Smells: from the nose to the brain and our vulnerability – 06/26/2023 – Luciano Melo

I was on duty at an emergency room, a few days before I finished medical school, when an external rescue team rushed into the emergency room pushing two stretchers. On each gurney, a victim of the same accident, a hit by a truck. We, who attended, followed orderly protocols and quickly exposed the lesions to our view, whose damage was incompatible with survival.

With no therapeutic options, efforts ceased, the scenario made clear the cruelty of fate. Soon after, the emergency room was cleaned and restocked with new supplies, once again ready to wait for the next emergency. Many hours later, I walked into that room more than once. Nothing that was seen signaled the hit-and-run. But there was a smell of iron, this metal is an important component of blood and had tattooed the air.

Years later I visited a museum where there was an invitation to touch a meteorite on display. After this visual and tactile experience, I smelled my hands. From space, the smell of iron remained on my skin. My memories made me feel again towards the end of my graduation. I felt the Proust effect, the rescue of living memories from the activation of a sense.

This phenomenon is named in honor of the French writer Marcel Proust, who in a passage of the work “In Search of Lost Time” transcribed the wonder of reliving, suddenly and powerfully, childhood, as if he had been transported to ancient times. All triggered by the smell and taste of Madeleine cake soaked in tea. Disparities aside, the mental process that made me remember college, technically, was the same that made the artist go back to childhood.

There is evidence that memories triggered by smell cause more intense emotions, clearer memories and stronger feelings of transport to the past than memories triggered by something that is seen.

But vision is powerful, it presents us with color, shape, texture, movement, speed, direction, distance, location, orientation and size. We recognize objects and people by looking, not smelling. When walking, we sweep the path with our eyes, without paying attention to the information captured by the nostrils. That’s why we often label smell as of little relevance and neglect it, despite the associations it commonly grants, such as those between cakes and childhood events; and between meteors, iron and blood.

Injustice. Our inattention to this sense makes us disregard that approximately 80% to 90% of what is perceived as “taste” is, in fact, a smell. Certain molecules, fruits of food, reach olfactory areas via the retronasal route, releasing electrical signals that will reach brain areas and, together with gustatory sensations, will build the flavor experience.

However, smell—possibly the first sense—did not arise in nature for the purpose of enhancing taste. Probably the primary function is navigation, guiding the being to move influenced by the distribution of odorants in time and space. Smells inform us of the past, so we know that someone just got off the elevator with a pizza and that the dog urinated on the currently dry mattress hours ago. Smelling also anticipates what we will find in places not reached by sight. We can use the nose to find the way.

From early hominids to humans, the skull increased in size and volume. The struggle against the pressure of the environment made brains with numerous areas of olfactory integration prevail. Thanks to this sophistication, a winemaker without being pressured can give in to the hedonic desire and analyze wine smells, including retronasal ones, compare the impressions obtained with other sensations, evaluate different olfactory constituents and name them separately. This cognitive process combines aromas, language, memory and different senses.

Smells facilitate connections between people, or dislikes. A female rabbit can kill her young if the offspring are contaminated with another female’s body odor. In humans, smells influence our social interactions, for example, we move away from people with bad breath. But you who read me must want less obvious examples. To please your patience I will be more technical.

The hexadecanal molecule emitted in feces, skin and human expired air increases aggressiveness in women and reduces it in men. Male brains have different activities than female brains in responding to the smell of substances chemically similar to androgen hormones.

However, it is difficult to say what are the behavioral consequences caused by smell disorders, because the human brain uses contingencies, such as compensation for other senses and established concepts. And, usually, events that damage our abilities to perceive smells cause other deficiencies, so it is not easy to say the isolated meaning of a failure.

We know that certain diseases run through the olfactory pathways, from the nasal sensory epithelium to the intracranial areas linked to or influenced by odors. The late negative impacts caused by the Covid-19 infection on the ability to decode scents in attention and memory follow this flow. Something similar is caused by Parkinson’s disease. Our brain opens up many avenues for smell.

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