The Science (and Neuroscience) of Scent

I stumbled upon a fascinating article on the science of scent, how we smell what we do, why we have scent differences, anosmia, and olfactory neuroscience. That led me down a rabbit hole to additional articles on: the shape and movement of odor molecules; the link between taste and smell; how scent, emotion, and memory are intertwined; “Super Tasters;” the role of ageing and even mental health; biophysiological differences; and so much more.  As I have plenty of time while waiting for the drydown of an attar test to finish, I thought I would share with you what I learnt.

Source: Thought Co.

Before I get to the Smithsonian essay which triggered this post, let me share basic background details from other articles on odor molecules, olfaction, the human body, and our ability to detect odors.

A Scientific American article from 2016 entitled “What a Smell Looks Like” on creating smellbots (yes, you read that correctly) provides some introductory explanations on scent beginning with the structure of odor molecules:

[S]ociety knows relatively little about the mechanics of how we smell. We know the nose is packed with olfactory neurons—gatekeepers that distinguish scents—but relatively little is known about what comes next. Which area of the brain pinpoints the location of a mother’s freshly baked cookies? Or distinguishes the smell of banana from that of peanut butter? Or what mind center judges the concentration of gas as it leaks from a stove? […]

An odor is a chemical molecule light enough to be swept around by the environment. Scents travel through air or underwater, before ultimately tripping sensors in our noses—known as olfactory neurons[Emphasis added by me.]

Photo: my own.

Aaron True, a postdoctoral researcher, has built the mechanical version of a nostril using a small tank and a tube in order to understand the movement and structural length of odor molecules. He is quoted in the Scientific American piece as saying:

“As you inhale, the odor essentially gets stretched out, so you end up with very thin regions of strong concentrated smells,” True said. “But then right next to it, you’ll have a region with a very low signal, very low odor.”

So when you’re in a garden and you stop to smell the roses, every sniff you make is changing the aroma for someone else. Inhalation creates large, blank pockets of space inside an odor cloud. These voids are known as intermittency, and they’re the olfactory equivalent of negative space in a photograph.

Now that we know more about the basics of odor molecules, let’s move onto our nose which is, obviously, the most important part of our body when it comes to scent. A 2021 Thought Co article by Regina Bailey entitled “The Olfactory System and Your Sense of Smell” explains in detail:

Our sense of smell is a complex process that depends on sensory organs, nerves, and the brain. Structures of the olfactory system include:

Source: Radiopeadia

  • Nose: opening containing nasal passages that allows outside air to flow into the nasal cavity. Also a component of the respiratory system, it humidifies, filters, and warms the air inside the nose.
  • Nasal cavity: cavity divided by the nasal septum into left and right passages. It is lined with mucosa.
  • Olfactory epithelium: specialized type of epithelial tissue in nasal cavities that contains olfactory nerve cells and receptor nerve cells. These cells send impulses to the olfactory bulb.
  • Cribriform plate: a porous extension of the ethmoid bone, which separates the nasal cavity from the brain. Olfactory nerve fibers extend through the holes in the cribriform to reach the olfactory bulbs.
  • Olfactory nerve: nerve (first cranial nerve) involved in olfaction. Olfactory nerve fibers extend from the mucous membrane, through the cribriform plate, to the olfactory bulbs.
  • Olfactory bulbs: bulb-shaped structures in the forebrain where olfactory nerves end and the olfactory tract begins.
  • Olfactory tract: band of nerve fibers that extend from each olfactory bulb to the olfactory cortex of the brain.
  • Olfactory cortex: area of the cerebral cortex that processes information about odors and receives nerve signals from the olfactory bulbs.

Graphic from “Textbook OpenStax Anatomy and Physiology,” published May 18, 2016. Souce: Wikipedia

A 2020 Harvard Gazette article entitled “What the nose knows” explains the link between taste and smell, a link that is closely intertwined with the pleasure of eating, as we all know from when we are sick and lose some of our sense of taste because our noses are congested. The Harvard article elaborates on the biophysical structural link:

When you chew, molecules in the food, he said, “make their way back retro-nasally to your nasal epithelium,” meaning that essentially, “all of what you consider flavor is smell. When you are eating all the beautiful, complicated flavors … they are all smell.” Murthy said you can test that theory by pinching your nose when eating something such as vanilla or chocolate ice cream. Instead of tasting the flavor, he said, “all you can taste is sweet.”

