Going viral: What Covid-19-related loss of smell reveals about how the mind works

A few months ago, a friend called me from New York in the middle of the day. “I just came out of the shower and thought of you,” she said. That exceeded our usual level of familiarity. “I cannot smell anything,” she continued. “I noticed I couldn’t sense my shower gel, so I ran into the kitchen and began to open and sniff all my spice jars. Nothing. This is spooky.”

My friend called because she knew I had just sent to my publisher the page proofs of a new book about olfaction, a subject I have been investigating for several years.

At the time, we had no clue as to why she had lost her sense of smell. Now we do: Covid-19.

Anosmia, the technical term for the once relatively unfamiliar loss of one’s ability to smell, is now all too common. It has become a critical diagnostic marker of Covid-19.

As we have come to learn, asymptomatic carriers — people infected with the novel coronavirus who don’t show visible symptoms — can still infect others. One telltale sign of otherwise asymptomatic carriers is that they often experience sudden-onset anosmia.

It took months of discussion before medical experts added loss of smell to the official list of Covid-19 symptoms. Such prolonged omission laid bare numerous popular if outdated opinions about the nose and how society, as well as science, has cultivated a highly distorted view of sensory perception by focusing almost exclusively on vision. That distortion delayed diagnostic progress for Covid-19.

A principal objection against using smell as a clinical marker was that it appeared merely anecdotal, meaning there hadn’t been sufficient systematic studies establishing a robust link between smell loss and Covid-19 infection. But what counts as sufficiently robust in this context? In comparison, the number of people with respiratory issues was far lower than the number of people with Covid-19-related anosmia. There’s no question that respiratory problems represent a more severe sign of the disease, but we seldom diagnose diseases only by their worst or end-stage symptoms.

At least half or more of patients worldwide with confirmed Covid-19 were diagnosed with full-blown anosmia. Germany reported that more than 2 in 3 patients with confirmed Covid-19 had anosmia. Even more striking is that 98% of Covid-19 patients were exhibiting a variety of smell dysfunctions, including reduced olfactory abilities (hyposmia) in addition to complete anosmia. It is hard to think of any other sensory disfunction with such shocking metrics to be dismissed that easily in clinical contexts.

The sense of smell has acquired a poor reputation over centuries. It has been viewed as too ephemeral, too fickle, and too subjective to be studied scientifically or considered a reliable sense. Olfaction also seems to lack the attractiveness of the visual system that has characterized modern Western science and culture. What you see is not always what you get, though.

Smell plays a far more prominent role in our lives than we give it credit for. Many people don’t immediately recognize they’ve lost their sense of smell, but instead report they’ve lost their sense of taste. Most of what you think of as the taste of your food and drink, however, is actually due to smelling.

When you chew, aromatic molecules are released from your food. These molecules travel up to your nose via the pharynx, the opening at the back of the throat that connects the mouth with the nasal cavity.

Think about it for minute. Your tongue detects salty and sweet, bitter and sour, umami (savory) and, according to recent research, fatty. There are no taste buds for mint or strawberry or vanilla. These flavors are created via “mouth-smelling,” a process known as retronasal olfaction. It acts as a second sense of smell.

That’s why everything tastes bland when you have a cold: When your sinuses are swollen or your nasal passage is blocked, aroma molecules from food can’t reach your nose. Given how much the common cold has been part of human history, you’d think we would have figured this out sooner.

This second nose reveals a lot about how the mind works. Why do we feel that flavor sensations of basil or garlic or orange are happening in the mouth, not in the nose? This enigma of what sensory scientists call oral referral has to do with the coupling of smell with the motor system. It makes sense for our sensory systems to be wired in a way that their sensations are mentally mapped onto where the action takes place in the body. In other words, when you eat something that’s potentially rotten, you want to spit it out fast, without a bigger margin of error for where to locate the course of action.

Linda Bartoshuk, a pioneer in gustation research at the University of Florida, pointed me towards this phenomenon when I interviewed her. Retronasal olfaction, she highlighted, has no directly voluntary action involved in its processing. Unlike orthonasal olfaction — inhaling or sniffing, the process we usually associate with smelling — retronasal olfaction isn’t something we initiate at will. You don’t inhale when you eat. You also don’t control the movements that bring the aroma molecules from your mouth to the nose; the air coming from your lungs does the job.

Test this on yourself: Hold your finger directly under your nose, then swallow. You’ll feel a subtle breeze coming out of your nostrils. That airflow is what makes aroma molecules travel upstream from your mouth to your nose.

What the workings of the nose show is that the mind’s experience is intimately linked to the body’s activity. Flavor perception tells us that conscious awareness is impossible to understand without the motor system and our sense of agency, meaning the experience of having control over our body with a certain feeling of ownership, in other words, this bodily sensation is part of my inner experience. This applies to all senses, including vision, as the treatment of pain in phantom limbs reveals. In 1994, neuroscientist V. S. Ramachandran famously used a mirror to trick the brains of people into having an experience of voluntary control over a limb that had been amputated. In one study, Ramachandran placed a mirror between an amputee’s intact hand and his missing one, then asked the patient to move both arms simultaneously while looking at his hand in the mirror. His brain soon adopted the mirror image as a representation of the missing limb, which now seemed to be back under Jimmy’s control as it seemingly could be clenched and unclenched. The phantom pain vanished.

Such examples are more than mere medical curiosities. They reveal how the mind works. Flavor is a more familiar experience than loss of one’s limb, however.

Olfaction offers an opportunity to rethink some of our traditional ideas about how the senses connect the mind with the world. The appeal of vision has always been the immediacy and directness with which it throws the world into our conscious awareness. That often leads us to underestimate the non-conscious elements that, while working in the backseat, actively shape our experience of the world and our sense of agency within it. Smell in particular works at the border of unconscious and conscious processing. That border may feel less real and less stable because it is under constant negotiation by the body’s nonconscious activities.

It is only when the sense of smell disappears that we suddenly realize we’ve lost an important texture of reality that is hard to describe, mainly because it is not always at the forefront of the conscious mind. “It’s just a decrease in quality of life,” is what doctors often say to people complaining about smell loss. What they mean is that you may “just” enjoy food and drink less, you may “just” not be able to smell your partner, you “just” lost a sense telling you things about the people and objects in your environment that are hidden from sight but, other than that, you will survive.

What Covid-19 is making clear is the absurdity of that perspective. Isn’t quality of life not also “just” the point of living, after all?

Ann-Sophie Barwich is a cognitive scientist, an assistant professor in the Department of History and Philosophy of Science and Medicine at Indiana University Bloomington, and author of “Smellosophy: What the Nose Tells the Mind” (Harvard University Press, July 2020).