Smell has a direct route to brain areas that encourage people to take action. Evolutionarily speaking, it has been useful to smell danger, for example. But what good is a particularly good sense of smell for modern humans?
Monique Siemsen27 January 2024, 12:12
It is often thought that humans have a bad sense of smell, especially compared to dogs. But that is not correct, says smell and taste scientist Sanne Boesveldt, associate professor in Wageningen. “We can smell more odors than we can see colors or distinguish sounds. We are just not always as aware of an olfactory perception as we are of, for example, a visual stimulus.”
Scientific research has shown that humans are also excellent smellers. Man is a star, especially when it comes to distinguishing certain odors. For example, we recognize geosmine out of thousands, in fact: people are able to distinguish 1 particle of geosmine in 250 billion particles of air. It is the smell that comes from, among other things, freshly mown grass. You can also smell it after a rain shower, if it has been dry for a long period of time.
Tetrahydrothiophene (THT) is another substance that does not pose a problem for humans. THT has a sulfur smell and is added to the odorless natural gas to warn us of a leak. We could still smell a few drops of expiration date in an Olympic swimming pool.
Fragrance molecule is encrypted lightweight
To understand what people benefit from that excellent nose, it is important to know which route the odor molecules take in our brain. Smell starts with a separated molecule that an olfactory organ knows how to find. To do this he must be able to float. Close to the source there are millions of molecules and smell is strongest. But especially in the open air, the molecules blow in all directions. A scent molecule is by definition lighter than other molecules.
In order to travel by air, a molecule must be lighter than 300 daltons, which is the unit we use to indicate the mass of a molecule. For comparison: the dalton of most proteins is expressed in kilo or megadalton. Once in the air, odor molecules can float for miles before reaching an olfactory organ. Smell therefore has major advantages over vision, for example. You can also smell in the dark, when something is far away or when an object is around the corner.
That volatility alone is not enough. An odor molecule must also have the right shape so that we can encrypt the olfactory signal. This happens in a journey from your nose to your brain, which takes place very quickly. Sometimes routes are even completed almost simultaneously. In any case, the first stop is the olfactory mucosa, or epithelium. This is not located in your nose, but where your nose ends and merges into your skull. Just below the brain. It is a horizontal area the size of a postage stamp, consisting of cilia. The nose provides protection and forms a channel through which the odor molecule can make its way to the epithelium.
Once there, the receptors located in the cilia are waiting. There are about 400 different receptors that encrypt the molecules. Boesveldt compares it to a key and keyhole. “It is slightly less strict, some receptors can decipher multiple molecules and vice versa: some molecules can reach multiple receptors. By calculating the combinations, science arrives at about 1 trillion different scents that people can smell. After that first encryption, an odor signal is delivered to the olfactory nerve, which carries the signal further into the brain.”
Direct connection between smell and emotion
The olfactory signal arrives first at the olfactory bulb, the olfactory bulb, which is an organ that dangles at the bottom of the brain and resembles a cotton swab. There an initial identification of the odor takes place. There is no direct identification of what you smell or another cognitive placement yet.
Yet you can do a lot with it and Boesveldt points out its evolutionary importance. “Certainly for negative things, you must be able to respond quickly if danger threatens. Before you know what you smell, you are already acting. If you smell a sulfur odor in your home, you literally immediately follow your nose to see where it is coming from. The fact that we act automatically and unconsciously is often in the interest of safety.”
After the olfactory bulb, the olfactory signal goes to the primary olfactory cortex, where your brain names the odor. From there the journey continues to the secondary olfactory cortex and certain parts of the limbic system, such as the hippocampus, hypothalamus and amygdala.
In the secondary cortex, your brain evaluates and combines the olfactory signal with other signals such as images and sounds. The limbic system is the part of the brain that regulates emotion and memories, among other things. So you become more aware of the smell, perhaps remembering where you smelled it before and what the smell means. It sounds like quite an undertaking, but the stations are passed in just milliseconds.
National highway to your brain
It is striking during this neuro-journey that olfactory signals skip the thalamus, while visual and auditory signals pass over it first. But smell has a direct route to brain areas that recognize primary emotions and compel you to take action. That is why smell is sometimes labeled as the highway to your brain. This also means that smell makes a more difficult cognitive and verbal connection than image and sound. These signals are first identified, recognized and only then processed emotionally.
