Thursday, August 4, 2022

Termite NASCAR - Bic Pens and Ants

2-phenoxyethanol is the chemical name of a solvent that helps ink dry quickly, but also mimics an ingredient in termite "trail pheromone;" it helps them follow each other. 

It's used in both blue and black ink, and from either Bic or Papermate pens. Its primary use is in middle school science projects, to teach kids how pheromones work. You use this pen to write your name on a piece of paper, then drop some termites on there, and watch as your name is spelled in ants. 

The Trail Pheromone of the Termite, Trinervitermes trinervoides. Tschinkel, Walter R.; Close, Peter G. (1972). J. Insect Physiol. Vol 19. pp. 707-721.

^This 1972 study says that "activity loss from filter paper is approximately exponential with a half-life of about 2 hr" in case you were wondering.

The Identification of 2-Phenoxyethanol in Ballpoint Inks Using Gas Chromatography/Mass Spectrometry – Relevance to Ink Dating. Laporte, Gerald & D Wilson, Jeffrey & Cantu, Antonio & Amanda Mancke, S & L Fortunato, Susan. (2004). Journal of Forensic Sciences. 49. 155-9. doi:10.1520/JFS2003217

^And why is this article published on the ASTM website? And with all the references listed as working for United States Secret Service, Forensic Division?  Because you can tell how old a document is by measuring the amount of this chemical that is left in the ink. 

Post Script:
Just pheromone things - "Seducin" - Some male cockroaches and crickets produce a pheromone called seducin from their bodies, on which the females nibble during copulation. This pheromone is an aphrodisiac. 

Tuesday, July 26, 2022

Avery Gilbert and the Terpene Revolution

He's calling the terpene revolution "the nucleus of the brand new field of cannabis psychophysics" (First Nerve, Feb 2021) and I can't argue because he is the first, and when you're the first, you get to name things.

Here's a quick run-down of Avery Gilbert's work circa terpenes since 2018. (Note that he was the first person to get federal approval for olfactory research on pot.)

Consumer perceptions of strain differences in Cannabis aroma, Feb 2018

The smell of marijuana (Cannabis sativa L.) is of interest to users, growers, plant breeders, law enforcement and, increasingly, to state-licensed retail businesses. The numerous varieties and strains of Cannabis produce strikingly different scents but to date there have been few, if any, attempts to quantify these olfactory profiles directly. Using standard sensory evaluation techniques with untrained consumers we have validated a preliminary olfactory lexicon for dried cannabis flower, and characterized the aroma profile of eleven strains sold in the legal recreational market in Colorado. We show that consumers perceive differences among strains, that the strains form distinct clusters based on odor similarity, and that strain aroma profiles are linked to perceptions of potency, price, and smoking interest.

Use of rating scales versus check-all-that-apply ballots in quantifying strain-specific Cannabis aroma, March 2019

Previous research using a check-all-that-apply (CATA) method to describe the strain-specific aroma of dried Cannabis flower revealed two major clusters, one characterized as woody, earthy, herbal and the other as citrus, lemon, sweet, and pungent. In this study, participants rated 10 strains (including seven strains not previously tested) using numeric rating scales and a slightly smaller set of odor descriptors. The results confirm the two major scent clusters, and indicate a possible intermediate cluster differentiated by a skunk note. We observed systematic variation in the use of descriptors and rating scales: evaluators who used more odor descriptors tended to assign higher scale ratings. Nevertheless, the CATA and rating scale methods yielded similar results.

Human olfactory detection of packaged cannabis, March 2020

Olfactory detection of cannabis aroma by police officers can be the basis for warrantless searches of motor vehicles in many jurisdictions in the United States. The odor source in these cases is often dried cannabis flower contained in various casual wrappings as well as in more elaborate packaging. Here we investigate whether packaging format alters the detectability of the cannabis. Two cannabis strains and five packaging formats were evaluated. Untrained observers were presented with two containers and asked to identify, based only on smell, the container that held a sample of packaged cannabis (the other container held identical, but empty, packaging material). The results showed that open and casually packaged cannabis was identified with high accuracy, while material packaged in doubly vacuum-sealed plastic was correctly identified at rates no different from chance. The results may help address issues involving the detectability of cannabis aroma in law enforcement and other scenarios.

Tuesday, July 19, 2022

Neural Waves Ahoy

We're getting a lot of amazing brain data from epilepsy science these days (like the first evidence of brain death under EEG). 

Because epilepsy patients undergo an entire week of EEG monitoring prior to treatment (so the doctors can "get familiar with" their brainwaves), other scientists ask to bother them with experiments that have nothing to do with epilepsy. Like olfaction experiments. So the patients volunteer to have the data from their brainwave monitoring used by researchers while they squirt smell molecules at their face. 

Olfactory processing in three distinct neural waves
Feb 2022,

Now we ask whether different oscillations represent distinct features of an odor, or if different odors are represented by different oscillations," Zelano said

They're talking about neural oscillations. 

Neurons in visual and auditory systems usually operate at a background hum of excitability, but when the brain is trying to see or hear something, these neurons are activated in sync.

But the olfactory cortex is hard to study because it's literally in the center of the brain (and that's because it is like the seed from which our big ass brain grew out of). Brainwaves can be detected non-invasively, so that's great. 

The low-frequency oscillations, termed theta waves, begin immediately after a volunteer sniffed and ended immediately afterwards. Theta waves were followed by two more sets of waves, beta (about 12-30 Hz) and gamma waves (above 30 Hz).

This raises the possibility of a two-step process, where the low-frequency waves "prime" the olfactory cortex and the high-frequency waves are responsible for olfactory processing.

"Low-frequency waves are used for communications between brain regions and high frequency oscillation is more involved in local computations, but it's very exciting to find a low-frequency oscillation motivating a high-frequency oscillation," said Guangyu Zhou, Ph.D., research assistant professor of Neurology and a co-corresponding author of the study.

Oh but this part is even better:

Further, the strength of the high-frequency waves was associated with volunteers' ability to correctly identify odors.

"This implies the higher-frequency oscillations are required to actually distinguish the odor one is smelling," Qiaohan Yang, MS, student in the Northwestern Interdepartmental Neuroscience Program (NUIN) and lead author of the study.

via Northwestern University's Comprehensive Epilepsy Center: Qiaohan Yang et al, Smell-induced gamma oscillations in human olfactory cortex are required for accurate perception of odor identity, PLOS Biology (2022). DOI: 10.1371/journal.pbio.3001509

Post Script, On Epilepsy:
Who knew that treating epilepsy would lead to such novel discoveries? Why is C. elegans or D. melonigaster so important for specific things, or how is the naming of the limbic system itself a kind of word-monster that grew out of our heavy reliance on rats during the concurrent explosion of olfactory science in the Behavioral era, and rats have a brain that is dominated by olfaction, and it basically controls the movements of their body, hence their limbs, and so the olfactory system was called the limbic system. Why rats? Why fruitflies? Why epilepsy? 

