CHAPTER 4

The Art of the Sniff

The smoke of my own breath;

Echoes, ripples, buzz’d whispers, love-root, silk-thread, crotch and vine;

My respiration and inspiration, the beating of my heart, the passing of blood and air through my lungs;

The sniff of green leaves and dry leaves, and of the shore, and dark-color’d sea-rocks, and of hay in the barn…

—WALT WHITMAN, Leaves of Grass

SOME SMELLS ARE MORE SUBTLE THAN OTHERS. THEY float up the nose on the tidal rhythms of normal breathing and may not reach conscious awareness until minutes later. When we want to pay attention to an odor, we don’t wait for the next lungful of air—we capture it with a sniff. Sniffing is an odd behavior—it has no analog in vision or hearing. (Dogs, mice, and deer can rotate their external ears to focus on sounds; we can’t.) Sniffing is ignored by students of “body language.” It can be done covertly, and in polite company it usually is; sniffing is considered rude, and audible sniffing is downright vulgar. It takes an uninhibited, bumptious soul like Walt Whitman to draw attention to it, much less revel in it. But there is no getting around it; sniffing is essential. Whether one is tracking down a dead mouse in the basement or savoring a newly opened bag of Doritos, the sniff is the prelude to a smell.

The purpose of a sniff is to get scent molecules to the place where we can smell them. The question that took philosophers and scientists thousands of years to answer was, Where exactly does smelling happen? Some ancient Greek philosophers argued that it took place in the nose, but the sievelike appearance of the cribriform plate—a bone at the base of the skull just above the nasal passages—led others to speculate that odor particles made their way directly to the brain through these tiny holes. In this view, the nose is a merely a tube and the brain is the sensory organ. The ancient nose-versus-brain debate wasn’t settled until 1862, when a German anatomist discovered the olfactory nerve cells in a cleft high in the nasal passage. Smell—at least the first physiological contact with odor molecules—clearly happens in the nose. The holes in the cribriform plate are there to allow nerve fibers from the sensory cells to reach the brain.

Because the olfactory cells were tucked away in a narrow olfactory cleft, they did not appear to be exposed to the main flow of air through the nose. Researchers were soon asking how much of air entering the nostrils actually made it to the olfactory nerve endings. Early experiments were ingenious and also a bit macabre. In one study, for example, the head of a cadaver was cut in half and tiny squares of litmus paper were placed throughout the nasal passages. The head was reassembled and ammonia vapor pumped through the nostrils and out the trachea. Color changes in the papers showed that very little ammonia-laden air made it to the sensory cells; most passed through the lower passages. A second, more grotesque experiment anticipated the slice-and-shock art of Damien Hirst by a century. A split cadaver head was pressed against a glass plate and smoke was blown into the nostril. Observers could see the currents and eddies as the smoky air flowed through the complex folds of the nasal chamber. The smoke patterns, like the ammonia vapor, showed that only a fraction of the incoming air made it to the receptors.

Today, sophisticated computer models can simulate nasal airflow. Researchers can see where the flow is laminar (smooth) and where it is turbulent. They can calculate how many scent molecules are deposited onto the sensory surface as air is drawn across it. For all the high-tech apparatus and numerical precision, the modelers reach the same conclusion as their head-splitting predecessors: only about 10 percent of inhaled air blows across the nerve endings in the olfactory cleft.

 

THE SNIFF—a short inhalation with a high rate of airflow—is an essential step in odor detection. By forcing more air past the olfactory cleft, we take a bigger sample of the external smellscape. So how did it come to be dismissed and even suppressed by serious scientists? This is a strange tale. The first scientist to pay much attention to sniffing was also the one who tried to eliminate it from smell experiments. In 1935, Charles A. Elsberg was a highly regarded neurological surgeon in New York with a flair for invention—he designed surgical instruments and had performed the first successful removal of a herniated spinal disk. Elsberg’s flair for promotion was even bigger. He had cofounded the Neurological Institute of New York, set up the country’s first Neurosurgery Service there, and later cofounded the Society of Neurological Surgeons. At the age of sixty-four, it occurred to Elsberg that brain tumors, by exerting pressure on the olfactory areas at the base of the brain, might lead to impaired odor perception. If he could measure odor sensitivity, he might be able to identify patients with brain tumors. Accordingly, he came up with a method that involved a bottle, a cork, a syringe, and some rubber tubing. The patient would hold his breath and Elsberg would inject odorized air into his nostril. Acuity was measured by how big a blast of air was needed for the patient to detect a smell. Elsberg found that a normal person needed six to nine cubic centimeters’ worth. Elsberg’s system was coldly efficient; it not only eliminated sniffing, it eliminated breathing.