Tongue papillae density of a Super Taster vs a Non-Taster. Source: Harvard.

On a related note, people categorized as “Super Tasters” probably have a heightened sense of smell. (Ergo, Super Smellers of a kind.) I looked into this a while back when a friend who is a Master Sommelier — a profession wherein scent and taste are closely intertwined — told me she was convinced I was a Super Taster because of the acuity of my nose.

Harvard’s School of Public Health has an article called “Super-Tasters and Non-Tasters: Is it Better to Be Average?” that discusses the issue in addition to providing more details on human beings’ ability to detect odors in general. To wit:

In contrast to the small number of basic tastes, humans are able to recognize more than 10,000 different odors. Unlike taste, humans are amazingly sensitive to smell.

  • We are able to detect the aroma of certain volatile compounds at the level of one part per trillion, and a few at levels even 1000 times lower. To give you a better “sense” of what this means, one part per trillion is equivalent to one second in 32,000 years!

  • Our exquisite sense of smell apparently evolved to help in locating food as well as avoid consuming spoiled food before tasting it.

  • You may have experienced your sensitivity to smell when you detected a natural gas leak. Gas companies add a trace of a very smelly volatile sulfur-containing compound called methyl mercaptan to natural gas so we can detect even very small leaks. Humans are able to detect this compound at 2 parts per billion, which is a very small amount, but still 1000 times more concentrated than one part per trillion.

  • Some of the compounds we can smell at levels of a part per trillion and lower include those in green bell pepper, mold, roasted oats, and, the record holder, another sulfur-containing compound formed in boiled seafood.

Returning to interplay between scent and taste, the Harvard School of Public Health Super Taster article explains further just HOW we smell things in food:

We sense the smell of food by two routes. Sniffing through our nose is called orthonasal smell, while the aroma released up through the back of our mouth into our nose when we chew and swallow food is called retronasal smell. Orthonasal and retronasal smell appear to be processed in different parts of the brain. The latter is the most important route for sensing the aroma of food and is believed to account for as much as 80-85% of the flavor of food (2). That explains why we can’t detect the flavor of food when we have a cold and our nose is blocked.

The taste and aroma of food are sensed through special receptors (proteins) on the surface of taste and olfactory cells in our mouth and nose. They provide a direct link between our brain and the outside world.

  • While the number of taste receptors is limited, it is estimated there are about 400 different types of receptors for smell.

  • Cells that contain the receptors for taste and smell are replaced every 10-30 daysAs we age the total number of these cells decline, especially after age 70.

  • Hyposmia, a reduced ability to smell and detect odors, is a common feature in some neurodegenerative conditions, such as Parkinson’s disease.  Individuals with Parkinson’s disease often experience a reduction in their sense of smell a few years before the appearance of the characteristic motor symptoms that lead to the diagnosis. (4)

  • Taste cells are clustered together in taste buds located throughout the mouth and back of the throat in structures called papillae. These are the visible bumps on your tongue. [Emphasis in bold added by me.]

Source: BBC

Scent is closely intertwined with memory and emotion as well. The Thought Co article by Regina Bailey entitled “The Olfactory System and Your Sense of Smell” explains the link between our nose and different parts of our brain:

Our sense of smell works by the detection of odors. Olfactory epithelium located in the nose contains millions of chemical receptors that detect odors. When we sniff, chemicals in the air are dissolved in mucus. Odor receptor neurons in olfactory epithelium detect these odors and send the signals on to the olfactory bulbs. These signals are then sent along olfactory tracts to the olfactory cortex of the brain through sensory transduction.

The olfactory cortexis vital for the processing and perception of odor. It is located in the temporal lobe of the brain, which is involved in organizing sensory input. The olfactory cortex is also a component of the limbic system. This system is involved in the processing of our emotions, survival instincts, and memory formation.

Source: Discover Magazine

Source: BrainFacts.org

Sense of Smell and Emotions

The connection between our sense of smell and emotions is unlike that of the other senses because olfactory system nerves connect directly to brain structures of the limbic system. Odors can trigger both positive and negative emotions as aromas are associated with specific memories.

Additionally, studies have demonstrated that the emotional expressions of others can influence our olfactory sense. This is due to activity of an area of the brain known as the piriform cortex which is activated prior to odor sensation.