Life may actually flash before your eyes on death
Feb 2022, BBC News

First-ever recording of a dying brain discovered by accident. 

"This was actually totally by chance, we did not plan to do this experiment or record these signals."

This is also one of the reasons why it is important to care for every human equally, regardless of what happened to them. You're born without an immune system? We're keeping you alive as long as we can. Paraplegic? We're giving you wifi for your body

You have epilepsy? We're going to slap some electrodes to your head and monitor your brainwaves for a really, really long time, and figure out how to help you. Unless you have a heart attack in the headset, in which case we'll watch what happens, and use your accident to further the advancement of science. 

via Department of Neurosurgery, Henan Provincial People’s Hospital, Division of Neurosurgery, Vancouver General Hospital: Vicente Raul et al. Enhanced Interplay of Neuronal Coherence and Coupling in the Dying Human Brain. Frontiers in Aging Neuroscience 14 2022. DOI: 10.3389/fnagi.2022.813531.

Why C. elegans? They have only 302 neurons, that's why.

Tuesday, July 12, 2022

What Have We Become

It appears that we could be evolving to better tolerate each other's body odor by having our noses become less sensitive to that smell over time. And the guy in the picture above has been "evolved" to withstand a car crash.

Humans and other primates have evolved less sensitive noses
Feb 2022,

The purpose of this study was to see if the genetics for smell remain constant across people from different backgrounds other than the Caucasians typically studied. 

The results showed that yes, they do, but also something unexpected.

"People with the ancestral versions of the scent receptors tend to rate the corresponding odor as more intense." And in opposition, the "newer" versions of those receptors lead to people having less intense odor detection capacity. And this suggests that we are evolving to be less sensitive to odors. 

In order to test their hypothesis, they used odors that were already known to be variable in the ways people perceive them. For example, some smells are very intense to some people but barely perceptible to others. Some of this is because of genetic variations in the way the receptor works. 

An interesting aside in the discussion -- "OR51B2 variation drives differences in the perception of human body odor component 3-methyl-2-hexenoic acid (3M2H) ... which could be a target for future studies interested in malodor blocking, or discovering the mechanisms underlying social communication from body odor."

Back to the big part of this study, which is the unexpected part (always the favorite part of any scientific endeavor). They measured the "age" of these genes, and found that the "newer" genes were less sensitive to intensity, and refer to this as "Degeneration of olfactory receptor gene repertoires in primates."

Image credit: Graham is designed to survive a car crash, Victoria’s Transport Accident Commission, 2016. Designed by Melbourne sculptor Patricia Piccinini, Royal Melbourne hospital trauma surgeon Christian Kenfield, and crash investigator at Monash University’s accident research centre David Logan.

Large genetic databases can be used to understand OR function, a proxy for general protein function.

In the discovery study, we may have the benefit of measuring olfactory phenotypes in a large, homogenous cohort (Fig 1) where genome-wide genotyping had already been conducted, giving us the statistical power of a large population without the time or expense. In this study, the novel signals do not have much population differences in MAF or effect size (Table 1 and Figs 3 and 4), suggesting that the large sample size rather than its genetic similarity might be the more important reason behind the findings. Given the increasing number of open databases of sequencing data, this method is becoming a more reasonable possibility for easily testing genotype/phenotype associations.

Olfaction is an excellent use of this new resource because of the ease of understanding the functional output of genetic variation in the protein. 
The human olfactory system has both robust assays to test the behavioral output of these proteins (psychophysics/rating odors) [5,6,10] and an established method for directly testing protein function in cells (heterologous cell-based assay) [42,43]. Genetic variation provides a strong tool for exploring olfactory coding and sheds light on how complex systems integrate information from variable sensors.

via Chinese Academy of Sciences Key Laboratory of Computational Biology at Shanghai Institute of Nutrition and Health, Monell Chemical Senses Center, Department of Neuroscience at University of Pennsylvania, and Sanghani Center for Artificial Intelligence and Data Analytics at Virginia Tech:  Li B, Kamarck ML, Peng Q, Lim F-L, Keller A, Smeets MAM, et al. (2022) From musk to body odor: Decoding olfaction through genetic variation. PLoS Genet 18(1): e1009564.

Some interesting facts about the variation of olfactory perception among populations, most of which was already known, but now confirmed for a more diverse population that includes Han Chinese:
  • Galaxide, a Musk molecule: Individuals can have specific anosmias to one or some, but not all musks, suggesting that there is not a single common coding mechanism.
  • Trans-3-methyl-2-hexenoic acid (3M2H), a Body Odor molecule: Almost 25% of the population has a specific anosmia to 3M2H [23–26], but this anosmia has not been connected to any olfactory receptor.
  • Aldehydes: Self-reported Asian populations rate aldehydes as more intense than Caucasian populations, but no specific genetic variants or receptors have been implicated. 

These are the receptors studied and their effects:
  • OR4D6 M263T and S151T ^ Galaxolide intensity
  • OR51B2 L134F ^ 3M2H intensity
  • OR5A1 D183N ^ β-ionone pleasantness (for the validation cohort and the meta-analysis, but not the discovery cohort)
  • OR7D4 R88W and T133M ^ Androstenone intensity and pleasantness (in the discovery cohort, for the validation cohort, only pleasantness)
  • OR2J3 T113A ^ Cis-3-hexen-1-ol intensity
  • OR1A1 =/= Caproic acid (although rs17762735 was associated with intensity in the validation study, the effect was in the opposite direction from the literature; there were no associations for 
  • pleasantness)
  • Aldehyde - There were no associations with aldehyde intensity or pleasantness 

On Body Odor:
3-methyl-2-hexenoic acid (3M2H) is also referred to as caproic acid, and as having a "hircine" odor, both of which refer to goats, because it smells like goats. Which means you smell like goats when you're hot and nervous and not wearing deodorant (although less likely if you're of Asian descent for whom one gene changes the production of body odor). Body odor in general is often characterized by thiolalcohols, which have sulfur molecules in them, although this one in particular doesn't have any sulfur in it. 

Related Post:
Social Deodorization

Tuesday, July 5, 2022

Fruit Flies Forever

Human sense of smell resembles that of insects
Oct 2021,

Good, because we would really like to use insect antennae to better understand human olfaction. They're easier to do experiments on, because their system is more simple than ours in many ways. Also good because the fruit fly is where so much smell science comes from.

