May 15, 2017

Study sniffs out myths about humans' sense of smell—it's better than you think

Daily Briefing

    This popular story from the Daily Briefing's archives was republished on Dec. 20, 2018.

    Smell has long been thought of as one of humanity's weaker senses, eclipsed by our senses of sound, vision, and touch—but new research published in Science suggests that we've been underestimating our sense of smell for more than a century.

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    A 19th century myth

    According to John McGann, study author and a sensory neuroscientist at Rutgers University, the idea that humans have a poor sense of smell, originated nearly 150 years ago with a neurosurgeon and anthropologist named Paul Broca. Broca—under pressure from the Catholic Church to distinguish humans from other animals—theorized that humans' sense of smell was diminished in comparison to the olfactory systems of other animals because humans had free will.

    Broca rooted his claim in his observations that the frontal lobe of the cerebral cortex—the part of the brain that Broca identified with rational thought—was larger in humans than among other mammals, while the two areas of the brain that process smell, olfactory bulbs, are comparatively smaller than those found in other animals.

    In 1879, Broca divided animals into two groups: Osmatics, or animals that used smell as their primary sense, and non-osmatics, whose primary sense was not smell. He categorized humans in the latter group, not because we could not smell, but because he thought we could choose whether to respond to a given smell.

    Over the years, several researchers—even Sigmund Freud—perpetuated Broca's belief that human's had an inferior sense of smell when compared with other animals, McGann writes. Freud contended that human's sense of smell is "usually atrophied," and claimed that people who took pleasure in scents were more animalistic and more inclined to have a sexual disorder.

    The science

    In his latest research, McGann explained that on first glance, human anatomy seems to support these hypotheses. After all, our olfactory blubs do take up less space in our brains than they do in other animals—just 0.01 percent, in fact, compared with the 2 percent of a mouse's brain.

    But McGann pointed out that the comparison is flawed. The olfactory bulb might take up a smaller amount of space in the human brain when compared with the total amount of space taken up by a mouse's olfactory bulbs, but our brains are much larger than those of a mouse. In fact, in direct comparison, the human olfactory bulb is more than four times as large as a mouse's.

    In contrast, McGann points to a range of recent research showing the comparative strength of a human's sense of smell. For instance, evidence suggests that humans can discern billions, and potentially even trillions, of different scents, debunking the widespread but untested theory that humans can distinguish only about 10,000 odors. And another study found that humans are capable of tracking a scent trail of chocolate odor outdoors in a manner similar to dogs.

    And yet more research shows that humans are more sensitive than rats or monkeys to certain smells, even as they are worse at other scents. For instance, McGann cites studies showing humans are as good as dogs at sniffing out amyl acetate, a chemical with a banana odor, and we're better than mice at sniffing out a scent found in human blood.

    Moreover, McGann writes, while the size of olfactory bulbs differ from species to species, humans have roughly the same number of neurons in that part of the brain as do mice, capybaras, and star-nosed moles.

    The human sense of smell is excellent, McGann concluded, and humans who have fully functional olfactory systems can discern nearly all volatile chemicals larger than an atom.

    But the real problem with the whole situation, McGann said, was less about the sense of smell and more about the long-lived assumption. "How could so many scientists have shrugged and went along with it, even though there was no actual testing?" he asked. "That's what really bothers me."

    Comments

    Johannes Reisert, who studies olfactory receptors at Monell Chemical Senses Center, praised the research. He explained that while olfactory scientists generally know that the human sense of smell is strong, the vast majority of people outside that area of expertise tend to underestimate humanity's capacity for smell. McGann, Reisert said, makes a "correct case that our sense of smell actually is pretty good in doing what we need it to do."

    Separately, Alexandra Horowitz, an expert at canine cognition at Barnard College, agreed that there hasn't been "exhaustive cross-species testing of sensitivity to different odorants." The research that has been conducted, she added, "often find[s] dogs able to detect at lower quantities [than humans], and do sometimes find humans surpassing or equal to non-humans (as on amyl acetate)." According to Horowitz, humans are particularly good at smells associated with food, such as using a sense of smell to determine which mixes of spice will taste good.

    But Horowitz only acknowledged that humans' sense of smell "is really not poor"—she stopped short of saying humans are particularly good smellers compared with other mammals. The situation is made more complicated because there's no metric by which to measure different scents, according to the Washington Post's "Speaking of Science."

    Meanwhile, McGann said he hoped his research spurs a greater appreciation for the role of smell in our overall health and wellbeing. Not only could we dedicate more resources to the role of smell in medicine, social communication, and emotional processing, he said, but we could also investigate the consequences of a malfunctioning sense of smell (Netburn, "Science Now," Los Angeles Times, 5/11; Guarino, "Speaking of Science," Washington Post, 5/11; Resnick, Vox, 5/11; Klein, New York Times, 5/11; Hadley, McClatchy/Sacramento Bee, 5/11).

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