by Eric Sorensen | © Washington State University
The act of smelling starts out as chemical detection but often ends up as an emotional trigger
Among all the modern variations on evolution are several hundred shoppers who two years ago wandered into a home decoration store in northern Switzerland. For most of them, it was just another chance to buy some plates or a basket, with the exception of a researcher asking them to fill out a questionnaire at the cash register. But after nearly a month of monitoring customers, researchers noticed that one group of about 100 spent on average significantly more money. The customers told the researchers as much, and receipts bore them out.
The bigger spenders shared one thing: They shopped while the air was filled with a simple, fresh orangey scent.
Other customers had shopped with no particular scent in the air, while a third group was exposed to a more complex blend of basil, orange, and green tea.
When it came time to buy, says Eric Spangenberg, dean of the WSU College of Business and the study’s corresponding author, “What we showed was that the simple scent was more effective.”
Throughout the living world, the nose leads the way, pioneering a course through the environment with a radar-like ability to spot perils and prizes. Pop open that Tupperware from deep in the fridge and the nose tells you in no uncertain terms that the contents are inedible. We find comfort in the musky nuzzle of a dog’s neck, discomfort in a dog’s breath, and in certain malodorous moments, reason to blame the dog.
In partnership with the tongue, our nose is a major arbiter of what we put in our mouth and how we enjoy it. Hold your nose while eating, or eat with a cold, and your meal becomes a bland, undistinguishable mush.
“Taste and smell are different sides of the same coin,” says Ken Kardong, professor emeritus in the School of Biological Sciences, “because both of them rely on sampling of chemicals brought to their sensory systems either in the air or in water.”
The tongue detects a limited suite of sensations: sweet, bitter, sour, salty, and the more recently confirmed savory taste called umami. The human nose is equipped with nearly 400 genetically distinct receptors attuned to specific chemical constructs. Working together, they can tease out an olfactory fingerprint separating one scent from the rest.
“You have no idea of the differences you can detect,” says Joe Harding, a medicinal chemist in the College of Veterinary Medicine whose decades-long career of probing the brain started with a look at the nose. “If we put a row of 1,000 chemicals out there on a table for you, you could tell the difference between every single one, and a lot of them would be one carbon different from the next. It’s insane.”
Our olfactory system is the latest in a long line of chemical detection systems that living things have deployed in the ancient and endless surveillance for food, foes, and mates. It is remarkably powerful. Writing in What the Nose Knows, sensory psychologist Avery Gilbert notes that two-thirds of the humans in a University of California-Berkeley study could follow a 30-foot chocolate trail using only their noses. The human sensitivity to methyl benzoate, a product of hydrolyzed cocaine, is as good as a drug-sniffing dog’s.
“Dogs have great noses,” Gilbert writes, “but it’s time to stop the trash talk and give ourselves more credit.”
The nose is also a powerful portal into the world of emotions conjured up by, say, the smell of fresh-cut grass, or moldering leaves, or your grandmother’s house.
Such moments are the end product of an odd transformation. The compound or mix of compounds that hits our nose can be a hardcore reality, a clinically verifiable product like methyl salicylate, C8H8O3. But in its conversion to an electrical signal and processing through the brain, it becomes the minty smell of a deep-heating liniment. Suddenly, the smeller is caught in a mix of memory and emotion, a bone-sore adolescent in a high school locker room.
Individual results will vary. The smell of a freshly cut pasture can evoke memories of a summer camp or time on a farm. But if it comes off a composting facility, it can trigger nausea and headaches.
It’s an odd outcome of our species’ refinement. Our noses can hold their own against some of the best in the animal kingdom. But one of the most profound things they betray is how easily our emotional judgments not only shape our world but are our world.
We have a lot in common with one of the world’s oldest known single-cell organisms, you and I. They are from the branch of life Archaea, and Cynthia Haseltine, an assistant professor in the School of Molecular Biosciences, studies how some of them use three particularly human-like proteins to repair their remarkably human-like DNA.
And even though her archaeons don’t have a nose, or a cell nucleus for that matter, they have an obvious chemosensory system.
“Archaea can definitely ‘smell’ things and are very chemotactic,” says Haseltine. “One of the best examples is halophiles, the salt loving ones. They will swim toward good things like food and nutrients and away from bad things like phenol.”