They modeled the brain of a cotton bollworm so they could inspect its operations, and found they're a pretty good match for humans (minus the phermomones, of course). This is good for helping us understand the inner-workings of a robust neural network, or should we call it the prototypical, the primordial neural network: 

"We find striking similarities in the structure and function of the olfactory system across different organisms," says Xi Chu, a researcher in NTNU's Department of Psychology and senior author of the new publication. The similarities are probably related to the fact that the olfactory system is evolutionarily the oldest of all sensory systems. ... "It's worth noting that the primary olfactory center in the mammalian brain is located only one synapse away from the outside world," says Dr. Chu. "This means that the incoming information goes directly into the primary olfactory cortex, unlike all other sensory signals, which travel through a different brain structure before dispersing to their respective cortical areas. -Steinar Brandslet, medicalxpress

via Norwegian University of Science and Technology's Chemosensory Lab: Jonas Hansen Kymre et al, Distinct protocerebral neuropils associated with attractive and aversive female-produced odorants in the male moth brain, eLife (2021). DOI: 10.7554/eLife.65683

Image credit: Antenna of a male moth by Dr. Igor Siwanowicz at the Howard Hughes Medical Institute in Virginia for the 2015 Nikon Small World Photomicrography Competition [link]

Post Script:
Mapping the olfactory system in fruit flies
Feb 2022,

They describe the fly's olfactory system as having "the ability to make quick assessments of odors in an unusual way that circumvents synaptic communication, which is metabolically expensive."

They have created a map of receptors based on variations in the functionality of the molecules, but with one extra step -- the activation-inhibition dynamic at the neuron level.

This is a feature of the olfactory system that has been researched a lot lately (see this post for example). It also sounds like the model for neuromorphic processor systems, where the advanced processing via a dedicated cortex is eschewed for a complexity-based, emergent phenomenon at the neuron level.

via University of California - San Diego: Shiuan-Tze Wu et al, Valence opponency in peripheral olfactory processing, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2120134119

Tuesday, June 28, 2022

Navigating the Information Gradient

Olfaction is so primitive in its function, that it's an ideal model for all kinds of things,  including navigation, but even moreso, information processing. The olfactory system might be the most effective information processing system we know of, and it's something we've barely begun to investigate. 

Chemotaxis doesn't make headlines often, but it should, because it's ultimately an information-processing problem (and the last time I checked, we were living in the Information Age).

Image credit: A smellmap of Amsterdam by Kate McLean circa 2017 at

Information processing constrains how E. coli bacteria navigate chemical gradients
Jan 2022,

Information that E. coli bacteria gather from their environment limits their performance at chemotaxis, the process by which they guide their movements in response to chemical signals.

And it's funny that they decided to use chemotaxis to test this, about using information efficiently, so in other words, chemosensation is a good model for testing and understanding how information is processed, biomimetically, if you will.

And why do we care? Because chemotaxis and olfaction are the same, at a primitive level. Not much has changed between the way E. coli navigates its environment and the way we do it.

"We wanted to test a broad biological hypothesis: that organisms make the best use of the information they acquire to perform behaviors and other functions. To investigate this, we needed a behavior simple enough that we could quantify how much information it needed and chemotaxis by the bacterium E. coli is a perfect example of such a behavior."

We realized we could measure the amount of information a bacterium was able to gather (in bits per second), while also understanding how much information they would need to navigate at the speeds observed."

To achieve this, they first set out to calculate the theoretical performance limit, which is the maximum speed at which a bacterium could navigate up a chemical gradient, based on a fixed rate at which it acquires information about chemical signals.

Finding the response strategy that maximized gradient-climbing speed with a fixed information cost resulted in the performance limit.

"We found that while climbing shallow gradients E. coli get very little information from their environment, about 0.01 bits/s.

via Yale: H. H. Mattingly et al, Escherichia coli chemotaxis is information limited, Nature Physics (2021). DOI: 10.1038/s41567-021-01380-3

Understanding how bacteria seek out and move towards food
Feb 2022,

Chemotaxis is the process of attraction in the direction of a chemical gradient. The primary way that organisms control their motion and progressively move toward a target is by inhibiting tumbling when sensing that the chemical concentration is increasing along their current direction.

The research team used stochastic optimal control theory (instead of linear control theory) to find the best possible fully nonlinear sensing and control strategy of run-and-tumble motion (of E. coli) in environments with noisy chemical gradients.

And it looks like chemotaxis, which is the progenitor of olfaction. It is not a stretch to say that olfaction is a form of chemotaxis, and we move through a room to locate a source by using the pattern of its vaporized chemical essence in the air in the room. We calculate its distribution pattern (by stochastic optimal control theory, apparently^), predict the source, and move towards it, updating as we go. The only difference here is that we use legs, and a pretty complex limbic system, whereas E. coli just tumbles and tumbles in the chemovoid. 

via University of Tokyo Institute of Industrial Science: Kento Nakamura et al, Optimal sensing and control of run-and-tumble chemotaxis, Physical Review Research (2022). DOI: 10.1103/PhysRevResearch.4.013120

Odour-Spatial Map - Diogo Matias - Champalimaud Foundation - 2021 [link]

Neurons in the olfactory cortex link smells to places
Feb 2022,

Sometimes it's good to have someone else say things like this, for a change: 

The researchers focused on the primary olfactory cortex. "The olfactory system is unique among the senses," said the study's senior author, Zachary Mainen, a principal investigator at the Champalimaud Centre for the Unknown in Portugal. "Only olfaction has direct reciprocal connections to the hippocampal system, which is involved in memory and navigation."

It looks like neurons in the posterior piriform cortex (part of the primary olfactory cortex) are encoding place information just like hippocampal cells, and especially behaviourally significant spots. So it's real -- smells are not just smells, they are places and smells at the same time; we can't extricate them from each other, at least not for some brain cells.  

via Champalimaud Centre for the Unknown: Cindy Poo, Spatial maps in piriform cortex during olfactory navigation, Nature (2021). DOI: 10.1038/s41586-021-04242-3

Post Script:
How the brain navigates cities: We seem to be wired to calculate not the shortest path but the 'pointiest' one
Oct 2021,

When people navigate through a city, they use not shortest path, but instead, pedestrians appear to choose paths that seem to point most directly toward their destination, even if those routes end up being longer, and this is called vector-based navigation.

via  Massachusetts Institute of Technology: Paolo Santi, Vector-based pedestrian navigation in cities, Nature Computational Science (2021). DOI: 10.1038/s43588-021-00130-y

Thursday, June 23, 2022

The Smell of Fear

Protecting gardens and crops from insects using the 'smell of fear'
Aug 2021,

They're using methoxypyrazines, such as isopropyl methoxypyrazine, isobutyl methoxypyrazine and sec-butyl methoxypyrazine. Methoxypyrazines smell like "green, herbaceous, vegetative, green peppers, freshly cut grass, and asparagus." 

But for aphids, methoxypyrazines smell like ladybugs. Aphids hate ladybugs. And farmers hate aphids. 

On a related note, the smell of cut grass is a defense mechanism for grass to tell other grass that it's being attacked, and to "brace yourselves." The next time you smell it, you can think of the sound of grass screaming.  

via American Chemical Society: Smell of fear: Harnessing predatory insect odor cues as a pest management tool for herbivorous insects, ACS Fall 2021.

Tuesday, June 21, 2022


This story keeps getting crazier the further you read. 

