A nose for good taste

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The nose is an expert in telling good from bad. Now researchers are trying to use the power of smell to detect the worst of the worst of the bad - diseases.

Bild/Illustration på näsor.Nasal turbines that create turbulence, seaweed-like hairs that swirl around and receptors that open a world of unlimited experience; poetry as olfactory researchers Johan Lundström and Mats Olsson enumerate things I did not know existed. This apparatus grants me access to the scent, smell and stench that the world has to offer, an ability that carries a lot of weight in nature. The ability to detect chemical substances, something we humans call smells, was the first sense to emerge in evolution and is found in all living organisms.

"Smells can go round corners and travel great distances, day or night. It's the optimum signal to use," says Johan Lundström, researcher at the Department of Clinical Neuroscience, Karolinska Institutet. His own interest in the sense of smell is something he traces to a dog he once had, the German Shepherd Ella. She had an ability to deduce information about other dogs by sniffing the ground, and he was intrigued.

"I began to wonder whether we humans have the same ability," he says. Ella later became a police dog and spent her last days sniffing out cocaine and bombs. Johan Lundström moved to the United States doing research on chemical communication in humans. Back in Sweden, he is now starting an olfactory laboratory at Karolinska Institutet together with Mats Olsson, Professor of Experimental Psychology, who is also studying how we are affected by the sense of smell.

As I listen, the aroma factory in my nose wafts library and steaming fresh coffee into my consciousness, a pleasant smell. But why is smell important? If I want to know what I have in my cup or where I am, there is no doubt that it is easiest to use my eyes.

Mats Olsson explains that smell does not compete with the other senses, but complements them. The fact is that a smell carries very little information that helps us identify what we are smelling. In surprisingly few cases, fewer than 50 per cent, do fully healthy people succeed in correctly naming common aromas such as coffee, lemon or vanilla if they do not already know what they are smelling.

"If someone looking at a pen isn't able to identify the object, you get worried. But with smell, it's different. Usually, it's the context that enables us to know what smell we are perceiving," says Mats Olsson.

Inability to identify smells is normal

So smell does not tell me that much about the world that I do not already know. On top of that, most novel odours smell bad. "Of the millions of olfactory substances perceptible to humans, the majority give rise to an unpleasant sensation," says Mats Olsson. But this is precisely the point: The sense of smell translates chemical substances into a qualitative experience, a feeling that is either pleasant or unpleasant. In this way, the nose lets us know what is good for us and what we should stay away from.

When we engage in something good, such as eating a well prepared meal, our sense of smell rewards us bounteously. This is because we eat with our noses - around 90 per cent of the taste we perceive when we eat is actually aroma sensations that arise when substances from the food stimulate the more than 400 types of olfactory receptors in the nose. However, substances that we breathe in and smell also travel in the opposite direction, from the nose and down into the throat.

"Everything you smell, you swallow. Every time you think the toilet smells bad, you're in essence swallowing a bit of faecal matter," Johan Lundström informs us.

And anyone who has had food poisoning knows how it feels when the olfactory system really puts its foot down. The distaste of what you ate that day can stay with you for years. It is known as smell aversion. But Johan Lundström says that our sense of smell can sometimes lead us the wrong way. If we get sick in the evening, a definite feeling of revulsion becomes associated with the dinner, which was the last thing we ate before getting sick. But since it takes some time to become sick, it is often the meal before that - lunch - which is the true culprit.

The sense of smell is closely linked to the amygdala and hippocampus, brain regions that are important in terms of emotions and memory. But it is not the smells that the brain attaches importance to, but what it believes is the cause of the smell. A nameless odour that is not associated with any object at all soon vanishes from our memory. It is virtually impossible to recall. And strangely enough, the name an odour is given determines how it smells.

"If you say 'smell this' and pass across a jar of parmesan cheese, a lot of people will wonder who's thrown up into the jar. If the jar is instead introduced with the words smell this cheese', the same person might think it smells deliciously good," says Johan Lundström, who in his research seeks to understand how smell works together with the other senses to create an overall experience.

Sensitivity to smells can be increased

Johan Lundström has shown that people are more sensitive to unpleasant smells than pleasant ones, and that sensitivity to specific smells can be increased significantly - if they are presented together with low-level electric shocks. The smell does then have the disadvantage of beginning to smell bad, but Johan Lundström hopes that the research will lead to a greater understanding of how olfactory learning works in the brain and to new ways of training up the sense of smell in people with impaired faculties.

Dogs are well known for their ability to smell, and sharks can detect a few tasty drops of blood in the water for miles. We humans, however, are not known for having sensitive noses. On the contrary - researchers concluded in the 1800s that the human sense of smell is almost non-existent. Since then, research on sight and other senses has made great advances, but the sense of smell has been almost completely ignored. Johan Lundström sees this as a real oversight.

