Subjective Olfactory Perception

Two reasons why we aren't good at talking about smells -- 1. we don't share a common percept, and 2.  

we don't share a common language for smells. 

We're genetically coded to perceive smells differently from one another, with a round number at 30% difference across a population. As far as smelling goes, you could say that many of us are mutants. Or you could say there is no "normal" and therefore no mutant. It's just part of the process of evolution. Because of this difference in smell receptors caused by changes in the genes, there are holes in our perception of smells, holes from a societal level, and so the statistics makes it so that the quality of the data, each individual's perception of an odor combined to that of a group of people, is not so good. 

Next, we don't share a common language for smells. Some of this comes from the above problem, but some of it comes from that fact that lots of smells are "social smells" and so the way we talk about them is first and foremost determined by social context, not by some objective characteristic of the smell itself. An example, sort of related, is that we don't talk about the smell of shit in public, it's just not good manners. Or another person's body odor, or the smell of semen, which is almost never ever ever written, not even the word, in general popular literature (in English; maybe this isn't the case in other languages but I don't know).

These two studies below bring some interesting additions to this idea, that the part of our brain that perceives odors is mediated by a pleasure-reward part of our brain, and that language itself comes in two kinds, social and non-social:

Image credit: Ryan Muldoon at University of Buffalo miscommunicating by Douglas Levere - 2023

Study sheds light on the neural underpinning of subjective odor perceptions

Sep 2023, phys.org

https://medicalxpress.com/news/2023-09-neural-underpinning-subjective-odor-perceptions.html

Participants rated 160 odors on 18 perceptual descriptors while under fMRI analysis -- the  orbitofrontal cortex is where the most detailed and subjective percepts reside, and not as much with the amygdala and piriform cortex, regions typically associated with odor processing. 

via Northwestern University, Rhodes College, University of Pennsylvania and NIH National Institute on Drug Abuse: Vivek Sagar et al, High-precision mapping reveals the structure of odor coding in the human brain, Nature Neuroscience (2023). DOI: 10.1038/s41593-023-01414-4

https://dx.doi.org/10.1038/s41593-023-01414-4

Social vs. language role: Researchers question function of two brain areas

Sep 2023, phys.org

https://medicalxpress.com/news/2023-09-social-language-role-function-brain.html

Language just got way more complicated:

"A research team led by Prof. Lin Nan from the Institute of Psychology of the Chinese Academy of Sciences found that during sentence processing, the neural activity of two canonical language areas—the left ventral temporoparietal junction (vTPJ) and the lateral anterior temporal lobe (lATL)—is associated with social-semantic working memory rather than language processing per se."

These regions were sensitive to sentences only if the sentences conveyed social meaning. 

These findings are likely to force a major reconsideration of the functional organization of the cortical language network.

via Institute of Psychology of the Chinese Academy of Sciences: Zhang, G. et al, A social-semantic working-memory account for two canonical language areas, Nature Human Behaviour (2023). DOI: 10.1038/s41562-023-01704-8.

https://dx.doi.org/10.1038/s41562-023-01704-8

Advances in Olfactory Perception

Scientists use machine learning to predict smells based on brain activity in worms

Jan 2022, phys.org

https://phys.org/news/2021-11-scientists-machine-based-brain-worms.html

Putting this here because they used graph theory aka network science to decode the otherwise cacophony of neuronal crosstalk involved in smelling.

Also, why C. elegans? It has only 302 neurons, that's why:

Chalasani's team set out to study how C. elegans neurons react to smelling each of five different chemicals: benzaldehyde (almond), diacetyl (popcorn), isoamyl alcohol (banana), 2-nonanone (cheese), and sodium chloride (salt).

The researchers engineered C. elegans so that each of their 302 neurons contained a fluorescent sensor that would light up when the neuron was active. 

