expected the worst actually experienced increased lung inflammation, while those who believed the odor was beneficial did not. Even more startling, the elevated inflammation levels persisted for 24 hours. Dalton presented the research at the 2010 meeting of the Association for Chemoreception Sciences in April. Dalton blames the reaction on stress. “We know there’s a pathway by which stress can produce those kinds of inflammation,” she says. “But we were frankly surprised that just a simple suggestion about what they were smelling could produce such a significant effect.”

The more closely researchers look, the more evidence they find that odors hold sway over our emotions, our cognition, and even our health. Slowly, they’re starting to tease out the specifics.

The importance of B.O.

One important finding by olfaction researchers is that odors aren’t all created equal. Some scents are actually processed differently by the brain.

Body odors, in particular, seem to belong to a class all their own. In a study published in Cerebral Cortex (Vol. 18, No. 6), Lundstrom found that the brain relies on different regions to process body odors versus other everyday scents. He used positron emission tomography scans to peer at women’s brains as they sniffed the armpits of T-shirts that had been slept in overnight by volunteers. They also smelled shirts infused with a fake body odor scent.

The test subjects couldn’t consciously tell which samples were real and which were fake. Yet the scans showed that true body odor scents fired up different brain pathways than artificial odors. Genuine body odors actually deactivated areas near the secondary olfactory cortex, Lundstrom says, and instead lit up several brain regions typically used not in olfaction, but in recognizing familiar and fearful stimuli. “It seems that body odors are processed by a sub-network in the brain, and not mainly by the main olfactory system,” Lundstrom says.

Back in the ancient, pre-Speed Stick era, sizing up body odors was critical for choosing mates and recognizing kin. “We believe that throughout evolution, these body odors have been tagged as important stimuli, so they’ve been given dedicated neural networks to process them,” he says.

Here too, however, there are individual differences in a person’s sensitivity to body odors. And sensitivity to these important odors may actually lay the groundwork for social communication. Denise Chen, PhD, a psychologist at Rice University, performed a version of the sweaty T-shirt test, which she published in Psychological Science (Vol. 20, No. 9). She asked each female subject to sniff three shirts — two worn by strangers, and one worn by the subject’s roommate. Chen found that women who correctly picked out their roommate’s scent scored higher on tests of emotional sensitivity. “Those people who are more sensitive to social smells are also more sensitive to emotional signals,” she concludes.

A sensory world

In addition to helping us navigate our social world, smell may join with sight and sound to help us make our way in the physical world as well. The bond between taste and smell is widely known. But increasingly, scientists are realizing that olfaction mixes and mingles with the other senses in unexpected ways.

Until recently, Lundstrom says, scientists mostly studied each sense in isolation. They used visual stimuli to understand vision, auditory stimuli to understand hearing, and so on. But in real life, our senses don’t exist in a vacuum. We’re constantly bombarded with bits of information from all the senses at once. Once researchers began studying how the senses work together, “we started to realize what we thought was true for each sense isn’t,” he says. “It could be what we thought true about the brain might not be true after all.”

In current research, he’s finding that people process odors differently depending on the other sensory inputs they receive. When a person looks at a photograph of a rose while smelling rose oil, for example, she rates the aroma as both more intense and more pleasant than she does if she smells rose oil while looking at a picture of a peanut.

While Lundstrom has shown that visual inputs influence our sense of smell, other researchers have found the reverse is also true: Odors affect our ability to process visual stimuli.

In a study published in Current Biology (Vol. 20, No. 15) last summer, Chen and her colleagues presented two different images simultaneously to a subject’s eyes. One eye viewed a permanent marker pen while the other eye was trained on a rose. Under those circumstances, subjects perceived the two images alternating back and forth, one at a time. When smelling a marker scent during the experiment, however, subjects perceived the marker image for a longer period of time. The opposite occurred when they smelled the aroma of rose. “A congruent smell prolongs the time the image is visible,” Chen says.

Alan Hirsch, MD, neurological director of the Smell & Taste Treatment and Research Foundation in Chicago, has also explored the link between scents and sights. He asked men to estimate the weight of a female volunteer while she wore different aromas or no odor at all. Some scents had no apparent effect on the men’s perceptions of her weight. But when she wore a perfume of floral and spice notes, the men judged her to weigh about 4 pounds less, on average. Even more intriguing, the men who described the floral-spice perfume as pleasant perceived her to be about 12 pounds lighter.

In a related study, Hirsch found that volunteers who sniffed grapefruit aromas judged women to be five years younger than they actually were, while the scents of grape and cucumber had no effect on the perception of age. Exactly why grapefruit had such a potent effect isn’t clear. The volunteers’ past experiences with citrus scents may have come into play, Hirsch suggests, or the grapefruit aroma may have come across as more intense than the milder scents of grape and cucumber. What is clear, though, is that scents convey a lot of information — true and otherwise — that help us make judgments about the world around us. “Smell is impacting us all the time, whether we recognize it or not,” he says.

Such studies are only beginning to unlock the secrets of smell. “Olfaction is a very young field,” Chen notes. Compared with vision and hearing, it’s poorly understood. True, humans are overwhelmingly visual creatures. Still, olfactory researchers seem to agree that the nose is much more important than most people realize.

It’s also an excellent instrument for learning more about the brain in general, Chen says, both because of its ancient roots and because of the unique way olfactory information weaves through so many intriguing parts of the brain. “Olfaction is a wonderful tool to study the functions and mechanisms of sensory processing, and their connection with things like emotion, cognition and social behavior,” she says.

Clearly, there’s a lot to learn. When it comes to cracking the mystery of olfaction, we’ve only gotten a whiff.


Kirsten Weir is a writer in Saco, Maine.