Coding for Information Overflow and Statistical Irregularity

Part of the "odor code" our brain uses to smell is tasked with overcoming the statistical irregularity caused by massive changes in airflow direction, speed, humidity, etc. as we pull that air through our nostrils. The cross-cancelling variables required in this effort are mentally exhausting to consider, never mind to calculate. But that's what we do when we smell:

How insects track odors by navigating microscale winds

May 2023, phys.org

https://phys.org/news/2023-05-insects-track-odors-microscale.html

"This is important because insects are typically tracking odor plumes in lower wind speeds, which indicates they are somehow making sense of the high directional variability they encounter," said Houle. "Turbulence intensity is strongly correlated with standard deviations in wind direction, which might be useful for future wind tunnel experimental designs aimed at recreating more 'natural' winds."

Based on their findings, Houle and van Breugel hypothesize an optimal range of wind speed and environmental surface complexity may exist to help insects locate an odor source.

via University of Nevada at Reno: Discovered near-surface wind direction is often highly variable over timescales of less than 10 minutes. They also found wind direction variability to be consistently higher in environments with greater surface complexity (urban areas) and lower at higher wind speeds.

Domestic cats' noses may function like highly efficient gas chromatographs

Jun 2023, phys.org

https://phys.org/news/2023-06-domestic-cats-noses-function-highly.html

Yet another example of how in olfaction nature is still ahead of technology:

Researchers created a 3D computer model of the cat nose and simulated how an inhalation of air containing common cat food odors would flow through the coiled structures. They found that the air separates into two flow streams, where one spreads slowly above the roof of the mouth on its way to the lungs, and a separate stream containing odorant moves rapidly through a central passage directly to the olfactory region toward the back of the nasal cavity.

In essence, the researchers suggest, the cat nose functions as a highly efficient and dual-purposed gas chromatograph.

via Ohio State University: Wu Z, Jiang J, Lischka FW, McGrane SJ, Porat-Mesenco Y, Zhao K. Domestic cat nose functions as a highly efficient coiled parallel gas chromatograph, PLoS Computational Biology (2023). DOI: 10.1371/journal.pcbi.1011

https://pubmed.ncbi.nlm.nih.gov/37384594/

Each nostril has a unique sense of smell, intracranial electroencephalogram study finds

Nov 2023, phys.org

https://medicalxpress.com/news/2023-11-nostril-unique-intracranial-electroencephalogram.html

10 subjects with intracranial depth electrodes were delivered an odor to the left, right, or both nostrils through an olfactometer device designed to deliver odors by computer control. Subjects had to identify the odor and indicate which nostril the odor came from. Subjects performed better in detecting and identifying odors in the bi-nostril condition compared to uni-nostril conditions.

Odor identity could be decoded from oscillations in the piriform cortex brain region via neural activity recorded from an intracranial electroencephalogram. The researchers observed that odor identity was encoded in two distinct, temporally segregated epochs in the bi-nostril condition, suggesting a separate smell interpretation occurs via each nostril, suggesting a possible computational advantage in processing odors in stereo. 

via University of Pennsylvania and the Barrow Neurological Institute of Phoenix: Gülce Nazlı Dikeçligil et al, Odor representations from the two nostrils are temporally segregated in human piriform cortex, Current Biology (2023). DOI: 10.1016/j.cub.2023.10.021

https://dx.doi.org/10.1016/j.cub.2023.10.021

Image credit: AI Art - Big Tunnel Full of Numbers - 2023

Promiscuous Pattern Recognition

Study reveals how smell receptors work

Aug 2021, phys.org

https://phys.org/news/2021-08-reveals-receptors.html

Big smell news - for the first time ever, using cryo-electron microscopy, we can see an olfactory receptor in action. And as expected, it doesn't work like any other receptor.

Odorant receptors are known for their 'promiscuous chemical sensitivity;' that's a scientific term, by the way. It means that any one receptor might be sensitive to hundreds of molecules, so it's been really hard  to figure out what makes any particular molecule match with a receptor.

They looked at the jumping bristletail (surprise - not the fruit fly) because it has only five types of receptors, and because one of those receptors (OR5) is really broad, responding to 60% of the smell molecules they presented to it (promiscuous).

So they look at this receptor in its default state, and then again as they expose it to smell molecules (either eugenol or DEET).