Elsberg touted his method as a major breakthrough: the first scientifically objective measurement of odor sensitivity. He either didn’t know of, or didn’t care to acknowledge, the olfactometer invented thirty years earlier by Hendrik Zwaardemaker. Every sensory psychologist in America was familiar with Zwaardemaker’s device, and most had one in the laboratory. It consisted of a glass sampling tube, curved at one end to fit into a nostril. A wider tube, containing an inner layer of scented material, fit snugly over the sampling tube. The farther the wide tube was pulled back, trombone-like, off the end of the sampling tube, the more scented surface was exposed. Sensitivity was measured as the length, in centimeters, that the scent tube had to be withdrawn in order to create a detectable level of odor. Zwaardemaker’s device, of which several versions were available, was reliable enough to explore the basic phenomena of odor perception and was used in laboratory demonstrations in colleges across the country. Nevertheless, Elsberg’s results were soon written up in Time magazine and on the front page of the New York Times. In the latter, the headline read, “Brain Tumors Detected by Scent with Device Keener Than the X-ray; Neurologists Hail Dr. C. A. Elsberg’s Discovery as Epochal—Based on Accurate Measurement of Sense of Smell, Which Was Viewed as Impossible Heretofore.” According to the credulous report in the Times, “Dr. Elsberg succeeded for the first time in measuring what had hitherto been considered universally as unmeasurable. He established a definite ‘scent yardstick.’”

Having nine cubic centimeters of air rammed up one’s nose is no barrel of laughs. However, blast injection proved to be a popular technique: most scientists prefer tight experimental control, even when precision comes at the cost of realism. Eventually researchers grew skeptical about the Elsberg method. They found that blast volume mattered less than blast force—this undercut the use of volume as a measure of smell ability. Even more troublesome, blast force was irregular—it depended on how abruptly the experimenter released the pinchcock on the rubber tube. The enthusiasm for nostril-blasting ended in 1953 when a psychology professor at UCLA compared odor sensitivity measured by Elsberg’s method and by natural sniffing. Blasting produced unreliable data, while natural sniffing produced very reliable data. The results blew Elsberg out of the water. Blast injection was not the scent yardstick he claimed it was. As the syringes and hoses were packed away for good, another psychologist ruefully wondered whether “we might be better off today if Elsberg had never publicized his creation.”

Mr. Natural: Keep on Sniffing

The physical characteristics of a sniff are smell dependent. Confronted with a weak scent, we take larger and longer sniffs, and more of them. We take smaller, shorter, and fewer sniffs to a strong odor. Considering how essential sniffing is to smelling, one might think this behavior would be studied by many scientists. Yet the bulk of what we know about sniffing is largely thanks to the work of one person, the Australian psychologist David Laing. He pioneered the natural history of the sniff.

In a series of elaborate studies beginning in 1982, Laing established how the dynamics of sniffing relate to smell. He controlled what people smelled with an air-dilution olfactometer, a device that generated a stream of air with precisely controlled odor levels. He measured how they sniffed by means of an oxygen mask with a tiny airflow probe concealed in it.

Laing found that natural sniffing took place in an episode of three and a half sniffs on average; some people used fewer, some many more. A person’s sniff episodes have a characteristic pattern that is stable across different odors and tasks. Sniff patterns were so stable and individually distinctive that Laing found he could identify a person by airflow data alone. He went so far as to liken sniff patterns to fingerprints.