Boesveldt explains that smelling happens more subconsciously than seeing or hearing, but that our sense of smell does not suffer from this. “Smell is not only important when there is danger. It also influences how we experience our food. What we taste comes largely from smell and not from taste. Think of a strawberry. Your taste buds register mainly sweet and a little sour. And that also applies to a cherry or a pineapple. What makes a strawberry for you is that very specific smell, which distinguishes it from other fruits.”
Image Bart Friso
Social function of smell
Moreover, our sense of smell has a clear social function. So people smell each other well. And they manage to extract an astonishing amount of information from it. Research shows that not only dogs, but also people can smell certain cancers, says Boesveldt.
“People appear to be able to smell Parkinson’s disease and infectious diseases. There is little margin for error. There is a scientific example of the Scottish nurse Joy Milne, who was known for her sensitive sense of smell and who had smelled Parkinson’s in one of the test subjects. It was an incorrect result, because at the time of this experiment this person had not been diagnosed with Parkinson’s. Yet this person was diagnosed with Parkinson’s a year later.”
How people can smell certain diseases most likely has to do with the changed biochemistry that can cause these diseases. The development of Parkinson’s, for example, is accompanied by increased and altered production of sebum. These excreted substances can be smelled by some.
Scent influences emotions
However, people also smell emotions in each other. Without being aware of it, we then show the same emotion. Professor of Psychology Monique Smeets is affiliated with the SmelLab at Utrecht University and investigates, among other things, how scent influences emotions.
“With anxiety, for example, we see a significant effect. In one study, we had one group of participants watch scary movies and another group watch regular movies. We then collected the sweat produced and had subjects smell it. The test subjects were fitted with sensitive measuring equipment on the face, which captures micro-expressions of emotional expression. When the test subjects smelled fear sweat, their facial muscles showed activity that we associate with fear. We saw the same thing happen with the emotions of happiness and disgust, but to a lesser extent.”
Modern humans still have to deal with scent and we are increasingly looking at whether we can use scent to increase our well-being. Because scent quickly evokes memories, dementia patients, for example, would benefit from scent therapy. However, the possibilities are limited because the sense of smell no longer works properly, especially in Alzheimer’s patients. They often smell much less well, which means that music, for example, could provide a better stimulus to the brain.
Yet Smeets does see applications in the field of well-being. “Whether scent can be used therapeutically: I say an unequivocal ‘yes’ to that. We call this branch of science aromachology. Research shows that aromachology works and activates a psychological mechanism, without pharmaceutical treatment. And that mechanism is what it’s all about. If you feel bad and you can feel a little happier again, through an aroma from nature: what’s wrong with that?”
Everyone smells different
Not all people have the same sense of smell. But we can train ourselves to smell better. The question is whether it will really be your sense of smell that will perform better. You probably won’t really smell better, because you have to make do with the receptors that you have been assigned. But your distinctive and naming ability of smell could increase. So you would actually train your brain.
Smelling is highly personal. Not everyone has exactly the same amount of receptors of a certain type. You may be missing specific receptors and then you cannot smell certain odors. In that case you have selective anosmia, or the olfactory equivalent of color blindness.
That difference in sense of smell also depends on your gender. Female test subjects are often involved in experiments and research, because they generally have a better sense of smell. When Smeets started experimenting with men’s sense of smell, she found something surprising. “Men do not so much distinguish emotion as gender. They could smell whether the collected sweat belonged to a woman or a man. This indicates a strong evolutionary importance of smell.”
Scent tells people exactly what they are eating, whether there is fresh water and food miles away and whether we have anything to fear from strangers. We also become happy or sad with each other. In modern humans, all this happens almost without us being aware of it. The question is what influence scent still has on us today. Does scent determine how we behave towards others?
Scientific research says yes. For example, men appear to be able to smell the difference between women’s tears and normal saline solutions. What was even more apparent: men who smelled women’s tears showed a reduction in aggression.
Smeets points out that women also respond to tears, especially children’s tears. “The opposite happens to them: aggression increases. The effects in these types of studies are not yet strong enough to be able to say with certainty, but it gives the impression that scent can have an aggression-regulating effect. Men show less aggression towards women and women get the boost to protect children if they are in need.”
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2024-01-27 11:12:00
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