"It's not true that humans have a bad sense of smell in general. It's actually extremely well developed, but our brains are wired so that we don't pay it that much attention," he says, referring to more recent scientific experiments that have gone to the bottom of the matter. In one of these, subjects were asked to crawl on all fours, blindfolded, and follow the trail of chocolate that had been drawn along the ground.

But capturing the fleeting nature of the sense of smell is not easy. An aroma often gives rise to different perceptions at different times - what smells good to a person in the morning may smell bad to the same person in the evening, when the sense of smell also tends to be up to 50 per cent lower. Even the subjects' noses are in a constant state of flux. Every month the millions of olfactory receptors disappear and are then pieced back together - in a way that no one understands.

"Every month you get a new sense of smell. How we can still experience the same smells is one of the mysteries of olfactory research," says Johan Lundström.

For some people, who have the olfactory disorder parosmia, their sense of smell appears to have been incorrectly assembled. The result is a badly wired sense of smell that mostly perceives pleasant aromas as negative - banana can smell of poo, and vanilla can smell of rotten eggs. The only thing that has been shown to help in some cases is to burn away the entire sense of smell and let it grow back.

"No one knows how it works, but sometimes the sense of smell is then restored," says Johan Lundström.

Smells are largely processed on a subconscious level in the brain - we detect and are influenced by many more smells than we are aware of. And the big difference between humans and other species is not how good a sense of smell we have, but what smells it is programmed to register.

"Animals hunting are really good at sensing the smell of meat. We humans are very sensitive to other scents, such as certain substances found in sweat," says Johan Lundström.

Sweat in particular is massively relevant in the research quest for olfactory substances that may play a role in human social communication - the equivalent of urine's key role in the canine world. A substance in sweat has been identified as the communication link behind the tendency of women living under the same roof to synchronise their menstrual cycles. Mats Olsson has shown that the same substance, found abundantly in male sweat, can affect the mood of women, even if its smell is masked so that it cannot be perceived.

A nose for age-old ailments

The composition of sweat can be affected by diseases, and it is well known that various diseases are associated with certain scents, which historically has been important knowledge for physicians. Mats Olsson wondered if humans might be extra sensitive to the smell of disease, so that it is possible for the nose to help detect a disease before it makes itself known in other ways.

He tested this by giving subjects a substance that activated their immune systems as if they were coming down with an illness. When others then got to smell the shirts of those simulating sick people, it did indeed turn out that they thought the shirts smelled worse than those worn by control persons.

"The effect is faint, yet fully measurable. You can imagine this as having been important during evolution, being able to pick up on which people are in the process of becoming sick and to give them a wide berth," said Mats Olsson.

Johan Lundström has been thinking along the same lines and has shown that people are sensitive to the smell of old people.

"It's not a bad smell, but there is a peculiar odour that can be perceived in old people's homes, for example. It appears that the smell of old people is actually the smell of immune reactions. They simply have more ailments," says Johan Lundström.

The researchers have not yet identified the scent molecules that convey the smell of disease. But the realisation that diseases and immune reactions leave scent trails has led researchers to try to develop new diagnostic methods based on the detection of olfactory substances. Johan Lundström, in collaboration with other researchers, are developing an electronic nose that is currently being trained to recognise olfactory substances indicative of skin cancer, breast cancer and even brain injuries. The project, funded by the United States Army, aims in part to be able to quickly sniff out signs of brain injuries sustained by soldiers in the field, much like scanning items at the supermarket.

"Smell detection can also have many applications in conventional health care because it is much quicker and simpler than the more common methods of diagnosis such as biopsies or blood samples," says Johan Lundström.

At present, Johan Lundström is examining how odours affect how we perceive strangers. The results have not yet been fully compiled, but the research suggests that the body odour of strangers makes us unfavourably disposed towards the outside world. A picture of a neutral face can suddenly be perceived as an angry face if the picture is first applied with a body odour, even if it is too faint to be perceived consciously.

So, the sense of smell makes us xenophobic?

"Yes, but it's a very good function to have. You can look at it as if the odours make us more on the ball and alert to what's going on around us when we encounter new people. In today's society, we might not need to be on our guard all the time, but in the past this has probably been crucial," says Johan Lundström.

When people rank their five senses in order of importance, smell usually ends up in last place. At the same time, olfactory research is gaining a lot of ground, and sometimes olfactory researchers extol it to the skies. But Mats Olsson calls for moderation.

"The sense of smell affects us in other ways than we usually think, but in today's society you can manage quite well without a sense of smell compared to how it is to live without sight or hearing, for example. But, sure, life would be duller without smells. Some of them I would miss a lot, like the smell of my children," he says.

Text: Ola Danielsson. Published in Medicinsk Vetenskap nr 2 2013