At the time of Laing’s work, I was beginning my first experiments on human odor perception at the Monell Chemical Senses Center in Philadelphia. My odor sources were plastic squeeze bottles with fliptop caps. I would sit behind a screen and hand one bottle at a time to my test subject, who would squeeze, sniff, and rate the odor. As I listened to the wheezing of the bottles, I realized each person had a typical sniffing style. I soon developed a private taxonomy of sniffers. There were the Delicates, who took tiny, barely audible sniffs. There were the Honkers—people who squeezed the hell out of the bottle and inhaled so forcefully I thought they might hurt themselves. I also observed different psychological profiles. There were Decisives—people who sniffed and promptly announced their rating—and there were the Agonizers, who sniffed and resniffed and sniffed again before summoning up a rating. Every combination of behavior and decision-making style turned up in my lab: Delicate sniffers who were very decisive, Honkers who were Agonizers, and so on. These patterns were so consistent that after two or three squeeze bottles I could predict how long the entire test would take. A diverse range of local oddballs answered our recruiting ads. Once, in the middle of a test, my research assistant handed a sample of patchouli around the screen. There was some squeezing and sniffing, followed by a long silence. Finally she looked around to find that her subject had poured the sample into his hand and was massaging it into his beard. He said he liked how it smelled.

Intuitively, it seems the more one sniffs, the better one smells. Like dogs at a fire hydrant, multisniffers must be extracting every last bit of information from a smell. But are they? David Laing systematically controlled sniffing to see how it affected a person’s ability to detect and describe a smell. Sometimes he allowed his subjects to sniff with their natural pattern; other times he told them exactly how many sniffs to take, how long to wait between sniffs, or how big a sniff to take. When subjects were limited to a single sniff, they took one that resembled the first in a natural sniffing episode. Whether the sniff was the first-and-only or the first-of-many, it did not appear to vary with odor strength. After many experiments he could state his findings in a nutshell: “a single natural sniff provides as much information about the presence and intensity of an odour as do seven or more sniffs.” A natural first sniff can’t be beat. (For the technically minded, the optimum sniff has an inhalation rate of 30 liters per minute, a volume of 200 cubic centimeters, and a minimum duration of .40 to .45 seconds.)

There are two aspects to sniffing that are reflected in how we use the verb “sniff.” It can refer to a purely mechanical act (the drawing of air “through the nose with short or sharp audible inhalations”) or to an olfactory experience (“to smell with a sniff or sniffs”). The dictionary’s dichotomy between physical and sensory sniffing is programmed into the central nervous system at a profound level. The brain is not a passive recipient of smells drawn up the nose; it actively manages the acquisition of odor by the nose, and it does so on a time scale of milliseconds.

UC Berkeley smell researcher Noam Sobel was puzzled to find smell-related activity in the cerebellum, a brain area principally involved in tactile discrimination and the control of motor movements. When he and his lab team followed up, they discovered that two parts of the cerebellum were involved in sniffing. One was a smell-activated area; it lit up when a person smelled an odor. The stronger the odor, the greater the activation. Normally this area is activated in the course of sniffing scented air. Sobel found it was also activated by passive smelling, where odors were puffed into the subject’s nose through a tube while they held their breath. The second area of the cerebellum is sniff-activated; it lights up during the physical act of sniffing, but not during passive smelling. The sensation of air flowing through the nose explains the activation in the tactile part of the brain. When topical anesthetic was applied to a subject’s nasal passages to numb the nose, brain activity plunged. Together, two brain areas adjust sniff size to odor strength. This feedback happens very quickly: less than two-tenths of a second into the sniff. (By measuring with far greater precision than was available to Donald Laing, Sobel’s group found that the first sniff of a series was not fixed—only its first 160 milliseconds were.) As a strong odor is detected, the cerebellum signals the respiratory muscles to throttle back on the sniff. What appeared at first to be anomalous brain activity led Sobel and his team to a new understanding of how the brain shapes our perception of smell. The cerebellum is doing what it excels at: monitoring sensory input (in this case odor strength), in order to control a motor action (inhalation).