The piriform cortex processes visual information and creates an expectation that a particular fragrance will smell pleasant or unpleasant. Therefore, when we see a person with a disgusted facial expression before sensing an odor, there is an expectation that the odor is unpleasant. This expectation influences how we perceive the odor. [Emphasis via bolding added by me.]

The previously discussed Harvard article “What the nose knows” explores the issue of scent and emotion in greater detail, scent and memory, scent and colour, and even the role of olfactory branding. For example, Nike‘s signature scent “was inspired by, among other things, the smell of a rubber basketball sneaker as it scrapes across the court and a soccer cleat in grass and dirt.” For help creating this, it turned to Dawn Goldworm, co-founder and nose, or scent, director of what she calls her “olfactive branding company,” 12.29. Ms. Goldworm elaborates on the connection between colour, memory and scent:

During the talk she explained that smell is the only fully developed sense a fetus has in the womb, and it’s the one that is the most developed in a child through the age of around 10 when sight takes over. And because “smell and emotion are stored as one memory,” said Goldworm, childhood tends to be the period in which you create “the basis for smells you will like and hate for the rest of your life.”

She also explained that people tend to smell in color, demonstrating the connection with pieces of paper dipped in scents that she handed to the audience. Like most people, her listeners associated citrus-flavored mandarin with the colors orange, yellow, and green. When smelling vetiver, a grassy scent, audience members envisioned green and brown.

Source: snhwallpapers.com

A 2017 Discover magazine article entitled “The Sense of Smell in Humans is More Powerful Than We Think” (one-time free reading link) also discusses the link between scent and emotion as it pertains to mothers and new baby scent:

If you’ve ever thought there is something special about the smell of babies, you’re right. In 2013, scientists from Germany, Canada and Sweden took fMRI scans of 30 women while they sniffed the cotton undershirts of newborns. The new moms’ thalamus lit up more than that of women without kids, suggesting the mothers’ increased attention. All the women showed activity in the brain’s neostriate areas, where the reward system lies.

Photo by Sickova Tatyana via Shutterstock

The fresh scent of newborns activates the same biological mechanism in women as a baby’s “very round eyes, the round face, the cute voice,” says Lundström, who was involved in the study. It is nature’s way of bonding mother and child. Although only women were tested in that particular study, Lundström suspects that similar results would be found in men.

For now, researchers haven’t managed to pinpoint the molecules responsible for that new-baby smell. Lundström and his colleagues have some chemicals under the microscope (figuratively and literally), and are even researching whether the newborn smell could be used to treat depression. The team is also investigating whether women who suffer postpartum depression lack receptors for newborn scent molecules or don’t receive the reward signals from the baby smell.

Source: hoax-slayer.com

The importance of our ability to perceive and experience the world through scent received heightened attention after Covid led people to lose their sense of smell. However, anosmia, as it is called, is something that afflicted a portion of the population long before Covid-19. A June 2022 article in Nature magazine entitled “How to bring back the sense of smell” discusses olfactory loss and the new focus on how to bring it back.

Although loss of smell received little public attention before COVID-19, “the reality is smell loss is something that is quite prevalent”, says Aria Jafari, a rhinologist at the University of Washington in Seattle. Between 5 and 15% of the general population, and nearly half of those between the ages of 65 and 80, have experienced impairment of their sense of smell.

The treatment for smell loss and the chances for recovery both depend on its cause. Ageing is one cause, chronic rhinosinusitis is another. There are treatments for the latter. “Loss of smell can be an early symptom of neurodegenerative diseases such as Parkinson’s disease or Alzheimer’s disease. There is no treatment for this form of smell loss, which probably involves the same mechanisms that cause damage elsewhere in the brain.”

A researcher operates an olfactometer that is connected to another person via electrodes and a nasal probe

One way to recover after temporary or Covid-related anosmia involves the basics of training one’s nose:

Training the nose

The mainstay of treatment for post-COVID smell loss is olfactory training — a procedure that many rhinologists compare to physical therapy for the nose. Individuals are instructed to sniff a sequence of four essential oils, deeply inhaling each one for 15 seconds while concentrating on their memory of the corresponding smell. They repeat the procedure twice a day over the course of months.

Olfactory training has its roots in the established understanding that the sense of smell is mutable and improvable. For example, people in professions that involve smell — sommeliers, perfumers, tea merchants and chefs — often develop keen noses, says Thomas Hummel, an olfaction researcher at the Technical University of Dresden in Germany, who conducted some of the first studies of the procedure in the 2000s4.

