The fascinating sense of smell and the consequences of its loss in Covid-19 – December 18, 2020 – Luciano Melo

The millennia of evolution have endowed the sense of smell, the ancestor of the senses, with strong connections to the brain centers of memory and behavior. However, the complexity of life has increased, and the rich visual experience has made smell a faint sense in humans.

This is how the first neuroscientists thought and, without much criticism, classified humans as an almost anosmic animal. For them, the sense of smell was something bestial, the opposite of what is necessary for “spiritual rationality”.

You got it wrong. The human sense of smell is excellent. We don’t distinguish well between urine smells on bars, but we do distinguish different types of perfume from the same food. Our noses can even detect volatile products made up of one or two atoms.

From the 1960s onwards, amazing accounts of odor formation were published. Including the story of a blind young woman, a laundry worker, who is able to separate the clean clothes of her customers and smell them. And also the case of Ms. Helen Keller, who is just as blind, but can recognize people by their smell. After the evaluation, nothing special was found in her sense of smell. Helen had just learned to judge smells better.

Smells also affect interpersonal relationships. It seems unlikely, but people affected by body odors understand other people’s personalities correctly. Natural human scents appear in various combinations of substances in the skin, saliva, urine and genitals. Pubic and armpit hair have the biological role of holding them. All of these smells are influenced by neurotransmitters and hormones.

Serotonin and dopamine modulate various physiological functions, including the intensity of sweat. Testosterone changes the function of the epithelial glands. These biological molecules also modulate aggression, fear, social closeness, and dominance. In other words, they regulate personality traits and ultimately the flavors that result from the breakdown of sweat on the skin. With these premises in mind, the psychologist Agnieska Sorokowska let volunteers smell strangers’ T-shirts and then guess who was using this piece via personality. The hits were significant.

We can also smell genetic variability. Our genes code for the major histocompatibility complex (MHC), an arrangement of amino acids that enables the immune system to recognize invading microorganisms.

The biologist Manfred Milinski showed in a study with pictures of the brain function that we humans can recognize the differences in the body odors caused by the addition of MHC amino acids. The practical meaning of this research is the disclosure of our ability to perceive MHC differently than our own, so that we “feel” genes that are different from our own. In this way, we have an unconscious means of avoiding sexual partners with genetic material similar to ours. Therefore we reduce the risk of inbreeding conjunctions.

Recent experiments have mapped the effects of smells on brain function. Then it was discovered that olfactory stimuli directly stimulate areas of the brain related to emotions, even before nerve regions that specialize in raising awareness of the importance of smell are activated. That is, smells can evoke emotions even before the brain interprets, evaluates and catalogs them. So “it stinks” occurs “it smells like used socks”.

The Sars-CoV-2 virus can get in the way, as it often causes anosmia, the inability to smell odors. Because smell and taste are constantly confused, infected people often complain of the inability to perceive the taste.

People with established anosmia suffer from the loss of the joy of eating. They feel more vulnerable because they cannot spot bad foods or harmful gases. They are also more anxious, insecure about personal hygiene, and prone to social isolation. We’re not sure if the anosmia caused by Covid-19 is always reversible, but luckily it looks like it.

References:

1-Sorokowska A, Sorokowski P, Szmajke A. 2012 Does the personality smell? Accuracy of personality
Assessments based on body odor. Eur. J. Personnel. 26, 496-503. (doi: 10.1002 / per.848)

2-Havlicek, J., Dvorakova, R., Bartos, L. & Flegr, J. (2006). Not advertised does not mean hidden: body odor changes during the human menstrual cycle. Ethology, 112, 81-89.

3- Fernando Poyatos (2002). Nonverbal communication across disciplines. Volume 1: Culture, Sensory Interaction, Language, Conversation.

4- Milinski M, Croy I, Hummel T, Boehm T. 2013 Important peptide ligands of the histocompatibility complex as olfactory cues in the assessment of human body odor. Proc R Soc B 280: 20122889. http://dx.doi.org/10.1098/rspb.2012.2889

5-Krusemark EA Novak LR Gitelman DR. Li W. 2013. When the sense of smell meets emotions: olfactory processing dependent on anxiety and adaptation of neural circuits. J Neurosci. 33 (39): 15324-15332.

6- McGann JP (2017) Bad human odor formation is a 19th century myth. Science 356: eaam7263. https://doi.org/10.1126/science.aam7263

7- Arnold TC, You Y, Ding M. Functional connectome analyzes show the organization of the human olfactory network. 2020 Jul-Aug; 7 (4): ENEURO.0551-19.2020. doi: 0.1523 / ENEURO.0551-19.2020

8-Croy I, Nordin S., Hummel T. (2014) Olfactory disorders and quality of life – an updated review. Chem Senses 39: 185-194. ttps: //doi.org/10.1093/chemse/bjt072

9- https://www1.folha.uol.com.br/colunas/luciano-melo/2020/10/a-perda-de-olfato-que-o-novo-coronavirus-causa.shtml

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