Woman with no left temporal lobe developed a language network in the right side of her brain
Apr 2022,

A woman without a left temporal lobe developed a language network in the right side of her brain that allowed her to communicate normally. She only came to realize she had an unusual brain by accident—her brain was scanned in 1987 for an unrelated reason. 

By all accounts she behaved normally, earning an advanced degree and excelling in languages—she speaks fluent Russian—which is all the more surprising considering the left temporal lobe is the part of the brain most often associated with language processing.

It was likely the woman had lost her left temporal lobe as a child, probably due to a stroke. The area where it had been had become filled with cerebrospinal fluid. To compensate, her brain had developed a language network in the right side of her brain that allowed her to communicate normally.

She also had a sister who was missing her right temporal lobe, and who also had no symptoms of brain dysfunction—an indication, the researchers suggest, that there is a genetic component to the stroke and recovery process in the two women.

via MIT, Carnegie Mellon University, UCLA and Harvard: Greta Tuckute et al, Frontal language areas do not emerge in the absence of temporal language areas: A case study of an individual born without a left temporal lobe, Neuropsychologia (2022). DOI: 10.1016/j.neuropsychologia.2022.108184

See this similar study on smell:
Left-handed women can have normal olfaction without apparent olfactory bulbs, and scientists are very confused.

via the Weizmann Institute: Human Olfaction without Apparent Olfactory Bulbs. Tali Weiss, Noam Sobel, et al. Neuron, volume 105,  issue 1, page 35-45.E5, Jan 2020. DOI:

Thursday, June 16, 2022

Word Avoidance

Did you just say what I think you said?

No, because this tree actually smells like semen, but nobody will ever say that out loud, or in print, so instead they call it rotting fish.

Tree that smells like rotting fish is so invasive states are offering bounties to kill it
Dec 2021, Bangor Daily News

And they're talking about the Bradford pear tree, which sucks as a tree, and is a great example of how good marketing can destroy the planet (or at least your charming downtown neighborhood).

Somebody was really successful circa the 1990's at selling these trees to urban planners looking to revitalize their downtowns with nice shade trees that have infrastructure-friendly root structures and "they even bloom pretty white flowers!" (that smell like a handful of hot semen for a couple weeks out of the year). The problem is that they only live about 30 years, they're brittle so their limbs break off easily, they're invasive so they make the soil inhospitable to other plants, and they multiply ferociously when you try to cut them down. Also their flowers smell like semen when they bloom, but nobody will tell you that, because nobody wants to say out loud that anything smells like semen. In fact, if you, for example, were anosmic to semen, you might go your entire life without knowing, because nobody ever talks about it. 

The Victorians would have called it "a man smell."

Post Script:
Also see the Tree of Heaven (Ailanthus altissima), which is another invasive and foul-smelling tree.

Monday, June 13, 2022

On Hedonic Consultation

AKA The Evolution of the Autobiographical Odor Encyclopedia 

This study copied below measures how fast we detect good smells vs bad smells (spoiler, bad smells are detected faster). 

But while reading through this, consider that bad smells can become good over a series of exposures matched with good feelings. Aged cheese, fermented cabbage, and burned cannabis are pretty well known examples of this. There's also people, who smell, each with our own odor fingerprint, although we may not realize it at times, as it might be below our limit of detection.

And then there's the reverse, where things (or people) that once smelled good, all of the sudden smell bad, such as with changes in birth control, or pregnancy, or after a viral infection like Covid (see the parosmia triggers study). In those cases, the whole olfactory system is rewritten, a kind of blank slate re-learning, where smells with strong odor components (like individually unique body odors, or coffee) are perceived as if for the first time, with the bad stuff up front. And all you can focus on is the bad, since you have to "re-learn" the smell, and how the good integrates with the bad to produce something that is neither good, nor bad, nor even identifiable by semantic description, but only by the name of the person. 

Nonetheless, there seem to be some good millisecond metrics here:

Seeing how odor is processed in the brain
Jun 2022,

  • Detection occurred before the odor was consciously perceived by the participant
  • Odor information in the brain is unrelated to perception during the early stages of being processed
  • Later, unpleasant odors were processed more quickly than pleasant odors

The participants wore an EEG cap while having smells shot at their face, and so that researchers could see when and where odors are processed in the brain.

"We were surprised that we could detect signals from presented odors from very early EEG responses, as quickly as 100 milliseconds after odor onset, suggesting that representation of odor information in the brain occurs rapidly"

Remember that the olfactory system has only a few synapse-steps, making it the most direct sensory system we have.

And then watch how they pretty much rehearse Proust's deep cookie immersion:

When unpleasant odors (such as rotten and rancid smells) were administered, participants' brains could differentiate them from neutral or pleasant odors as early as 300 milliseconds after onset. However, representation of pleasant odors (such as floral and fruity smells) in the brain didn't occur until 500 milliseconds onwards, around the same time as when the quality of the odor was also represented. From 600–850 milliseconds after odor onset, significant areas of the brain involved in emotional, semantic (language) and memory processing then became most involved.

via University of Tokyo: Mugihiko Kato et al, Spatiotemporal dynamics of odor representations in the human brain revealed by EEG decoding, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.211496611

Post Script:
Professor Robert Sapolsky Stanford Lecture - On Recognizing Relatives (with smell)

Learning to Smell: Olfactory Perception from Neurobiology to Behavior, by Donald Alan Wilson and Richard J. Stevenson, Johns Hopkins University Press (2006)

Thursday, June 9, 2022

Covid's Parosmia Uncovered

Covid has a smell. Not that you can smell sick people, although you can actually. Dogs have proven that it's possible; only that humans have never been trained, and for obvious reasons.

Instead, we mean that things "smell like Covid." That's because everyone who got pre-Omicron Covid also got hit with a neuroplastic time bomb where their olfactory neurons got attacked and then reconfigured. Half the people who got Covid, and its anosmic introduction, knew they had it, and the other half didn't. (This according to a May 2022 study, linked here, and repasted below.)

Lost of people would argue with that stat. And I would have to argue back. The absolute worst source of data for testing anosmia etc is the subjective reporting of the people themselves. We don't even know we have a sense of smell in the first place, nevermind detecting that it's been removed. There are congential anosmics (can't smell from birth) who don't realize they're anosmic until they're teenagers! How do you not realize that? Because we don't talk about smells. They're outright lingua-phobic. And so when we lose it, especially in the midst of a respiratory infection that messes with our breathing, we don't even notice. We also think it's our sense of taste, so when someone asks "have you experienced any change in your sense of smell," you say no, but I did lose my sense of taste. In that study, they used Sniffin' Sticks, so an objective measure, and so they got the 100% stat. 

Next, things "smell like Covid" because after a bout of anosmia, your olfactory system needs to reboot, retrain, pick your computer analogy, and in the process, there's some bugs in the code. Eggs smell like Covid, coffee smells like Covid. Shit? Smells like biscuits. Wait, what? Yes, the Faeces Paradox, see below, it's all been explained for us. Thanks, Flavor Center at University of Reading:

Researchers find cause of disordered smell
May 2022,

Among the 29 volunteers with post-viral parosmia, scientists found 15 commonly identified compounds that triggered parosmia. They also found reduced sensitivity in some people, via lower TDI scores from Sniffin’ Sticks, although some who were considered functionally anosmic could still detect some of the trigger smells...