 

SO CLOSELY IS sniffing tied to odor perception that people routinely sniff when they are asked to imagine a smell. Without prompting, they take larger sniffs when imagining pleasant odors and smaller ones when imagining malodors. During visual imagery the eyes explore an imagined scene using the same scan paths made when viewing the actual visual scene. Preventing eye movements during visual imagery—by having people stare at a stationary target—reduces the quality of the image. Sobel found that, similarly, imagined odors were more vivid when people could sniff than when they were wearing nose clips and unable to sniff. Actually sniffing increased the unpleasantness of an imagined bad smell (urine) and increased the pleasantness of a good one (flowers). Sniffing at an imaginary odor isn’t an absentminded habit—it’s a behavior that improves the mental image we are trying to create. Sobel’s claim that “the sniff is part of the percept” would have outraged Charles Elsberg, but it sounds reasonable to most neuroscientists today.

We have in fact done a complete about-face since Elsberg’s attempt to measure smell without sniffing. Because smelling is sniffing, we can now test odor perception by measuring sniffing alone. We can take advantage of the fact that people naturally and unconsciously take smaller sniffs when an odor is present: the stronger the odor, the smaller the sniff. People with no sense of smell fail to adjust; they keep inhaling as if the air were unscented. A new smell test, developed by University of Cincinnati psychologists Bob Frank and Bob Gesteland, is simplicity itself. The patient wears a pair of standard-issue medical nose tubes connected to an electronic console, and sniffs at half a dozen cylinders in a row. That’s it—test over. No need to identify smells by name, no multiple-choice questions, no rating scales, no fancy odor generators. Here’s how it works: Each cylinder is the size of a can of beans and may or may not contain a slightly unpleasant odor (in pilot testing, Frank and Gesteland used methylthiobutryate, which has the character of feces, putridity, decay). The test console records airflow into the patient’s nose and computes the size of each sniff. It compares sniffs made when the patient was smelling scented cylinders with those made to an empty cylinder. If the two types of sniff are of similar size, the patient almost certainly has an impaired sense of smell.

Remedial Sniffing

We have glasses to help those with defective vision, hearing aids for the partly deaf, and who now will produce an artificial device to improve the smelling ability of people with subnormal noses?

—Popular Science Monthly, 1931

If perception and sniffing are inseparable, what happens to people who can’t sniff? The most extreme case of nonsniffing is the person with a total laryngectomy, or removal of the voice box (larynx), a procedure that disconnects the upper and lower respiratory airways. After laryngectomy, a person breathes through a hole in his throat, rather than through the mouth or nose, so he is unable to sniff or even activate his vocal cords to speak. Adding to their misery, about 85 percent of these patients are smell-impaired. Fortunately, some can be helped by a simple physical maneuver that resembles a polite yawn, or in other words, yawning with the mouth closed. This pseudo-sniff technique pulls air through the nose (though not the lungs) and allows about 50 percent of patients to score in the normal range on a smell test. A device called a tracheostomy valve, which directs exhaled air upward past the vocal cords and into the back of the nasal passages, restores speech function and also improves odor perception.

Impaired sniffing also occurs in Parkinson’s disease and contributes to the smell loss found in these patients. Because the disease affects motor movement, the sniffs of a Parkinson’s patient are weak and small. The worse their sniffing, the worse their performance on olfactory tests. The patients with the worst deficits can improve their test scores by simply taking bigger sniffs. While part of the problem lies in the physical action of the sniff, Parkinson’s patients often develop cognitive impairment, which registers on smell tests; in fact, smell deficits are an early symptom of the disease.

A 1996 U.S. patent describes a device to help the sniff-impaired. It resembles a double-ended turkey baster, with the bulb in the middle equipped with one-way valves. The user positions one end of the device over, say, a bowl of chili, then squeezes and releases the bulb, and it fills with air. Now the user inserts the other end in his nostril and squeezes again, forcing a bulb full of chili-scented air up his nose. The device is sort of an Elsberg self-blaster, a nose trumpet for the hard of smelling.