You can read more about the olfactory training treatment program, anosmia, hyposmia, phantosmia, and parosmia in the lengthy piece, if you’re interested. I want to move onto what triggered my descent down the olfactory neuroscience rabbit hole.

How Do You Smell? | Science | Smithsonian Magazine

Smithsonian magazine‘s October 2022 issue has a lengthy, detailed article entitled “Sniffing Out the Science of Smelling – From the lab to the art gallery, the latest efforts to understand the fragrant, musky, stinky and utterly baffling world of your nose.” Written by Abigail Tucker, the article begins by focusing on Dr. Andreas Keller, a neuroscientist turned olfactory art gallery owner, who explains many of the basics of scent, scent perception, the diverse personal responses to a scent from one person to another, and much more. (I was interested to learn that “In one 2011 survey, more than half of young adults admitted that they would rather forfeit their ability to smell than their smartphones.”)

“At a New York City gallery called Olfactory Art Keller, smelling is believing. Touching this painting by Luiza Gottschalk releases aromas of a pasture and a wet night.” Photo by Caroline Tompkins via the Smithsonian article.

At his NYC olfactory art gallery, visitors who “ponder a scent by the name of ‘Sinner’ often bicker about what they are smelling—is that hospital disinfectant? Church incense? No: tandoori chicken!”

These diverse personal responses are exactly what Keller has devoted much of his academic life to studying: why various individuals seem to perceive the same odors differently, or interpret the same odors in different ways, or not notice some of them at all. Such questions are at the heart of the larger problem of how smells are coded by the brain. How, for example, do a bunch of snorted-up proteins and carbohydrates result in our tangy awareness of just-baked sourdough, that pandemic crowd-pleaser?

Well, I’ve covered that a little already in the earlier section of this essay, but it all goes back to “how the nose—or rather, the clump of sensory neurons high inside your nostrils—boasts about 400 receptor types.”

In 2004, Richard Axel and Linda Buck were awarded the Nobel Prize in Physiology or Medicine for discovering the genes that encode these receptors, which sit on the surfaces of specialized neurons, allowing them to shoot information down leggy axons much deeper into the brain, to one of the two olfactory bulbs where smells are processed. But we still can’t fathom which odors many of the individual receptors recognize, let alone how they work in concert.

Scent is, as noted earlier, comprised of odor molecules. For example, the smell of a banana “is a conglomerate of something like 20 or so molecules, each destined for overlapping sets of receptors, activating some and perhaps inhibiting others, and striking all chords in the concerto that we somehow register as banana.”

Androsterone via Wikipedia

But our receptors may not work like someone else’s:

you can expect about 30 percent of your receptor arsenal to function differently than your neighbor’s, which explains why conversations starting with “Do you smell that?” so frequently devolve into farce. A substance like androsterone—a musk found in human sweat, truffles and elsewhere—can smell like sandalwood or urine or nothing at all, depending on the nature of the smeller. For a long time researchers believed that “asparagus pee” wasn’t universal, because only certain people reported a malodorous bouquet after consuming the spiky veggie. In fact, only some unlucky noses can detect it, but for those who can, the odor is universal—a fact those subjects realized “only when they smelled each others’ urine,” Keller says. Even professional perfumers sometimes need a helping nose when it’s time to dose the cinnamon, for instance.

I was amused to read the various odors which are captured and stored at Philadelphia’s Monell Chemical Senses Center, particularly “a man-made compound called ‘U.S. Government Standard Bathroom Malodor,’ based on real-life military latrines and colloquially known as ‘Stench Soup.’” More traditional olfactory materials are also stored there, like ambroxan, my nemesis galoxolide, guaiacol, and various musks. Musks are one of the categories of raw materials which some people cannot detect easily and it turns out that the Smithsonian reporter was one of them.

I’ll let you read more, if you’re interested, into research into musk sensitivity referenced in the piece, as I’d like to move onto a study between Keller and the Philadelphia Morell Center on the genetic and olfactory perception differences between 1,000 Han Chinese and 100s of New Yorkers. The gist of the outcome:

while the study revealed these genetic and sensory differences between people, the variations did not shake down along ethnic lines. Rather, all of the key receptor types were sprinkled throughout New York and China alike. This is probably because our odorant receptor variants are quite ancient, with the mutations predating the relatively recent divisions between Asian and European and African populations.