Some of the most cited food and drinks that set off parosmia in sufferers include:
  • Coffee
  • Onions
  • Garlic
  • Chicken
  • Green peppers

The most common trigger molecules are grouped into four distinct categories based on structure: 
  • thiols
  • trisubstituted pyrazines
  • methoxypyrazines
  • disulfides
  • (and as always with smell-things: some less common triggers did not fall into any one of these categories)
  • (these molecules tend to be potent, have very low olfactory detection thresholds and, in isolation, are neither distorted nor unpleasant for nonparosmics)

Trigger Molecules and their Parosmic Descriptions:
  • 2-furanmethanethiol ("coffee") is the most frequently reported trigger. Whereas NONPAR (non-parasmics) used a range of food-related terms to describe it (coffee, roasty, popcorn, smoky), PAR (parosmics) often struggled to find suitable descriptors, as they were unable to relate it to anything they had smelled before. PAR typically used words describing its hedonic quality (disgusting, repulsive, and dirty) or new coffee (relating to the altered smell of coffee since onset of parosmia) as described previously. Four PAR described it in the same way as NONPAR (biscuit, toasty or roasty) indicating that it is not universally parosmic, but certainly an important and frequent molecular trigger of parosmia.
  • 2-methyl-3-furanthiol and its corresponding methyl disulfide  ("meaty") were detected but reported less frequently as distorted. 
  • 2-Ethyl-3,6-dimethylpyrazine (also "coffee") was the second most frequent trigger in coffee; described with a variety of food terms by NONPAR, but by “new coffee”, “unpleasant” and “distorted” by PAR.
  • 2,3-diethyl-5-methylpyrazine, 2-ethyl-3,5-dimethylpyrazine and trimethylpyrazine (found in roasted, fried and baked goods) were common triggers. These compounds also triggered a parosmic response to cocoa, grilled chicken, and peanut butter
  • 2-Ethyl-3-methoxypyrazine, 2-isobutyl-3-methoxypyrazine and 2-isopropyl-3-methoxyprazine (green peppers) were common triggers in coffee.
  • 3-methyl-2-butene-1-thiol (pungent and weedy) was reported as a trigger 9/29 times.
  • 3-mercapto-3-methylbutanol and its formyl ester are potent aroma compounds in coffee, and were detected in half the cases, but only reported as distorted 5 or 6 times.

  • The unknown compound has been tentatively identified as 4-methylthio-4-methyl-pentan-2-one, but this is yet unconfirmed. (What the heck is "the unknown compound?)
  • Although thiols and disulfides seem to effectively trigger a parosmic response, there are two notable exceptions.
  • Methanethiol, detected by some NONPAR, was not detected by any PAR. Likewise, dimethyl trisulfide is detected by 12/15 NONPAR but only by 4 PAR, and only reported once as a trigger.
  • A few compounds were detected but never reported as triggers.
  • 4-Ethylguaiacol was detected by 7 PAR and always described as spicy, sweet and smoky, but never parosmic.
  • Similarly, (E)-β-Damascenone, a key odour-active compounds in coffee, was detected by 6 PAR and always described as jammy and fruity.

The Faeces Paradox
  • Foods smell of faeces yet faeces smell of food (biscuity or pleasant)
  • Two parosmic researchers did not detect these compounds in a faecal slurry and were unaware of any foul smells.
  • However, they detected several other compounds, many of which they had also detected in coffee, and only some of which triggered parosmia.
  • In comparison, a normosmic scored the intensity of indole and skatole as close to the strongest imaginable. 
  • This provides a neat explanation as to why the changes in valence for faecal samples is reversed. 

How did they do it?
They GCMSd different sources, like coffee or onions, so that the individual molecules could be separated and presented one-at-a-time to the volunteers, so they could detect the specific molecules in the source that repulses them.

Why did they do it?
Prior to the global pandemic caused by COVID-19, parosmia was a rare condition known to occur after infections such as cold, flu or sinus infections, with very little awareness about the causes and treatments for the disease.

During the pandemic COVID-19 symptoms included loss of smell and taste in 50–60% of cases, of which about 10% developed parosmia. Since the omicron variant, loss of smell and taste has become a less common symptom (estimated to occur in about 10–20% of cases) and parosmia cases are likely to be fewer in number, parosmia is still estimated to affect 2 million people in Europe.
Not exactly. See below.

via the Flavor Center at University of Reading: Jane K. Parker et al, Insights into the molecular triggers of parosmia based on gas chromatography olfactometry, Communications Medicine (2022). DOI: 10.1038/s43856-022-00112-9

Study finds sensory loss in ~100% of active COVID infections, which is twice as high as self-reports
May 2022,

In participants with active infections during the delta surge, a majority (22 of 25) had been vaccinated. Objective screenings found that 100% were experiencing a diminished or lost sense of smell—but only 54.5% self-reported any problem with odor detection.

via Ohio State University: Kym Man et al, Chemosensory losses in past and active likely Delta variant break-through COVID-19 cases, Med (2022). DOI: 10.1016/j.medj.2022.05.004

Image credit: Free Photos at img freepic dot com [link]

Post Script:
Can 'smell' trigger tumors?
May 2022,
"Now that glioma preferentially emerges in the OB, will neuronal activity in the olfactory circuit affect the emergence of glioma?" the researchers wondered. This "mind-blowing" flash of inspiration became a turning point in this study. The research team attested to this hypothesis through a series of experiments.

In this study, they employed a cutting-edge chemogenetic technology to specifically manipulate the neuronal excitability of ORNs. They found that inhibiting the activity of ORNs reduced the size of the tumor significantly, whereas activating their activity increased the size of the tumor. It was therefore concluded that the neuronal excitability of ORNs was the root of gliomagenesis.

To further verify this conclusion, the researchers suppressed olfactory inputs through naris occlusion by using small plugs. They found that with naris occlusion, tumors were significantly hindered in the olfactory bulb, indicating that olfactory stimuli could regulate gliomagenesis.

via Zhejiang University: Pengxiang Chen et al, Olfactory sensory experience regulates gliomagenesis via neuronal IGF1, Nature (2022). DOI: 10.1038/s41586-022-04719-9

Post Post Script:
Clinical trial led by Thomas Jefferson University Hospital paves the way for innovative topical treatment
Mar 2022, Jefferson Hospital

Platelet-rich plasma (PRP) is a common restorative therapy used to regenerate cells, heal tissue, and address an array of medical conditions from healing injured muscles and tendons to increasing hair growth and reducing the appearance of scars. Animal studies have shown that PRP helps regenerate the olfactory epithelium, which may be the site affected in COVID-19 induced olfactory dysfunction (OD). As smell and taste are closely interrelated, improved sense of smell can help with sense of taste as well. Until now, PRP has been used as a nasal injectable in several small clinical trials for smell loss. Although the results were promising, nasal injections can be uncomfortable and invasive for patients.