On top of that, they will swim toward oxygen, and the halophiles, which are photosynthetic, will also swim toward light. Haseltine might be stretching the definition a bit, but says this “is sort of like ‘smelling’ the good stuff.”
It is not stretching things to say plants can smell.
Back in 1990, Bud Ryan and Edward Farmer in WSU’s Institute of Biological Chemistry found that plants under attack will emit methyl jasmonate, an oil with the jasmine scent that Humphrey Bogart recalls smelling before he is knocked out in Dead Reckoning. Bogie had nothing on plants, who, in addition to emitting methyl jasmonate, put up defenses the moment they smell it.
David James, an associate professor of entomology at the WSU Research and Extension Center in Prosser, has documented plants giving off both methyl jasmonate and the minty methyl salicylate when attacked by predatory insects.
In essence, it’s a bouquet, with a stronger eau de jasomonate arising in attacks from chewing insects and more methyl salicylate produced by attacks from sucking insects.
“People used to say trees talked to each other,” says James. “It’s true. Plants do talk to each other through smelling each other.”
One way to appreciate the versatility and sensitivity of our noses is to look back to these and other early innovators in the evolutionary chain. One organism will have a set of systems that, for the most part, work, but over the ages will add on new features that work better. As the saga turns, increasingly specialized organisms emerge based on the previous iteration, but they often hang on to pieces of the past.
“Evolution is not a lot of new construction, in small steps anyway,” says Kardong, who devoted much of his career to teasing out the 35 million-year evolution of a snake’s venom-injecting fangs. “It’s really a lot of remodeling, because remodeling is usually a lot easier because you’ve already solved a bunch of problems. So why invent another backbone when the fish did it pretty well?”
The sensing systems of our forebears did remarkably well.
Migratory fish, as Northwest residents tend to know, can literally smell their way from the ocean up their native stream, switching on the olfactory guidance system after using a compass-like magnetic sense to home in from the ocean.
“In the absence of olfactory cues, salmon move downstream, presumably trying to re-encounter the cues,” says Jen McIntyre, postdoctoral research associate at the Puyallup Research and Extension Center.
Their terrestrial descendants, amphibians, developed a rudimentary vomeronasal organ that was made more sophisticated in reptiles and most mammals. Snakes can smell airborne odors, but some odors get embedded in the ground. The vomeronasal system lets a reptile tap the substrate, flick its tongue to collect a sample, and pass it across the organ on the roof of its mouth.
A snake’s olfaction and vomerolfaction can be particularly handy in not only finding prey but tracking it down after it is injected with venom. A wounded animal is a dangerous thing, so a snake will attack, then retreat to wait for the animal to die.
A few years ago, Kardong tested snakes’ ability to follow the trail of a stricken mouse and found they preferred a trail with the highest concentration of odor. It probably helped that the snakes could detect a change in their prey’s odor brought about by something injected, or the very process itself.
“They were simply amazing with what they could pick up even after 24 and 48 hours,” he says. “The success rate went down in tracking the odor, but they were still able to do it. Which also by the way says something about their chemical memory, that they remembered. Because they tracked the trail of the mouse they struck from all other mouse odors out there. So they have some kind of memory of the particular odor gestalt of the mouse they struck.”
Some smells are so compelling, they seem to leave no choice but to follow a certain course. Vince Jones, a professor of entomology, calls this a “programmed behavior.”
“If a housefly lands on a sugar solution, almost automatically mouth parts start to dab the substrate to try to get at it because the tarsi’s picked it up,” he says.
The tarsi, by the way, is the lower part of an insect’s leg, roughly equivalent to our toes. That’s right, an insect smells with its toes.
But to continue:
“There are some very programmed behaviors that they have when they encounter certain smells and volatiles, and the way they move upwind to locate the exact source of these things is actually pretty spectacular.”
With just a whiff of pheromone, a chemical attractant that says a female is upwind and ready to mate, a codling moth will start flying across the wind and up and down, zig-zagging from one edge of the plume to another, smelling its way to the female.
But as programmed as the behavior may be, says Jones, pheromones aren’t destiny. Young males are more willing to fly, but are less successful. Older males find the female more often, but fly less. Not all males respond.
“It is an evolutionarily important drive for them so it tends to be a very conserved behavior,” says Jones. “When they do respond to it, they can be very persistent or not. It just depends.”
Moreover, insects can be trained to associate prey with a particular volatile.