Boosting nasal airflow even improves odor perception in normal people. The Breathe Right nasal dilator was first marketed in 1993 to help reduce snoring by increasing nasal airflow, but got attention as an athletic aid the following year when Herschel Walker of the Philadelphia Eagles wore one for the first time in an NFL game—he had a cold. When Jerry Rice of the San Francisco 49ers followed suit, the Breathe Right gained locker-room cred, and commercial success followed in drugstores across the country. The dilator is placed on the bridge of the nose just above the fleshy portion of the nostrils, where it exerts a springlike action that prevents the sides of the nasal vestibule from collapsing inward during an indrawn breath. (The nasal vestibule is the space behind the opening of the nostril; it’s the finger-pickable part of the external nose.) Testing shows that wearing a dilator makes odors smell stronger, improves odor identification ability, and helps the wearer detect an odor at significantly lower concentrations. These benefits are the result of more air getting up into the nose. The nasal dilator increases the intensity of food aromas in the mouth but, weirdly, decreases the pleasantness.

 

THE ACT OF SNIFFING, overlooked by many scientists and politely ignored by well-mannered people, is critical to how we generate a mental image of the smellscape. The rapid sampling of odor-laden air is managed by a precisely timed interplay of sensory and motor function. In many instances, sniff improvement results in smell improvement. Seventy years after Charles Elsberg set out to suppress the sniff, we have finally begun to appreciate its value.

Even as it makes midsniff adjustments to the smell stream entering the nose, the brain is actively fine-tuning the mental impression it creates from an odor through a process called adaptation. Everyone is familiar with visual adaptation: after being in bright sunlight, it takes a minute or two for your eyes to adjust as you enter a darkened room. The reverse happens when you leave a movie theater in midday: the sunlight is unbearably bright at first, but gradually you adjust. Olfactory adaptation works on a similar principle: a new odor smells strong when we first experience it, but the longer we’re exposed to it, the more it fades into the background. In the extreme, the smell may be undetectable for a while.

It’s easy to overstate the practical importance of this phenomenon. Adaptation is a temporary change; it doesn’t permanently erase the ability to smell. Fragrances are not written in disappearing ink: if women stopped smelling an eighty-five-dollar perfume within a few days of buying it, the fragrance industry would have collapsed long ago. The extent of adaptation depends on the nature of the smelling being done. Perfumers I know insist they can only smell half a dozen fragrances before they notice a dulling of perception. For these professionals, olfactory fatigue is a real obstacle. They sample trial perfumes from blotters, five-inch strips of filter paper dipped in the liquid. The professional takes a quick sniff or two and moves the blotter away, ever conscious of overdoing it.

In contrast, an amateur sniffer holds the blotter in front of his nose and inhales continuously, a sure-fire way to dull the nose. Even one minute of such deep breathing makes an odor immediately harder to detect. When I run a consumer smell test, I let the panelists sniff at their own natural pace. I’ve found they can easily assess a couple of dozen scents without a noticeable decline in performance. That’s because they are sampling a variety of scents and doing so to make a quick thumbs-up or thumbs-down opinion—the typical objective of consumer and market research. This poses much less risk of adaptation than does the perfumer’s repeated study of minor differences between related samples. The average person making rapid-fire judgments does not need to worry about the smellscape fading from view.

 

THE LONGER YOU are exposed to an odor, the more you adapt to it. Step into a garlic factory and the reek will overwhelm you. A few minutes later its intensity fades, and after an hour you might not be able to smell garlic at all, no matter how hard you try. Work there a few months and this adjustment will happen almost as soon as you step in the door. That was how I once became oblivious to Safari. Early in my career, the company I worked for was developing the perfume for Ralph Lauren. As we tweaked the formula, ran stability tests, corrected the color, and did the million other chores needed to ensure a successful launch, the entire building was steeped in Safari. A few weeks into the job, none of us noticed it.