A recent phase I clinical trial of eight patients who had at least six months of olfactory disturbance has shown preliminary success with 50 percent of participants (4 people) experiencing clinically significant improvements in smell and taste.

Autopsies suggest COVID’s smell loss is caused by inflammation, not virus
Apr 2022, Ars Technica

via Johns Hopkins: Ho C, Salimian M, Hegert J, et al. Postmortem Assessment of Olfactory Tissue Degeneration and Microvasculopathy in Patients With COVID-19. JAMA Neurol. Published online April 11, 2022. doi:10.1001/jamaneurol.2022.0154

Monday, June 6, 2022

Consistently Inconsistent

What's in your weed? You might be surprised
May 2022,

  • 90,000 samples across six states
  • Leafly database of chemical analyses compiled from cannabis testing centers
  • Largest analysis to date of the chemical composition of marijuana products
  • Commercial labels "do not consistently align with the observed chemical diversity"

"Our findings suggest that the prevailing labeling system is not an effective or safe way to provide information about these products," said co-author Brian Keegan, an assistant professor of Information Science at CU Boulder. "This is a real challenge for an industry that is trying to professionalize itself."

"A farmer can't just pick up an apple and decide to call it a Red Delicious. A beer manufacturer can't just arbitrarily label their product a Double IPA. There are standards. But that is not the case for the cannabis industry," said co-author Nick Jikomes, director of science and innovation for the e-commerce cannabis marketplace

Products do tend to fall into three distinct categories:
  • high in the terpenes caryophyllene and limonene
  • high in myrcene and pinene
  • high in terpinolene and myrcene
  • (similar to Avery Gilbert's study; Citrus vs Earthy)
  • (but those categories do not neatly correspond to the indica, sativa and hybrid labeling scheme)

Some strains, such as one called White Tahoe Cookies, were surprisingly consistent from product to product, while others, such as one called Durbin Poison, were "consistently inconsistent," said Jikomes.

"There was actually more consistency among strains than I had expected," he said. "That tells me that the cultivators, at least in some cases, may not be getting enough credit."

via University of Colorado at Boulder: Christiana J. Smith et al, The phytochemical diversity of commercial Cannabis in the United States, PLOS ONE (2022). DOI: 10.1371/journal.pone.0267498

Thursday, June 2, 2022

Scientists Make Discovery

Olfactory neurons adapt to the surrounding environment
May 2022,

This title could have had way more clickbait. The news is that we have no idea what's going on with olfactory neurons. 

Not like it's ever been claimed; they just never seem to obey any sensical laws, and they're always doing things we can't understand, but now it seems that even basic assumptions about how they work are way off.

The scientists discovered an unsuspected variability in gene expression profiles depending on the expressed olfactory receptor and previous exposure to odors. 

And in a previous study, these scientists found that after stimulation of a receptor by an odorant molecule for less than an hour, the expression of the gene coding for this receptor decreased in the neuron, indicating a very rapid adaptation mechanism.

"While it was thought that the binding of an odorant molecule would only lead to the activation of the corresponding receptor, we discover that olfactory neurons drastically change their identity by modulating the expression of hundreds of genes after activation. And this new identity is again dependent on the expressed receptor. We are facing an unexpected, massive, rapid and reversible adaptation mechanism," explains Ivan Rodriguez, co-corresponding author of the study.

This work reveals that olfactory neurons are not to be considered as sensors simply passing from a resting state to a stimulated state, but that their identity is in permanent evolution, not only according to the expressed receptor but also according to past experiences. This discovery adds another level to the complexity and flexibility of the olfactory system. 

via Faculty of Science and the Faculty of Medicine of the University of Geneva: Luis Flores Horgue et al, Transcriptional adaptation of olfactory sensory neurons to GPCR identity and activity, Nature Communications (2022). DOI: 10.1038/s41467-022-30511-4

Image credit: Unexpected Outcome, by shironosov and Getty, 2022 - Three shocked scientists looking at the obtained substance expressing intense emotions [link]

Post Script:
Consider that the olfactory family of genes is evolving within us in real time. It's a huge group of genes, being the largest family there is, at 2% of the genome, and it's highly variable in the population, with a 30% variation from person to person. 

Do you remember that item we learned back when the Human Genome project came out, that we share over 90% of our genome with monkeys, and almost as much with bananas? Now consider that our individual olfactory apparatus is 30% different from that of other humans. There's a lot of action here in the olfactory system.

Just wait until we figure out that it's running our immune system, because that's when things will get really crazy. 

Friday, May 13, 2022


The world of smell goes unnoticed to most of us. There are more books in the Library of Congress on perfume bottles than there are about smell itself. The year 2020 will mean lots of things to lots of people, but most of us will not remember it as the year that changed the literary world of smell forever.  

Image credit: Camillo Golgi's image of a dog’s olfactory bulb from his Sulla fina anatomia degli organi centrali del sistema nervoso, 1885. [link]

Together, Smellosophy by Ann Sophie Barwich and Nose Dive by Harold McGee have advanced the written record of our sense of smell by 40 years. Trygg Engen wrote The Perception of Odors in 1982, and we haven't seen a real update since. Obviously there have been scores of researchers writing journal articles since then, but an article is not a book. Also, there have been books written about smell, by well-regarded scientists and writers like Avery Gilbert, Rachel Herz, Synnott, Classen and Howes, Wilson and Stevenson, Alain Corbin, to name a few that come to mind.

But these books, both of them, are something else. They are dense, they are exhaustive, and they offer the most detailed explanation of how your sense of smell works, and what things smell like, than any other book you could read about the topic. 

*For third place in the most important smell-books of the past 40 years, maybe we should add The Essence: Discovering the World of Scent, Perfume and Fragrance by artbook publisher Gestalten, also in the year 2020/2021, and runner-up to Nose-Dive for The Perfumed Plume's best fragrance book of 2021

I'll save Harold McGee for another day, since I'm still parsing his almost 700 page codex. I thought it more important to finish Barwich's book first; it's the more scientifically hardcore of the two, and I was afraid my attention might wane the other way around. And this is not a book you want to breeze through. Again, it's probably the most comprehensive book on smell ever written. I took notes, some of which I'll paste here:

Notes on the author: Sophie Ann Barwich is a a cognitive scientist and empirical philosopher with a doctorate in odor classification and a background in philosophy and history, and spent time in Stuart Firestein's Columbia lab, and also interviewed a ton of perfumers, fragrance industry professionals, and just about anyone else who's important in the olfactory world that's still alive (except Asifa Majid, although she is cited in the book). She's an Assistant Professor at Indiana University Bloomington, between the Department of History & Philosophy of Science and the Cognitive Science Program.