Many variables beyond receptor type influence how, whether, and what people smell. Some factors, for example, are:

Biological sex is one—females are more sensitive to smells and better attuned to stenches like body odors. Mental health is in the mix, with some conditions, like autism, linked to an enhanced sense of smell, and others, like Parkinson’s disease and depression, related to reduced sensitivity. [Bolding emphasis added by me.]

There is a wealth of additional details in all the articles referenced and linked above if you’re interested in learning more. The Smithsonian one has quite a lengthy section on Covid’s impact on scent, if that is something which you’ve experienced, as well as olfactory neuron research and elaboration on the role of scent in triggering memories.

I want to end this post with a few related and completely unrelated points. The first pertains to the issue of universality. If there is one thing that the research I’ve shared shows is that there is no universal truth when it comes to what something “smells like.” This is a sore subject for me and one which I’ve mentioned a few times in the past because I frequently encounter blanket statements that seem to suggest there is “One Scent Fits All” or, to put it another way, “this is what a fragrance smells like.” No, it is what it smells like to you and on you based on a multiplicity of factors, including poorly-known ones like your biophysiology and genetics.

So when guys write to me asking “What is the best masculine fragrance?”, I’m always floored by the underlying assumption that there is one calculable, definitive answer. There is not. Not ever.

Similarly, when someone dismisses what another person smells or when they claim that someone is a bullshitter in detecting olfactory nuances that they personally did not, the underlying assumption is that we have the identical scent capability. We do not. We cannot all be the same even biologically — from the number of papillae that you may have on your tongue to your roughly 400 different types of scent receptors, your skin chemistry, the state of your health, the impact of ageing on your odor sensitivity, and the way your brain filters and processes odor molecules.

On the plus side, however, as one of the articles elaborates with regard to olfactory nose training following Covid-related anosmia, we can always improve or heighten our ability to detect various odors. If you’re beyond the basics listed in my “Beginner’s Guide to Perfume,” you can take the next step by daily scent therapy where you buy raw materials by category and genre and follow a daily training regimen.

There are a few places that I would recommend when looking for raw materials. One is Eden Botanicals which focuses on naturals and botanicals essential oils because they have a section of samples by aroma family. They offer international shipping. Another is The Perfumer’s Apprentice which offers Educational Kits that go beyond naturals and also include aromachemicals. They, too, ships worldwide. Given the shipping problems faced by the UK after Brexit, UK readers may want to turn to Pell Wall which offers a variety of perfume-making raw materials kits, organized by scent category or type, that you can use to train your nose in addition to making perfume.

On that note, I shall be off to layer and sniff more attars. Have a good weekend, everyone.

8 thoughts on “The Science (and Neuroscience) of Scent

  1. Great article! While I do think we are all on a spectrum when it comes to how well we smell scents, I know that when I had to be trained to describes flower scents in my employment it really opened up the world of scent for me and I became much better at detecting scents. Some of the improvement came from not being able to smell as much as being able to describe what I was smelling. Although I think what improved the most was my ability to distinguish the various layers of scent coming from a flower. My job was to write copy on rose scents. I am still amused when people describe rose as a single scent as there are so many complicated scents in roses and so many rose to smell.

    • Hi, welcome to the site. I very much agree that learning to describe what you’re smelling tends to hone and even heighten one’s ability to detect odor nuances. Completely true! The reasoning is similar to why I would recommend a note exercise that forced people to really analyze and categorize what they loved and hated as notes in perfumer. Questions 9 and Questions 10. I think the very act of analysis creates better neural connections or more immediate neural links between our scent receptors, the odor molecules we’re picking up, how we describe them, the nuances we perceive, how we feel, etc.

  2. Pingback: A Beginner's Guide To Perfume: How to Train Your Nose, Learn Your Perfume Profile, & More – Kafkaesque

  3. As always, thank you for another fascinating and educational post. I always brew up some tea or coffee and settle down to enjoy one of your posts. I’ve now realized there are some musks I can’t smell, but alas I am assailed when anyone around me eats asparagus. My partner thinks I’m a Super Smeller.

    • How funny about your intense reaction to asparagus, Shiva. I’ve never heard that one before. I wonder how you’d react to the infamous durian, lol. xD

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