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Nov 3, 2020

Lingering questions about human smell

Depending on whom you ask, either bears or elephants have the best sense of smell in the animal kingdom. Black bears have demonstrated that they can accurately pinpoint food smelled from over a mile away, and elephants use olfaction—or, the sense of smell—to distinguish between two ethnic groups who live near them in East Africa. Dogs, in addition to their known tracking prowess when trained to find a certain scent, have also demonstrated remarkable smelling abilities by spontaneously ferreting out small numbers of cancer cells in their owners.

The perception of smell takes place between the olfactory sensory neurons on the roof of our nasal cavity and the olfactory cortex—the part of the brain that processes the information the sensory neurons pick up. Between these lies the olfactory bulb. Dogs have about 300 million sensory neurons, while we have closer to 6 million. Bears have up to seven times as many sensory neurons as dogs. Compared to humans, bears have significantly more surface area on which the neurons can live—their noses take up a significant part of their faces. Elephants’ noses are remarkable for a variety of reasons, and elephants are thought to possess the most genes (upwards of 2,000) dedicated to olfaction. Dogs’ snouts also provide a lot of surface area, but their smelling apparatus has an additional structural difference: when dogs breathe in, air is channeled either towards the lungs or to a recessed area dedicated to olfaction. According to NOVA’s Peter Tyson, “Within the recessed area, the odor-laden air filters through a labyrinth of scroll-like bony structures called turbinates. Like a whale's baleen sifting out krill, the turbinates sieve odor molecules based on different chemical properties” that the olfactory receptors help the brain recognize.

A similar physiological adaptation is found in rats, who smell in stereo, with one nostril picking up one scent, while the other is busy smelling something else. Rather than having a separate chamber devoted to smell, air flow is directed separately to the two nostrils, which are “almost completely isolated from each other and supply two distinct sheets of olfactory sensory epithelia,” according to a 2006 study. Amazingly, the rat brain can process each independent smell simultaneously. While physiological adaptations explain some differences in smell, olfaction remains the least understood of the five senses. Indeed, whether the animals cited above are actually better at smelling than humans are is a matter of some dispute, and that dispute is influenced by factors that are not limited to the nose.

Within the first few days of life, newborn humans can only see a limited range of color and can’t focus on objects more than a few inches from their eyes. They can’t control their limbs or intentionally touch with their fingers. They can smell just a few things. But babies can identify their mothers’ scent from at least 2 feet away. And, a breastfed baby will show a preference for the smell of her mother’s milk. Because there hasn’t been good evidence that humans produce pheromones that communicate between mother and baby, it’s thought that amniotic fluid familiarizes babies in utero with their mothers’ scents, suggesting that smell develops before birth. (This would account for why babies can distinguish their mothers from other mothers: a pheromone is the same across people, but amniotic fluid and breastmilk are specific to individuals). Although smell is important to children in infancy—perhaps even having survival implications—the sense of smell in humans is generally thought to be a “minor sense.” That is, a person is thought to be only 3% impaired when they lose the sense of smell, but 85% impaired due to blindness, according to the American Medical Association’s Guide to the Evaluation of Permanent Impairment as cited by Robert J. Stevenson.

One of the reasons that human olfaction is considered the least important of the senses is that smell is associated with weak “post-perceptual processing,” which refers to the ability to imagine a smell when you’re no longer smelling it, or to break smell down into units that would allow for you to, say, combine different units together. While you can imagine variations of what a cat-dog hybrid might look like (dog ears plus cat nose, or dog nose plus cat eyes, for instance) it’s harder to imagine what a rose smell combined with a bread smell might be like. Indeed, a rose smell is no longer a rose smell when combined with something else: it loses its inherent character.

It’s also hard to describe new smells without referring to known smells, making novel ideas difficult to express. For this reason, smell is considered a relatively inflexible, ineffectual means of communicating with fellow humans whereas language, for instance, is made up of sounds and words that can be combined to communicate brand-new ideas. Social communication is crucial to human survival, and—it is postulated—that’s the reason that our brains developed a fairly circumscribed neural apparatus with which to process smell. As this article puts it, “The rapid expansion of neocortical tissue in our human ancestors left olfaction languishing as a minor sense” when, recognizing that smell was not geared to facilitate communication between people, our evolving brains devoted fewer and fewer resources towards smell over time.

While the idea that smell is a minor sense has been generally accepted for centuries, a 2004 article by Gordon Shepherd and a 2017 review by John McGann hold the view that human smell has been mischaracterized as inferior to other animals’. Shepherd argues that there is little real information gleaned when we try to compare different animals’ smelling abilities, because the way we measure each animal’s sense of smell can never quite account for the different influences that affect how smell is perceived. Shepherd also argues that, for a fuller picture of human olfaction, there is a need to look at facets of smell that aren’t always taken into account, including the sinuses, the sense of taste, and the role of language in helping us conceptualize and understand what we’re smelling.

McGann likewise acknowledges the problem with comparing olfaction in different animals, writing that “the results are strongly influenced by the selection of odors tested, presumably because different odor receptors are expressed in each species.” Indeed, he notes that humans can distinguish between millions of odors, and actually outperform dogs in detecting certain smells. But our tracking abilities, while not absent entirely, are fairly weak, and McGann’s argument primarily addresses discriminatory abilities rather than sensitivity. That is, humans may be able to distinguish between as many smells as other animals do—and many more than some human olfaction researchers have acknowledged—, but that doesn’t mean that a human can be used the same way a bloodhound can.

Looking at the perception of human smell through history, it’s evident that the notion that humans smell poorly is partly a holdover from 19th century prejudices—Freud’s in particular— which associated smell with animality. Indeed, there is historical evidence that, since St. Augustine’s writing in the medieval period, dwelling on smell has even been dismissed as shameful (Le Guerer, 2002). We can see this phenomenon persist in the things that disgust us: flatulence has been considered gross (and funny), because smells—unlike sounds and images—to some degree invade our bodies. Once something has been smelled, the chemical that produced the smell is already inside us.

Whether or not humans are excellent smellers isn’t likely to be finally resolved for some time, but researchers can agree that it’s an important gauge of brain health. Humans’ sense of smell continues to expand until age 8 or so, but it declines in old age, correlating with decreased cognitive abilities. The sense of smell is a good measure of cognition, because smell is so closely associated with memory, and it’s considered challenging for anyone to describe smells accurately. Given that the perception of smell is also based in “selective responsiveness as a function of individual experience”—or, the ability to tune out or into certain scents based on their familiarity—, it makes sense that it might be a gauge for a person’s awareness of and adaptability to the world around them. Finally, smell appears to be related to executive functioning, though the link may be more of a correlation than cause-and-effect. But, whether your sense of smell is strong or weak, a change in your ability to smell is something to pay attention to.

By Aimee Fountain
















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