After a long vacation, I opened my closet to grab a suit for work, and got an overpowering faceful of Safari. The sensory truce between my nose and my workplace had fallen apart in less than two weeks. Similarly, long-term adaptation is what keeps plumbers and pig farmers from going insane.

Adaptation is a two-way street: when the odor source is removed, the nose gradually regains its sensitivity. This time-course of recovery is almost the mirror image of adaptation. Step outside after your visit to the garlic factory, and the recovery begins. If you were inside for just a few minutes, recovery will take a matter of minutes. If you were there for hours, it will be hours before full response returns. Odor strength is another factor in adaptation. The stronger the smell, the more you adapt. Ten minutes on the processing floor of the garlic factory will cause more adaptation than ten minutes talking to someone with garlic breath.

Adaptation is also odor-specific. If you work in a garlic factory, your nose will selectively tune out garlic, but your sensitivity to roses, sour milk, beer nuts, and other un-garlic-like smells will be unaffected. The narrowness of adaptation is sometimes exploited by perfumers when they try to match one fragrance to another. A perfumer will use saturation sniffing as the final step in comparing the target and the make. He sniffs the sample to the point of total adaptation, then smells the target; with his brain filtering out any sign of the original, any remaining minor differences will stand out.

Adaptation is a useful feature of any sensory system; it preserves our ability to detect small differences between stimuli against enormous variation in overall intensity. Just as auditory adaptation lets us have a whispered conversation but also talk in the middle of a rock concert, olfactory adaptation constantly recalibrates our noses to background conditions. It also selectively tunes new smells into the background, freeing our attention for the next new scent that may be creeping our way.

The Spin Doctors

In a lecture hall at the University of Wyoming in 1899, a chemistry professor named Edwin E. Slosson played a prank on one of his classes. He explained that he wanted to demonstrate the diffusion of odor through the air. He poured some liquid from a bottle onto a wad of cotton, making a show of keeping it away from his nose. He started a stopwatch and told the students to raise a hand as soon as they smelled something. Here’s what he reports happened:

While awaiting results I explained that I was quite sure that no one in the audience had ever smelled the chemical compound which I poured out, and expressed the hope that, while they might find the odor strong and peculiar, it would not be too disagreeable to anyone. In fifteen seconds most of those in the front row had raised their hands, and in forty seconds the “odor” had spread to the back of the hall, keeping a pretty regular “wave front” as it passed on. About three-fourths of the audience claimed to perceive the smell, the obstinate minority including more men than the average of the whole. More would probably have succumbed to the suggestion, but at the end of a minute I was obliged to stop the experiment, for some on the front seats were being unpleasantly affected and were about to leave the room.

Slosson’s experiment vividly demonstrated the potency of olfactory suggestion, for he was holding a cotton ball soaked in nothing but water.

The sensory expert Michael O’Mahony revisited the phenomenon in the late 1970s. During a British television documentary on taste and smell, he showed viewers an electronic device that he claimed could capture and broadcast odors using “Raman Spectroscopy.” The machine played a ten-second audio tone that viewers were told would evoke a “pleasant country smell.” They were encouraged to call in or write and describe what they smelled. Many did. They reported smelling new-mown hay, freshly cut grass, lavender, honeysuckle, and so on. O’Mahony repeated the trick on a BBC radio show using a supposedly inaudible “ultra high frequency tone”—actually no sound at all. Some listeners reported smell sensations when it was played.

While amusing, these stunts by Slosson and O’Mahony raise serious questions for scientists conducting smell studies, because they show that just expecting a smell can trigger an odor perception. Thus a purely psychological expectation might have the same consequences as a real smell. For researchers the question becomes, How can we be sure the results of an odor experiment are really due to the smell and not to expectations about the smell? What is needed is an olfactory placebo: a test condition in which people are led to believe an odor is present when in fact it is not. To truly have an effect, an odor must outperform the placebo. This was the reasoning behind a study I did with Susan Knasko, a postdoctoral fellow of mine at the Monell Center, and the late John Sabini, a psychology professor at the University of Pennsylvania. We sprayed water mist in the air and told people it had a smell. The test room was actually scent-free and remained so. People who were told the smell was unpleasant later rated the room as smelling bad. When told the smell was pleasant, they liked the smell of the room. A supposedly “neutral smell” produced intermediate results. Interestingly, physical symptoms such as headache and itchy skin were also affected by the “good smell” and “bad smell.” Our study was the first to confirm in the laboratory that the power of suggestion, by itself, could produce odorlike effects.

