Perfume or Trash: Understanding Our Brain’s Judgment of Scents

Personally, I would say that I like the smell of lemon, but I don't like the smell of garlic. We have just taken a tour of our individual olfactory preferences. Indeed, for each odor, we attribute a sensory appreciation to it, like a cerebral cursor, ranging from pleasant to unpleasant, including neutral, and all these positions define our relationship to odors. But our olfactory preferences are not limited to this “I like/I don't like” and come from complex and fascinating cerebral mechanisms that are very studied by neuroscientists.

[Un article issu de The Conversation écrit par Hirac Gurden, Directeur de Recherches en Neurosciences au CNRS, Université Paris Cité]

We feel differently throughout our lives

Olfactory preferences appear very early in our lives, from birth. Innately, odors containing sulfur, for example, indicate the presence of putrefaction or toxic plants in nature. They are therefore repulsive to the newborn, who has never smelled them before. The reason here is evolutionary, since beings unable to detect and perceive this type of odor did not survive. We are now all equipped with a brain circuit that links the smell of rotten eggs to a characteristic facial expression of disgust.

However, sulfurous odors will only remain partially repulsive for adults: we are very sensitive to the sulfurous odor of city gas which alerts us, but the sulfurous odors released by cooking garlic are no longer aversive for people who enjoy eating them in a dish. Conversely, a few very rare odors such as vanilla or banana can be perceived instantly and pleasantly by the newborn. But, as we have seen with the example of garlic, these innate perceptions and preferences remain limited and will quickly and strongly evolve with each person's experience. The social and cultural context (family, school, etc.) will influence and place our sense of smell at the crossroads of individual and cultural paths.

Garbage cans versus perfumes

Rejection reactions to an odor are related to its concentration in the air. Declaring “it smells too strong” is generally accompanied by a facial disgust reaction. Thus, garbage cans give off large quantities of odors, strongly marked by sulfur and nitrogen and molecules of the butyric acid family, compounds that always smell bad to everyone.

For odors that are not in these olfactory categories, such as perfumes, it is the associations between odors and positive or negative experiences that will mark the pleasure or displeasure of these odors in memory. Thus, a majority of people will declare that lavender smells good and a minority that it does not smell good, the same for this or that perfume, all this depending on the events of each person's life.

The pleasure of smell perception based on our genes and experiences

Olfactory sensitivities related to the quality and quantity of odors therefore vary on both a genetic and cerebral basis. Indeed, each individual does not have the same number of families of odor receptors, nor the same exact quantity of these receptors, which can greatly change our perception and preferences. For example, it has been proven that some people detect the smell of coriander much more intensely than others. This hyperstimulation has the effect of evoking the smell of crushed bedbugs or the taste of soap. These people indeed have a large number of receptors specific to this odor, which causes this disgust. In addition to this genetic variability, life experiences are added.

A birthday or a vacation, our memories are marked by specific olfactory signatures: the smell of chocolate cake, or the smells of the beach and the sea on the coast, which will always be perceived in a pleasant way. Same places and same smells, but this time another person unfortunately experiences an accident. The smells of the coast then take on a negative valence, because they have been associated with a dangerous or risky situation. The brain therefore constantly builds associations between our sensory perceptions and our experience, a mechanism that greatly guides our behavior.

Thus, it is both our genetic heritage and the context of the perception of new odors that give us our own abilities to detect and appreciate odors. Odors are categorized according to their valence (positive, negative or neutral), which means that we perceive them as pleasant, or neutral or unpleasant… And everything can change, under the effect of new experiences: the brain is an organ that is constantly adapting.

The complexity of the olfactory brain

Olfactory pleasure is strongly associated with the activation of the reward circuit in the brain, which involves neurotransmitters, i.e. molecules that allow communication between neurons. This is particularly the case for dopamine, which plays a crucial role in the sensation of pleasure and reward. When an odor is perceived as pleasant, the reward circuit is activated, releasing dopamine in regions such as the nucleus accumbens. This release of dopamine reinforces the association between pleasant experiences (birthday + chocolate cake + family + friends + gifts) and motivates behaviors seeking similar pleasures. We then eagerly await our birthday to relive this pleasant situation marked by positive odors.

Pleasure and displeasure are also based on emotions such as joy or disgust that are expressed through the activation of a structure called the amygdala, not the one at the back of our throat, but a group of neurons in our brain. Thus, it is the complex activity of a large set of brain regions that determines our olfactory preferences.

Finally, back to the little olfactory game. Let's remember the two smells we thought of at the beginning of the article and try to describe the emotions and memories associated with them. We won't be surprised to realize that we chose them because of the context and the events and feelings they evoke in us: a beautiful example of the olfactory brain in action, connecting our personal history to our social and cultural environment, by paying attention to the world of smells around us.