By looking at basic properties of the datasets—such as how many cells were active at each time point—Chalasani and his colleagues couldn't immediately differentiate between the different chemicals. So, they turned to a mathematical approach called graph theory, which analyzes the collective interactions between pairs of cells: When one cell is activated, how does the activity of other cells change in response?

The algorithm was able to learn to differentiate the neural response to salt and benzaldehyde but often confused the other three chemicals.

via Salk Institute, Cold Spring Harbor Laboratory and UC San Diego: Javier J. How et al, Neural network features distinguish chemosensory stimuli in Caenorhabditis elegans, PLOS Computational Biology (2021). DOI: 10.1371/journal.pcbi.1009591

https://doi.org/10.1371/journal.pcbi.1009591

Image credit: AI Art - AI Makes a Nose Face - 2022

a highly detailed, macro shot of a human nose, 8k, depth of field

The art of smell: Research suggests the brain processes smell both like a painting and a symphony

Apr 2022, phys.org

https://medicalxpress.com/news/2022-04-art-brain-symphony.html

"These findings reveal a core principle of the nervous system," using a model to simulate the workings of the early olfactory system. This is a reminder that the olfactory system is an ideal model for understanding the brain.

In their computer simulation, they found that centrifugal fibers switched between two different modes -- one worked on a specific instant in time, while the other worked on the neural patterns across time.

This is where I make a further interpretation, which might be incorrect, but it seems like one is for comparing a smell to the body's repository (is this good or bad for me? have I smelled this before? where? who was I with?) and the other mode is for comparing the smell against itself, over time, perhaps to learn whether it's getting stronger or weaker. One uses autobiographical, physiological memory, and the other uses basic chemotaxis. One ontogeny and the other phylogeny?

Anyway, another reminder by one of the authors that the olfactory system is a good model: "Computational approaches inspired by the circuits of the brain such as this have the potential to improve the safety of self-driving cars, or help computer vision algorithms more accurately identify and classify objects in an image." -Krishnan Padmanabhan, associate professor of Neuroscience at University of Rochester School of Medicine and Dentistry

via University of Rochester Medical Center: Zhen Chen et al, Top-down feedback enables flexible coding strategies in the olfactory cortex, Cell Reports (2022). DOI: 10.1016/j.celrep.2022.110545

https://dx.doi.org/10.1016/j.celrep.2022.110545

Sniffing out the brain's smelling power

Oct 2022, phys.org

https://medicalxpress.com/news/2022-09-sniffing-brain-power.html

(Out of order but seemingly related to the above) Here's another way of thinking of the two processes to smelling -- We said mitral cells are what do the smelling, but mostly because those were the ones we could see. Tufted cells were harder to see, up until now -- they find that the mitral cells were faster, more discriminating, and more broadly-tuned. 

The authors think the mitral cells only enhance important smells, but the tufted cells are part of a background process for identity and intensity. 

via Cold Spring Harbor Laboratory: Honggoo Chae et al, Long-range functional loops in the mouse olfactory system and their roles in computing odor identity, Neuron (2022). DOI: 10.1016/j.neuron.2022.09.005

https://dx.doi.org/10.1016/j.neuron.2022.09.005

AI Art - AI Makes a Nose Face 2 - 2022

a straight smooth vertical tube with the texture of human skin, highly realistic, hyper-real, 4k, Octane render 

Researchers map mouse olfactory glomeruli using state-of-the-art techniques

Apr 2022, phys.org

https://medicalxpress.com/news/2022-04-mouse-olfactory-glomeruli-state-of-the-art-techniques.html

While other research teams previously examined the organization of glomeruli in the olfactory bulb, so far they only identified the positions of a limited subset of these clusters. As a result, the relationship between the location of glomeruli and odor discrimination has been very difficult to infer.