Your nose is tailored to measure the world as calibrated by your mental life and physiological conditions. (p12)

Important terms for the olfactory enthusiast: Combinatorial and combinatorics refers to the combination of massive datapoints into a single datapoint, like how the olfactory bulb takes thousands of chemicals and pulses out a single signal in response; Foregrounding like from this sentence, "Smell is frequently embedded in the conscious experience of the world without being foregrounded as an olfactory experience" (p91); juxtaglomerular cells means next-to-glomerulus cells, juxta-anything sounds interesting, so...

Referencing Asifa Majid: Odor language is strongly contingent upon the rules of conventionalization (p102)

  • The Lingua Anosmia is Perishable: Leslie Vosshall - One central problem with these new computational studies were the data; "Most of the theoretical work has been based on a single 30 yr old dataset. Why has no one done an update?" (p173)
  • Andrew Dravnieks Atlas of Odor Character Profiles: "a great list in the early 80's, for use in the Northeast of the United States, for people who are baby boomers." (p173)
  • But so many of the words on that list have no frame of reference for the people who come to our studies. Any of these lists...they are perishable, highly culturally biased lists, that will work for some specific period in history, for a specific target audience. (p173)
  • The Odor Atlas didn't map odor quality space, "they mapped the odor quality space of Dravnieks." (p173)
  • The DREAM Project did provide a strong case for data mining, but only had a 0.3 correlation. (p175)

Polar surface area is a key metric for olfactory receptors (p185) -Poivet et al. "Functional Odor Classification through a Medical Chemistry Approach", Science Advances 4 n2 (2018)

Terry Acree's potato chips, only 3 odorants do the trick: methanethiol (rotten cabbage), methionol (potato), 2-ethyl-3,5-dimethylpyrazine (toast) -Computing Odor Images. Rochelle MM, Prévost GJ, Acree TE. J Agric Food Chem. 2018 Mar 14;66(10):2219-2225. doi: 10.1021/acs.jafc.6b05573.

Stimulus Representation Beyond the Map: Gordon Shephard and Thomas Cleland: Olfactory perception is more like the feature coding of face recognition in the visual system. The olfactory bulb does not represent chemical classes but the chemical environment; it tracks the statistics of a changing odor environment (Shephard and Cleland, p233) [And I would say that society and culture are part of that environment, and that given enough data, the language of smell can be a map of our changing social environment.]

"The brain evolved from the body and not the other way around" -Terry Acree (p237)

Predictive powers of the nose, Walter Freeman studying neural networks at Berkeley in the 1980's:
How brains make chaos in order to make sense of the world. Skarda, C. A., & Freeman, W. J. (1987).  Behavioral and Brain Sciences, 10(2), 161–195.
Simulation of chaotic EEG patterns with a dynamic model of the olfactory system, Walter J. Freeman, Biological Cybernetics (2004) v56 p139-150.
Model of biological pattern recognition with spatially chaotic dynamics, Yong Yao and Walter J. Freeman, Neural Networks (1990), v3 p153-170.
Neural networks and chaos. Freeman WJ, J Theor Biol. 1994 Nov 7;171(1):13-8. doi: 10.1006/jtbi.1994.1207.
Characterization of state transitions in spatially distributed, chaotic, nonlinear, dynamical systems in cerebral cortex. Freeman, W.J. Integrative Physiological and Behavioral Science 29, 294–306 (1994).

Unknown Odors (Covid) - based on Walter Freeman's work: "known odors elicit an established spatiotemporal signature of activity. Unknown odors first evoke chaotic activity before acquiring their own spatiotemporal signature for future recall. Chaos here was a condition for learners so that the brain would not confuse a novel odor with the signature of an already known one." (p238-239)

"The spatiotemporal activity in the bulb should thus be seen as an expression of the dynamic coding space -- not a fixed representation of odors, since odorants can be assigned various meanings, and, in turn, patterns." (p242)

The wide distribution of decorrelated signals in the olfactory cortex "allows olfactory signals to be integrated and synchronized with parallel processes in neighboring cortical domains..." Sparse coding is less detailed but faster in processing and recognition. Temporal patterns is where it's at, not topographical patterns, so it's about measurement, not mapping. "dynamically encoded signatures" (p242-243)

"Rather than molecules, your brain depicts transient information patterns, extracted and weighed in a given context, without a superimposed matrix of chemical classes to accommodate for countless permutations." (p246)

"Olfaction becomes an ideal model for higher-order processing other modalities.

Higher-brain integration is notoriously tricky to understand; it's signaling is not topographic, seemingly random, and autoassociate, just like in the olfactory system." (p247)

Relearning to Smell, Post-Covid: Mark Stopfer: "The first time you present an odor, there are no oscillations, you have to present an odor two or three times before the oscillation begins to build up. That's because there's this activity-dependent plasticity that takes place within the antennal lobe [we're talking insects here]. The local neurons that are activated become more effective over repeated activations. The inhibitory local neurons become more and more effective at synchronizing the projecting neurons over the course of repeated odor presentations. We think that's enabling the system to become more specific as the odor remains present." (p257)

"You go from a very general response to a more specific response. At the very beginning, you get this big burst that tells you there's something novel in the environment. Then right after that, you start to categorize it: it smells floral versus savory, for example. If the odor is still present, the system becomes more and more specific as this process builds up, the response downstream becomes more specific, and then you can identify exactly what it is. The same circuit at first will give you this generalization: it's something [fruity] -- and then the same circuit over time will say: oh well, it's cherry, not strawberry. It only happens if the odor is there long enough to perhaps be of interest to the organism." [this is why we can't name bad smells]

Beautiful description: The olfactory brain measures "odor situations" to evaluate how cues are related to each other (temporally, combinatorially, causally) and to attribute these perceptions a specific value (pleasant, putrid) and behavioral response." (p260)

Odor images are not encoded in the stimulus; they constitute mental impressions that arise from the categorization of sensory information. (p268)

One of a Kind: "When we manufactured a standard solution of an odorant, all we have to do is make another one, and it will smell different every time." -Terry Acree (p269)

  • Christophe Laudamiel's description of what a perfumer does - refinement of observation through cognitive engagement: "We don't have a super nose, but we notice things; it's our job to recognize a lot of smells, we pay attention, we recognize what we smell. We know how to describe things, and we know how to compose." (p272)
  • Christophe again: Perfumery is more than the sum of its parts: "What about black olive? when you want black olive, it's burnt rubber with wood." (p286)
  • Christophe again: Odorants convey more than one qualitative note: "I don't know a single molecule. You say cut grass? Cut grass is a whole world. In cut grass is wet dirt. There is a pear note. There is a green note, which you would say is a green, leafy note. But then how do you define a green, leafy note? That's the one that smells like cut grass. So it's a catch 22." (p297)

Visuocentric theories often follow the idea that perception is all about the stable representation fo objects [but odors are constantly changing.] (p303)

The brain is dynamic; it measures the world rather than mapping it. (p304)

