Smell: a feeling in stereo

Scent is perhaps the feeling we have underestimated the most so far, because we think it is a bit slow. Compared to a sound or a flash of light, an odor takes longer to enter your nose and be noticed. Try it this morning: you probably heard your coffee maker boiling long before you smelled the coffee.

A nasal cycle

And the nose, like other senses, has a preference: when you breathe, you have one nostril dominant. The same goes for the eye, you have a dominant eye that we call director. Place your hand under your nose, exhale, blow through your nose, one nostril exhales more air than the other. But unlike the eyes, the dominant nostril changes sides every 2 to 5 hours, this is called a nasal cycle.

One nostril opens slightly wider, allowing a greater flow of air to reach the lungs. The other nostril, the more closed nostril, is responsible for draining the mucus from our nasal cavity, for cleaning the particles, dust or microbes that accumulated there when the nostril was open. And this asymmetry of airflow, this nasal cycle, affects our sense of smell.

Gabriel Lepousez is a research fellow in the Perception and Memory team at the Institut Pasteur, affiliated with the CNRS and the University of Paris Cité: “In fact, the nostril is very open, the air flow is very fast and intense, so there are many volatile molecules that enter this nostril. But the volatile molecules that enter this nostril and join our olfactory mucosa that is between our two eyes is located, the airflow is fast, so the molecules that need to attach to the receptor have a short time to attach to the receptor. So only the molecules that are attached to it have a lot of affinity to the receptor.”

The other nostril does not receive the same volatile molecules. “Conversely, the nostril that is closed is not completely closed, as you have noticed from your nose, some air still flows through it, the airflow is more restricted, but the airflow is therefore slower, and as a result the few molecules that still manage to enter the nostril that is closed or slightly occluded, have much more time to circulate and therefore have more time to adhere to the sensor. And as a result, what we feel when sensing The level is that if I feel certain molecules on the left or right, I will not feel them with the same intensity, but in fact, the nature of the molecules I smell on the left and right will not be the same. almost the same.”

Without daring to ask

Electrodes in the brains of people with epilepsy

If the nature of the molecules we smell on the right and left are different, is the odor information that each nostril perceives processed differently in the brain? Or do the two nostrils form a single detector? To investigate this question, we need to study the activation of a particular brain area: the piriform cortex – it is the seat of olfactory memory – present in each of the cerebral hemispheres, so there is a piece on each side of the brain.

The specialness of this new study lies in the methodology, there is no MRI or electroencephalogram… but electrodes placed directly in the subjects’ brains. We therefore understand that the process is invasive, but much more precise. The volunteers are in fact people with epilepsy who needed this type of recording to locate the source of their pathology. The scientists took the opportunity to conduct this other experiment.

The scientific method

We feel in stereo

After the operation, they allowed the subjects to smell odors through one nostril, the other, or both, and watch the activation of each of the hemispheres of this piriform cortex. As a result, we smell in stereo: the brain perceives odors from both nostrils differently.

Gabriel Lepousez : “And what they were able to observe is that the cortex listening to his respective nostril has information coming from that nostril quite early. And the information from the opposite nostril also arrives a few hundred times milliseconds later, so there is a temporal shift As if the olfactory cortex converts information related to space, left-right, into temporal information, before-after. Not only is there with the stream a qualitative diversity of the odor and also in its intensity, but that this qualitative information that is actually purely related to the mechanics of the fluids at the level of our nostrils, this difference at the qualitative level, it is also used by the brain, it is retained later, the brain and the piriform cortex will recompose the smell, will resynthesize things, but it will not form an overall average, it will maintain this temporality to better know what happens first, what happens next.

The Scientific Journal

A challenge for research

Gabriel Lepousez: “And what’s interesting is that this is also typically what happens when we smell a perfume, for example: the top notes or the base notes are notes that also arrive consecutively. When you smell a perfume, the top notes are always very fast, the base notes are much slower and more progressive, because there is a more limited rate of evaporation.”

This speed is a research challenge because until now, as I said, we had somewhat underestimated the vividness of our sense of smell. Now we realize that the olfactory system is fast and that it uses spatial information to enhance the precision of this sense. The brain imaging techniques we commonly use therefore lose a lot of information because they are too slow to capture the temporal architecture of the olfactory message. And this opens certain perspectives, in particular to refine olfactory tests and diagnostic tools, better characterize olfactory disorders in certain pathologies, COVID-19, neurodegenerative diseases or even depression, also associated with a failing sense of smell.

The editorial report

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