Odors and sensitivity

Odors and sensitivity

Perhaps the most primitive of the senses, smell has a surprising influence on cognition, emotion, and even other senses.

The warm, nutty scent of baked cookies; the strong sting of bleach; the clean, green scent of the first spring lilac blossoms - these scents may seem simple, but the scent isn't limited to the nose.

The smell is an old sense. All living things, including unicellular bacteria, can detect odors from chemicals in their environment. Odors are molecules, after all, and smell is just the vertebrate version of chemical sensing.

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

Second, we can blame the brain. For all other senses, sensory memos are delivered directly to the thalamus, "the great standard of the brain," he says, and from there to the primary sensory cortices. But the olfactory supply makes its way through other areas of the brain, including memory and emotion centers, before reaching the thalamus. “In neuroscience, we say a little casually that nothing reaches consciousness unless you've passed the thalamus,” he says. "For the smell, you have all this basic treatment before you are aware of the smell."

However, this basic treatment is not the whole story. An assortment of internal and external factors influence how we perceive a particular scent. And as more and more researchers turn to this often overlooked meaning, the more interesting the olfactory image becomes.

A cheese under another name

On a fundamental level, the quirks of physiology can affect your sense of smell. Some people are "blind" to certain chemicals. Take asparagus, for example. Many people notice an unpleasant sulfur-scented tint in their urine after eating a few stalks. But not everyone. Recently, several of Monell's colleagues from Lundstrom reported in Chemical Senses, (Vol. 36, No. 1) that some lucky people with some single letter change in their DNA are unable to smell this particular scent.

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

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

Expectations also play a role. Try this, Lundstrom suggests: hide aged Parmesan cheese in a mug and tell a friend someone has vomited in it. They will recoil at the smell. But tell them it's fantastic cheese, and they'll pass out. Obviously, there is top-down brain processing at work. “You can go from extremely positive to extremely negative just by changing the label,” he says.

This phenomenon has implications beyond practical jokes. Pamela Dalton, PhD, MPH, also a faculty member at Monell, recently discovered that expectations about a smell actually affect physical health. She presented a synthetic odor to asthmatics, who often signal a sensitivity to strong aromas. She told half of the volunteers that the smell could reduce asthma symptoms, while the rest thought the chemical odor could make their symptoms worse.

In fact, the volunteers smelled a rose scent known to be harmless even at high concentrations. Still, people who thought the smell was potentially dangerous said they experienced more asthma symptoms after sniffing it. What Dalton expected. What surprised him was that it was not all in their heads. Volunteers who expected the worst actually experienced an increase in lung inflammation, while those who thought the smell was beneficial did not. Even more surprisingly, the high inflammation levels persisted for 24 hours. Dalton presented the research at the 2010 meeting of the Association for Chemoreception Sciences in April. Dalton attributes the reaction to stress. “We know there is a way that stress can produce this type of inflammation,” she says. “But we were frankly surprised that a simple suggestion of what they smelled could have such a significant effect.”

The closer researchers look, the more they find that smells influence our emotions, cognition, and even our health. Slowly, they begin to spell out the details.

The importance of body odor

An important finding of olfaction researchers is that not all smells are created equal. Some scents are actually processed differently by the brain.

Body odor, in particular, seems to belong to a class of its own. In a study published in Cerebral Cortex (vol. 18, no. 6), Lundstrom found that the brain depends on different regions to process body odor compared to other everyday scents. He used positron emission tomography scans to observe the brains of women sniffing the armpits of T-shirts volunteers had slept in overnight. They also smelled shirts imbued with fake body odor.

Test subjects could not consciously know which samples were real and which were fake. Yet analyzes have shown that real body odor triggered different brain pathways than artificial odors. Authentic body odor actually turned off areas near the secondary olfactory cortex, Lundstrom says, and instead lit up several areas of the brain typically used not for smell, but to recognize familiar and frightening stimuli. “It appears that body odor is processed by a subnet in the brain, and not primarily by the main olfactory system,” Lundstrom explains.

In ancient times, measuring body odor was essential for choosing mates and recognizing loved ones. “We believe that throughout evolution these body odors were identified as important stimuli, so they were given dedicated neural networks to process them,” he says.

Here too, however, there are individual differences in a person's sensitivity to body odor. And sensitivity to these important smells can actually lay the foundation 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 selected their roommate's scent had higher scores on emotional sensitivity tests. “People most sensitive to social smells are also more sensitive to emotional cues,” she concludes.

A sensory world

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

Until recently, Lundstrom says, scientists have primarily studied each sense in isolation. They used visual stimuli to understand vision, auditory stimuli to understand hearing, etc. But in real life, our senses don't exist in a vacuum. We are constantly bombarded with snatches of information coming from all the senses at once. Once researchers began to study how the senses work together, “we started to realize what we thought was true for each sense,” he says. "It might be what we thought was true about the brain, maybe not true after all."

In current research, he finds that people process smells differently depending on what other sensory input they receive. When a person looks at a photo of a rose smelling rose oil, for example, they rate the aroma as both more intense and more pleasant than if they smell rose oil while looking at a photo. of a peanut.

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

In a study published in Current Biology (Vol. 20, No. 15) last summer, Chen and his colleagues presented two different images simultaneously to a subject's eyes. One eye looked at a permanent marker while the other eye was trained on a rose. Under these circumstances, the subjects perceived the two images alternately, one at a time. By smelling a marker odor during the experiment, however, subjects perceived the image of the marker for a longer period of time. The opposite happened when they smelled the aroma of rose. “A congruent odor prolongs the time that the image is visible,” Chen says.

Alan Hirsch, MD, neurological director of the Smell & Taste Treatment and Research Foundation in Chicago, also explored the connection between scents and sites. He asked the men to estimate the weight of a volunteer woman while she was wearing different scents or no scent at all. Some perfumes had no apparent effect on how men perceived her weight. But when she wore a scent with floral and spicy notes, men judged her to weigh about 4 pounds lighter, on average. Even more intriguing, the men who described the floral-spice scent as pleasing perceived it to be about 12 pounds lighter.

In a related study, Hirsch found that volunteers who sniffed grapefruit aromas judged women five years younger that they really were, while the scent of grapes and cucumber had no effect on the perception of age. It is not known exactly why grapefruit had such a powerful effect. The volunteers' past experiences with citrus scents may have played a role, suggests Hirsch, or the grapefruit aroma may have appeared more intense than the milder scents of grape and cucumber. What is clear, however, is that perfumes convey a lot of information - true or not - which helps us to make judgments about the world around us. “The smell touches us all the time, whether we recognize it or not,” he says.

Such studies are only beginning to unravel the secrets of smell. “Olfaction is a very young field,” notes Chen. Compared to seeing and hearing, it is misunderstood. To be sure, the vast majority of humans are visual creatures. Yet olfactory researchers seem to agree that the nose is much larger than most people realize.

It's also a great instrument for learning about the brain in general, Chen says, both because of its ancient roots and because of the unique way in which scent information weaves its way through so many intriguing parts of the brain. . “Olfaction is a great tool for studying the functions and mechanisms of sensory processing, and how they relate to things like emotion, cognition and social behavior,” she says.

Obviously, there is a lot to learn. When it comes to unraveling the mystery of olfaction, we have had only one puff.