Individual variation is not at odds with the notion of objectivity in perception; rather it is an expression of the core mechanisms of sensory systems ... The traditional dualism between objectivity and subjectivity in sensory perception presents itself as an artifact of older philosophical framing. It is time to change... (p311)

Smellosophy: What the Nose Tells the Mind
Ann Sophie Barwich, Harvard University Press, 2020

Nose Dive: A Field Guide to the World's Smells
Harold McGee, Penguin, 2020

Personal criticisms:
If you're not already familiar with the science of olfaction, this book is not an easy read. That's mostly understandable, because it is so comprehensive, airtight in fact. Less excusable is another pattern I noticed, which is that it's hard to follow the quotations and remember who is saying what, and that's because people are referred to by their first names; this is a multidisciplinary crowd, from philosophers to chemists to perfumers to neuroscientists; it's very unlikely that the reader would be on a first name basis with all these people. Using their full names would give the reader a little bit more to hang onto in their working memory. For example, in another, completely unrelated book Chimpanzee Culture Wars, author Nicolas Langlitz continues to write linguist Michael Tommasello's full title after already having mentioned him like 300 times throughout the book.

Next, although less severe of a criticism, is that at one point the book changes in tone quite dramatically, almost as if it were two different books (circa p210). And last thing, which isn't a criticism but a simple note: She opens her chapter 9 with Parmesan Vomit (p264), which I called "Quantum Hedonics" in Hidden Scents in 2015.

Monday, May 9, 2022

Ant Ink and Infotaxis

The ant secretion methyl-4-methyl-pyrrole-2-carboxylate - "innocuous, faintly grassy, sulphurous, or fruitlike with a hint of naphtha", "an ichor of extraordinary power for the ants.

"They sweep their antennae back and forth in advance of the head to catch the odorant molecules. When a forager takes a long turn to the left and starts to run away from the track, its left antenna break out of the odor space first and is no longer stimulated by the guiding substance. In a few thousandths of a second, the any perceives the change and pulls back to the right." (p30-31) 

Biophilia: The Human Bond with Other Species 
E. O. Wilson, Harvard University Press, 1984

Tuesday, May 3, 2022

Downwind Odor

It's called the Rolling Unmasking Effect: "The source is a complex mixture of odorants, yet it is simplified to a single impactful odorant at the receptor downwind. The odor frontal boundary represents the farthest downwind reach of a single compound, while the internal colored ovals represent the boundaries of sequential odor unmasking as the secondary-impact odorants are diluted below their detection/masking concentration levels."

Qualitative Exploration of the ‘Rolling Unmasking Effect’ for Downwind Odor Dispersion from a Model Animal Source. Donald W. Wright et al. International Journal of Environmental Research and Public Health, 2021,18,13085. DOI: 10.3390/ijerph182413085

It's already hard enough to identify odors by their source, but these researchers show us that matching an odor "in the field" to one you think is the source, needs to account for the differentiated dispersal of odorants as the odor plume moves through space.

Just because one odorant scores high on the sniff test --at the source--, doesn't mean that odorant won't be the first to disappear at 10 yards. And just because you smell rotten eggs, doesn't mean that odorant is found in more abundance relative to others at the source, it could be that the rotten egg parts of the smell are better at avoiding dispersal, riding the edge of the odor plume as it emanates from its source. 

Some good terms:
  • rolling unmasking effect
  • downwind odor frontal boundary
  • odorant prioritization 
  • downwind odor impact
  • dynamic dilution olfactormetry

From the paper:

We propose solving environmental odor issues by utilizing troubleshooting techniques developed for the food, beverage, and consumer products industries.

While the composition of environmental odors, as detected by human receptors, carries the potential for extreme complexity, the reality is that there is a high degree of compositional simplification, which typically develops with increasing distance separation from the odor source.

We refer to these two effects as the Rolling Unmasking Effect (i.e., RUE). 

Odors generated from rural and agricultural sources are lowered by "downwind diultion" dispersion strategies, and monitored by dynamic dilution olfactormetry.

There is also broad recognition of a challenge to link specific compounds to resulting downwind odor [10,11]. In one notable example from an odorant prioritization study to the rendering industry [12], just two odorants (trimethylamine (TMA) and dimethylsulfide (DMS)) were identified as the impact-priority odorants downwind of a fish meal processing plant. 

In a more recent study [14,15], these authors were able to identify the specific chemical odorant that is believed primarily responsible for the reported ‘skunky’ odor downwind of dense cannabis-growing operations. ... The compound 3-methyl-2-butene- 1-thiol (i.e., 321 MBT), was the primary source of this ‘skunky’ odor of cannabis [14,15].

This has been shown for p-cresol as a 'sognature' [signature?] downwind odor from confined animal feeding operations (CAFOs), recognizable at a great distance from the source. 

Odors from a large colony of Mexican fee-tailed bats: ammonia, "rat nest",  and "bat cave" or "taco shell", which was dominated by 2-aminoacetophenone, upon approach to the outer ‘odor frontal boundary’; enabled by the decline of odor masking by the quinazoline odorant.

Figure 3. P.T. porcupine encounter in Moody Gardens. (1) Wind direction; (2) odor frontal boundary; (3) approximate secondary (near-source) boundary; (4) investigator's approximate location upon initial encounter and (5) location of outdoor enclosure of the odor source.

PT Porcupine Urine Sampling:

Unfortunately, the panelist (D.W.W.) was unable to confirm the chemical identities of the two character-defining ‘grilled onion’ odorants from the P.T. porcupine environments. Therefore, in a further attempt to identify these unknowns, collaborations with experts in the food flavor/aroma field were engaged.

The near-source smell was perceived as ‘phenolic,’ ‘industrial,’ and ‘foul.’ The dramatic difference in character was particularly surprising considering that only a few paces separated the pleasant 'gilled onion' at the odor frontal boundary and the 'foul' odor deeper into the plume. 

Although the PT porcupine and swine barn sources generate distinctly different odor characteristics at their respective odor boundaries, despite sharing much in common through their VOC emission profiles at the source. 

Focusing on all compounds present at the source often expands the study to include background noise, an unnecessary expenditure if the goal is to reduce downwind environmental odor impact.

One team member did not characterize the odor as 'onion' specifically; instead, it had reminded her of a favorite sauce that her grandmother frequently made. The second team member called the odor character ‘stale onion’.

Post Script:
Here's an odor network for all the odor complaints in southern California circa 2012, via the South Coast Air Quality Management District (SCAQMD)and UCLA post-grad Jane Curren:
Local Odor Vocab

Post Post Script:
Odor wheel for drinking water:
Torrice, M. (adapted from Suffet, M.). The scientists who sniff water. Chem. Eng. News 2017, 95, 16–19.
Suffet, I.H., and P.E. Rosenfeld (2007). The Anatomy of Odour Wheels for Odors of Drinking Water, Wastewater, Compost and the Urban Environment, Water Science and Technology 55(5), 335-344.