“They’re not little robots,” says Jones, whose lab does work on insect-chemical associations. “They learn to associate different things with different odors. It’s a very complex environment, a lot more than we would have thought maybe 15, 20 years ago.”
Like bugs, humans find certain smells can be an acquired taste.
Pius Ndegwa grew up on a small farm in Kenya with cows, goats, sheep, pigs, and their attendant odors. For the most part, the smells bring up good memories.
“It’s something you grow up with,” says Ndegwa, now an extension specialist in Biological Systems Engineering and a specialist on air-quality issues around agriculture. “It’s normal to you. It’s the people who come to the farm—you see them holding their noses.”
The same can be said for people who have the farm brought to them, like Central Washington residents who are seeing dairy operations move to their part of the state.
“Odor is an important problem,” says Ndegwa, “Actually, it’s the most important problem for animal agriculture, because that’s what attracts attention to these facilities. So we try to keep an eye on it. When we do our outreach engagement, we try very much to sensitize producers that if you can keep odors down, you’re going to have less problems from citizens.”
When complaints emerge, sorting out the offensive odors from officially regulated odors can be problematic. The Environmental Protection Agency regulates individual gases like ammonia and hydrogen sulfide, which smell, but don’t necessarily account for the full body or aroma coming from, say, a feedlot.
“Odor is characterized by many gaseous emissions,” Ndegwa says. “It’s not just ammonia and hydrogen sulfide. So if you just measure ammonia and hydrogen sulfide, you can’t really say you’re measuring the odor.”
Ndegwa is currently studying whether anaerobic methane digesters, which convert manure and other waste to fuel, are a plus or minus in the odor equation. To that end, he and his team collect air samples in 10-gallon bags around a digester site and ship them overnight to Purdue University, where a panel of trained sniffers will gauge the contents’ intensity and offensiveness.
“That’s the most subjective,” Ndegwa says, “because you’re talking about how pleasant or how unpleasant a smell is. And this comes from your own experience. But with this particular panel, they’re trained so there’s at least some sense of homogeneity.”
In the end, there may be no such thing as an objective smell. But you could start looking for one on the top floor of Dana Hall. There, members of the Laboratory for Atmospheric Research tease out the contents of air samples with not one but two machines: a proton transfer reaction mass spectrometer and a gas chromatograph mass spectrometer.
The first machine adds a proton to organic compounds in an air sample and separates the compounds by their mass-to-charge ratio. It won’t identify specific compounds, but can get researchers in the neighborhood. Lukas Märk, CEO of this machine’s manufacturer, Ionicon Analytic, says food companies use it to quantify aroma and flavors, “sniffing” the aroma of instant soup, for example.
The second machine blasts molecules with an electron beam, producing ionized fragments whose unique spectral signatures can be matched to a National Institute of Standards and Technology library.
As a demonstration, Tom Jobson, an associate professor in civil and environmental engineering, opens a large brown-tinted bottle of methanol, the most abundant organic compound in our atmosphere after methane. I smell nothing.
But ten feet away, a small tube has picked it up and the proton transfer spectrometer is showing it as a spiking blue line on a graphic display. Jobson does a quick calculation and figures the spectrometer smelled about a quadrillion molecules.
We take turns breathing into the tube. He has more acetone in his breath. I have more methanol.
“You don’t have much isoprene,” Jobson says. “What I’ve come across is people who are missing gall bladders don’t produce much isoprene.”
Were I a smoker, the sensor would have picked up acetonitrile, a byproduct of burning vegetation, like tobacco.
“Insurance companies can nab you right away if you tell them you’re not a smoker,” Jobson says. “All they have to do is sample your breath.”
In some cases, the nose is more sensitive than the machine. But regulators need numbers, and the two spectrometers can give them that. The smells of individual chemicals will also be masked and altered as they combine and enter the body’s chemoreceptor-brain nexus.
“We can take a look at a mixture, separate it into its components, figure out what the concentration in the air was, figure out from the literature what the odor threshold is, and assess: OK, what part of this mixture are people really smelling?” says Jobson. “But that’s kind of the problem. You mix a lot of things together, you get a unique perfume.”
Two years ago, Jobson was part of a team of WSU researchers tapped to look into odors generated by industrial compost facilities. For the most part, the facilities deal with things whose smells we like: leaves, pine needles and branches, lawn clippings. But in certain quantities, like tens of thousands of tons, they can be a bit much, especially if you add food waste, which Seattleites now have to do.