They used a combination of single-cell RNA sequencing, spatial transcriptomics and machine learning techniques. This allowed them to create a map that outlined the brain regions where most of the sensory neurons in the mouse olfactory bulb sent odor-related information.

via University of Massachusetts Medical School, Broad Institute of Harvard and MIT, and Stanford University: I-Hao Wang et al, Spatial transcriptomic reconstruction of the mouse olfactory glomerular map suggests principles of odor processing, Nature Neuroscience (2022). DOI: 10.1038/s41593-022-01030-8

https://dx.doi.org/10.1038/s41593-022-01030-8

How mosquito brains encode human odor so they can seek us out

May 2022, phys.org

https://phys.org/news/2022-05-mosquito-brains-encode-human-odor.html

Of the two nerve centers, one responds to many smells including human odor, essentially saying, "Hey, look, there's something interesting nearby you should check out," while the other responds only to humans. Having two may help the mosquitos home in on their targets, the researchers suggest.

First genetically engineer mosquitos whose brains lit up when active, and then deliver human-flavored air (with decanal and undecanal).

"When I first saw the brain activity, I couldn't believe it—just two glomeruli (out of 60) were involved. That contradicted everything we expected, so I repeated the experiment several times, with more humans, more animals. I just couldn't believe it. It's so simple."

via Princeton: Carolyn McBride, Mosquito brains encode unique features of human odour to drive host seeking, Nature (2022). DOI: 10.1038/s41586-022-04675-4

https://dx.doi.org/10.1038/s41586-022-04675-4

On Fruit Flies and the History of Brain Science

Researchers uncover brain mechanisms in fruit flies that may impact future learning

Jun 2020, phys.org

https://medicalxpress.com/news/2020-06-uncover-brain-mechanisms-fruit-flies.html

I was going to write something about the trifecta between much of the basis for modern neuro- and behavioral science and fruit flies and olfaction, but this researcher sums it up pretty well:

Paul Sabandal said olfactory conditioning in fruit flies has greatly contributed to overall understanding about the mechanisms underlying associative learning and memory. Historically, in fruit flies, dopamine is implicated in both punishment- and reward-based learning while octopamine is widely considered to be essential only for reward.

When he says "historically", he implicitly refers to the fact that fruit flies, along with the elegant roundworm C. elegans, are prime biological models for studying the brain and translating that information to humans.

via the University of Texas at El Paso: John Martin Sabandal et al, Concerted Actions of Octopamine and Dopamine Receptors Drive Olfactory Learning, The Journal of Neuroscience (2020). DOI: 10.1523/JNEUROSCI.1756-19.2020

image credit: Ovary of a Fruit Fly, Dr. Yujun Chen, Nikon Small World 2020

Biology blurs line between sexes, behaviors

Aug 2020, phys.org

https://phys.org/news/2020-08-biology-blurs-line-sexes-behaviors.html

Never heard this one before:

Typically, C. elegans males prefer searching for mates over eating, in part because they can't smell food as well as females do. But if a male goes too long without eating, it will dial up its ability to detect food and acts more like a female. The new research shows that TRA-1 is necessary for this switch, and without it hungry males can't enhance their sense of smell and stay locked in the default, food-insensitive mate-searching mode.

via the University of Rochester Medical Center: Hannah N. Lawson et al, Dynamic, Non-binary Specification of Sexual State in the C. elegans Nervous System, Current Biology (2020). DOI: 10.1016/j.cub.2020.07.007

Scientists may have found one path to a longer life

Jul 2020, phys.org

https://medicalxpress.com/news/2020-07-scientists-path-longer-life.html

Aaaand now they're immortal. Just kidding but we're getting there:

Studying one of the most common laboratory models used in genetic research—the fruit fly Drosophila—John Tower, professor of biological sciences, and his team found that the drug mifepristone extends the lives of female flies that have mated.

via University of Southern California: Gary N Landis et al, Metabolic Signatures of Life Span Regulated by Mating, Sex Peptide and Mifepristone/RU486 in Female Drosophila melanogaster, The Journals of Gerontology: Series A (2020). DOI: 10.1093/gerona/glaa164