Thursday, December 21, 2023

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,

"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,

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

Each nostril has a unique sense of smell, intracranial electroencephalogram study finds
Nov 2023,

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

Thursday, December 14, 2023

Insects for Olfactory Insight

Insects are such an important part of olfactory science because they smell with their antennae, which are outside their bodies, making it easier to study. Also, their brains are pretty simple, which makes it easier to study how the most complicated and least understood sense works.

Another reason insects are so important to olfactory science isn't really about olfaction, it's about malaria, and Zika, and West Nile, you name it. Mosquitoes are one of the main drivers of infectious disease around the world. And if we could only figure out how they use their sense of smell to find us, we could stop them from finding us and infecting us.

The first article shows you just how important this effort is --

Researchers build mosquito testing arena to discover how they find us over long distances
May 2023,

"This is the largest system to assess olfactory preference for any mosquito in the world."

Using an ice-rink-sized (1,000 m3) outdoor testing arena in Choma District Zambia, researchers found that human body odor is critical for mosquito host-seeking behavior over long distances. They had six people sleep in single-person tents surrounding the arena over six consecutive nights, and they used repurposed air conditioner ducting to pipe air from each tent—containing the aromas of its sleeping occupant.

The testing arena contained a ring of evenly spaced landing pads that were heated to human skin temperature (35ºC). Each night, the researchers released 200 hungry mosquitoes into the testing arena and monitored their activity using infrared motion cameras.
  • mosquitoes were not attracted to heated landing pads unless they were baited with CO2 
  • human body odor was a more attractive bait than CO2 alone
  • some people were more attractive to mosquitoes than others
  • one volunteer with a strikingly different odor composition from the others consistently attracted very few mosquitoes
  • people who were more attractive to mosquitoes consistently emitted more carboxylic acids probably produced by skin microbes
  • the person who was least attractive to mosquitoes emitted less carboxylic acids but triple the amount of eucalyptol, which may be related to the person's diet
  • the team identified 40 chemicals that were emitted by all of the humans, though at different rates.
  • "It's probably a ratio-specific blend that they're following" 

via Johns Hopkins Bloomberg School of Public Health, Johns Hopkins Malaria Research Institute, and Macha Research Trust: Conor J. McMeniman, Human scent guides mosquito thermotaxis and host selection under naturalistic conditions, Current Biology (2023). DOI: 10.1016/j.cub.2023.04.050.

Washing with different soaps could make you more or less attractive to mosquitoes
May 2023,

"It's remarkable that the same individual that is extremely attractive to mosquitoes when they are unwashed can be turned even more attractive to mosquitoes with one soap, and then become repellent or repulsive to mosquitoes with another soap," says senior author and neuroethologist Clément Vinauger.

"What really matters to the mosquito is not the most abundant chemical, but rather the specific associations and combinations of chemicals, not only from the soap, but also from our personal body odors," says Vinauger.

via Virginia Tech: Clement Vinauger, Soap application alters mosquito-host interactions, iScience (2023). DOI: 10.1016/j.isci.2023.106667.

Perfume component helps lure male moth pests
Apr 2023,

Smells are so complicated: "Nonenal is a universal attractant that, by itself, doesn't have much of an effect, but when a certain percentage is added to the multi-chemical attractant mixture discovered nearly 40 years ago, it has a highly stimulatory effect."

(The researchers started examining ways to attract and then trap armyworm moths as part of a "mating disruption" strategy.)

via North Carolina State University: Ahmed M. Saveer et al, Nonanal, a new fall armyworm sex pheromone component, significantly increases the efficacy of pheromone lures, Pest Management Science (2023). DOI: 10.1002/ps.7460

Good smells, bad smells: It's all in the insect brain
Aug 2023,

I don't think I've ever heard the sense of smell referred to in this way: "While it is more of an aesthetic sense in humans, for insects, including locusts, the olfactory system is used to find food and mates and to sense predators."

It certainly is different, because in insects, their "palp" mouth triggers automatically to eat food just from the presence of some specific odors. I think we would usually see this difference in the context of the pheromone-receptor parts of our olfactory system, which don't actually work anymore in humans. Lots of animals, insects too, and beyond of course, have their behavior very strongly (could you call it irresistibly?) controlled by smells. 

Back to the study:

Interestingly, some of the locusts showed no response to any of the odors presented. They found that locusts only associated appealing scents with a food reward. Delaying the reward, they found that locusts could be trained to delay their behavioral response.

"All information received by our sensory apparatus, and their relevance to us, has to be represented by electrical activity in the brain. It appears that sorting information in between positive and negative happens as soon as the sensory signals enter the brain."

via McKelvey School of Engineering at Washington University in St. Louis: Rishabh Chandak et al, Neural manifolds for odor-driven innate and acquired appetitive preferences, Nature Communications (2023). DOI: 10.1038/s41467-023-40443-2

A non-invasive way to turn a cockroach into a cyborg
Sep 2023,

First, we made remote control roaches by smashing an electric circuit through their head. But now, it's as simple as slipping over their antennae a sleeve made of gold and plastic, and fixed in place by a blast of ultraviolet light, like plastic shrink-wrap.

Note to self -- insects don't get "injured," they get "damaged" -- "damaging cockroaches during attempts to control them results in a very short life expectancy, which then results in very little payoff for a lot of work".

via Nanyang Technological University in Singapore: Qifeng Lin et al, Resilient conductive membrane synthesized by in-situ polymerisation for wearable non-invasive electronics on moving appendages of cyborg insect, npj Flexible Electronics (2023). DOI: 10.1038/s41528-023-00274-z

Post Script: For a cockroach, and for all insects, their antenna is their nose, and so this is how we'll do it for humans too. (Except laser pulses through the retina are a likely candidate as well. Why not both?)

Thursday, December 7, 2023

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:

Study sheds light on the neural underpinning of subjective odor perceptions
Sep 2023,

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

Social vs. language role: Researchers question function of two brain areas
Sep 2023,

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.