Then things start to smell to high heaven, or at least to Marysville. Laurie Davies, program manager for the Department of Ecology Waste Resources Program, describes the smell on a bad day as “acrid” and “amazingly bad.”
The Laboratory for Atmospheric Research found the waste facilities were emitting formaldehyde, acetaldehyde, methanol, and benzene. They are hazardous air pollutants regulated by Ecology and the EPA, but it is unclear if they were at problematic levels. Davies says the samples are, in effect, “one data point.” Ecology now plans to do more sampling.
But the Puget Sound Clean Air Agency, meanwhile, can use a different sniff test.
In 2009 and 2010, agency inspectors responded to complaints from neighbors of Cedar Grove composting facilities in Maple Valley and Everett. Like moths flying upwind to a pheromone, the inspectors zig-zagged from the homes to the compost facilities and graded the smells, variously described as “sweet silage” and “putrid compost,” on a scale of zero to four. Zero is no smell; four is “so strong that a person does not want to remain present.”
All rated the smells as being at least two: “odor is distinct and definite, any unpleasant characteristics recognizable.”
The agency fined Cedar Grove $169,000, reduced in a settlement to $119,000.
The complaining neighbors won by the nose.
Joe Harding wasn’t really interested in olfaction.
But he was interested in how peptides could act as signaling molecules, and three decades ago, one of the best places to look at them was in primary olfactory neurons. His work led to a paper with Jay Wright in the prestigious journal Science showing how neurons in the main olfactory pathway of a mouse regenerated after being cut.
Harding has since moved on to other studies, including the development of a drug that builds new neuronal connections, a huge finding with implications for Alzheimer’s and Parkinson’s disease and traumatic brain injury.
But all these years later, he can easily sketch the bewildering cascade of processes behind odor signaling, or describe the signal’s journey over nerve fibers of the olfactory epithelium, through the bony cribriform plate into the olfactory bulb, where mitral cells carry signals to the rest of the brain for processing.
“Somewhere, and this is even more magic, somewhere all of that is integrated, and you say, ‘Oh, that’s amyl acetate, that’s the smell of banana,’” says Harding. “What? It’s an incredible, absolutely incredible feat of nature’s engineering.”
And here’s a particularly intriguing part: Signals go to the neocortex for storage and memory, but some go to the limbic system, home of the hippocampus, steward of short-term memory, and other brain structures loosely tied to emotionality.
“I think a lot of us have experienced this, where a smell triggers a really powerful memory and it’s usually an emotional memory,” says Harding. “I think that’s the dual effects of the hippocampal and limbic function working.”
Think of it as a value judgment system. In a modern world that prizes rationality, that seems counterproductive. But in an organism whose ancestry reaches across the eons, a fast, subjective sniff test could come in handy, if not keep you alive.
“That’s what emotion is, it’s a value judgment system,” says Harding. “How important was this to me? Did I even notice it or did I not notice it? If I noticed it, it was emotional in one way or another. It was either good or bad. That’s how the system works.”
Which brings us back to stores that smell good.
Eric Spangenberg first grew intrigued with the notion when he went into a wine store in Moscow, Idaho, and said to himself, “God, this place smells bad.”
But it sold cheese. The store was supposed to smell that way, a term researchers call congruence.
“I like to go to Les Schwab, smell the tires,” says Spangenberg. “Now if you went to Les Schwab and it smelled like the cheese store in Moscow, you’d say, ‘Oh, geez. I’m buying my tires at some other tire store. This place just didn’t wash its toilet or something.’ But if you went to the cheese store and it smelled like the tire shop, you’d say, ‘I don’t want to buy my cheese here.’”
Smell-oriented marketing isn’t all that new. Real estate agents have been baking cookies and cinnamon rolls before open houses for years. But Spangenberg took a scientific approach, comparing cookie-scented home sales to cinnamon-scented home sales, so to speak.
It’s still hard to say what was going through the Swiss shoppers exposed to the simple scents and the more complicated ones. Mapping brain activity with functional magnetic resonance imaging, or fMRI, might help, and Spangenberg is considering it. For now, he thinks the more complicated scent simply took up too much of some shoppers’ mental bandwidth, overloading their minds.
Or, for that matter, their limbic systems. It’s possible that they didn’t actually think that hard about their purchases as much as had a certain feeling about them, and that feeling was supported by a clean, fresh scent. For some, when it came time to spend a bit more, it just smelled right.
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