The psychologist Pamela Dalton and her colleagues took this result and pushed it much further: they showed that expectations alter the perception of actual odors. She had volunteers sit in a test chamber for twenty minutes while exposed to odors that were neither pleasant nor unpleasant. Some subjects were told nothing about the odor. Others were told it was a potentially harmful industrial chemical or, alternately, that is was a distilled, pure natural extract. To use the Clinton-era term for expectation management, the experimental conditions differed only in spin. By the end of the test, all three groups had higher detection thresholds—their noses had been dulled by adaptation to the real odor. However, their perception of odor intensity was spin-dependent. With positive spin or no spin at all, the odor seemed less intense as time went on; with negative spin it smelled as strong or stronger. In other words, odors we think are benign fade from awareness, while those we believe to be hazardous hold our attention and stay strong.

It may not even matter whether the actual smell is good or bad. Spin can alter these perceptions as well. Dalton tested odors that were pleasant (wintergreen), unpleasant (butyl alcohol, a solventlike smell), and neutral (isobornyl acetate, a balsamlike note). Negative spin made all three smell stronger. Information bias is very effective at distorting the clear evidence of our senses—the brain easily trumps the nose.

Biasing information doesn’t have to come from an authority figure in a lab coat. Dalton tested two people at a time in the environmental chamber. One was an unsuspecting volunteer, the other a carefully scripted actor pretending to be naive. The actor kept up an ongoing verbal and behavioral commentary about the odor in the air. This peer-to-peer kibitzing worked splendidly. When the spin was negative, 70 percent of volunteers reported health symptoms (everything from throat irritation to dizziness to stomachache); when it was positive, only 12 percent did so. Given a scent in the air—any scent—acquaintances can literally talk you into feeling sick.

The commonly acknowledged power of scent derives in large part from the power of suggestion. Negative placebo effects may exacerbate the symptoms of “sick building syndrome”—for example, if you believe that the musty smell in your office is from a toxic mold—while positive placebo effects explain the popularity of aromatherapy treatments. Beneficial mood change is one of the biggest claims made for aromatherapy. For example, lavender is usually extolled as relaxing and neroli as stimulating. A recent study showed that positive spin can completely reverse the aromatherapeutic effects of these two scents. When told the lavender they were smelling “has relaxing properties,” people did in fact relax, as measured by changes in heart rate and skin conductance. Yet when told it “has stimulating properties,” the same measures showed—presto change-o—that people were stimulated. The same reversal happened with neroli. It takes only the slightest waving of hands to create a positive placebo effect in aromatherapy.

The effects of spin often play out in everyday life. When the crew of a Norwegian air ambulance noted a cabbagelike smell in flight, they figured the patient they were transporting had passed gas and they ignored it. When the smell reappeared on another flight later that day, the crew was puzzled; it was unusual for two patients to be so extraordinarily gassy. Soon flames were shooting through the cockpit and the pilots were forced to make an emergency landing. The fartlike smell was smoldering insulation on electrical wires. The crew was in a medical mind-set, not a mechanical one, and their preexisting expectations led to a near-fatal misreading of what their noses were telling them.

Smells don’t happen to a passive nose alone. The brain actively regulates the physical and cognitive aspects of odor perception: it exerts moment-by-moment control of sniffing to govern how much scent enters the nose; it systematically dials down the intensity of one smell to prepare us for the next; it automatically makes a provisional interpretation of a smell, based on context cues, to prime us for a behavioral response. From sniff to spin, the nose and brain constantly reshape our awareness of the smellscape.