And? Its ion channel pore dilates. That's it. Both of the competing theories about how smells work were wrong. It turns out they work via nonspecific chemical interactions -- they are not recognizing a specific chemical characteristic, but something more general about the molecule itself.

And there you have it! Olfaction is still one of the strangest senses we have.

Don't forget to thank cryo-electron microscopy, and the hundreds of scientists who have been trying to figure this out over the past hundred years.

via Rockefeller University: del Mármol, J., Yedlin, M.A. & Ruta, V. The structural basis of odorant recognition in insect olfactory receptors. Nature (2021). https://doi.org/10.1038/s41586-021-03794-8

The Chemosensitive Organ

Diving off the deep end, I had an olfactory moment the other night that was a bit amazing, a bit scary. Amazing because I may have unlocked a new dimension of olfactory experience for myself, scary because I might be detecting the first real glimpse of neurological dysfunction that awaits my aged, exasperated mind.

Smells are like that. There are moments when I catch a whiff that is so faint, and so impossible to detect (like the mold that grows on the metal of your umbrella), that I often have to consider the myriad olfactory hallucinations that beset our chemosensitive organ, of which there are many.

It was a hot summer night. The crazy smell experiences always start out like that. Odors love heat, because evaporation is what makes them mobile; heat is the odor motor, if you will.

On this hot, sweaty night, I was eating dinner, hot and sweaty, resting my arms on my high-topped table, when I became fussed over the cleanliness of the surface of my table. My resting forearms sheathed in a thin film of slightly sticky, waxy sebum, and my tabletop layered in a similar compound from previous nights of the same scenario. "I need to clean this damn table. It's gross."

I try to keep my arms off the table. But it's a high top, and it's more comfortable for me to hunch over my bowl of gruel, resting on the underside of my forearms. I keep losing the battle, touching my sticky gross arms to the sticky gross table, then remembering not to, sitting straight up, only to capitulate again moments later.

All the while, I'm sampling my dinner, and very thoughtfully. I tried a new recipe last night, and today are the leftovers. The leftovers always taste different, and for some things, even better. The lemon juice in the marinade, the Maillard on the cast iron; What happened there? I used soy sauce and lime last time. What's the difference now? The acid, more sour; The surface texture, less caramelized. I am thinking, but with my mouth, my nose, lost in gustatory, olfactory thought. Shit! Dirty sticky table! And then it happened. Mid-bite, mid-thought, I could smell the table with my forearm.

That's right, mid-bite, the sensations of my dinner coalesced with the sensations of my forearms, and all of the sudden I could taste the table. Not even sure how to describe the taste, it didn't last long enough. My friend who knows a bit about these things says it's synaesthesia, hallucination. I'm not sure. I don't recall ever reading or hearing about this, but the science of smell is more mystery than science.

Our skin is covered in sensory receptors. Our olfactory cortex is entwined with our immune system. There is a hell of a lot going on here. I'm convinced that the reason I can smell mold in ways nobody else seems to be able to is because I trained (inadvertently through paranoia) my immune system in concert with my olfactory system to recognize it, and after a severe and prolonged exposure event in a basement apartment. The two -- immune and olfactory system -- now work together to alert me to its presence in the most impossibly minute airborne concentrations.

And now, I am suspicious that it goes way deeper. Then again, as I said at the outset, I may be slowly and steadily rolling into cognitive decline, my neural tendrils twisting, knotting, brittle and breaking. At least I'm taking notes.

Notes:
Pattern recognition receptor (PRR)   
A receptor present on the surface of keratinocytes and other cells of the innate immune system that recognizes microbe-specific molecules that are recognized by a given PRR are called pathogen-associated molecular patterns (PAMPs) and include fungal glucans...They are also called primitive pattern recognition receptors because they evolved before other parts of the immune system, particularly before adaptive immunity...The innate immune system is an older evolutionary defense strategy, relatively speaking, and it is the dominant immune system response found in plants, fungi, insects, and primitive multicellular organisms. 

Language Models

image Paramount Comedy Channel

Yann LeCun in this talk for the Wired Business Conference 2016, title AI Arms Race, notes how the best natural language models are now deep learning based. At Limbic Signal we find this funny because our sense of smell works akin to the deep learning model, and yet the language of smell is an unwieldy concept. Alas, it is unwieldy because we measure wieldliness by classical means. We have entered the age of approximation, however.