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When we smell coffee, we’re really smelling more than 100 chemicals blended together.


Your nose knows more than scientists thought

You’ve probably heard that your sense of smell isn’t that great. After all, compared with a dog or even a mouse, the human olfactory system doesn’t take up that much space. And when was the last time you went sniffing the ground alongside your canine companion? But now, in a new review published today in Science, neuroscientist John McGann of Rutgers University in New Brunswick, New Jersey, argues that the myth of the nonessential nose is a huge mistake—one that has led scientists to neglect research in a critical and mysterious part of our minds. Science checked in with McGann to learn more about why he thinks our noses know more than we realize.

Q: Many of us assume our sense of smell is terrible, especially compared with other animals. Where did this idea come from?

A: I traced part of this history back to 19th century [anatomist and] anthropologist Paul Broca, who was interested in comparing brains across lots of different animals. Compared to the olfactory bulbs [the first stop for smell signals in the brain], the rest of the human brain is very large. So if you look at whole brains, the bulbs look like these tiny afterthoughts; if you look at a mouse or a rat, the olfactory bulb seems quite big. You can almost forgive Broca for thinking that they didn't matter because they look so small comparatively.

The smell testThe relatively small size of the human olfactory bulb—compared to other animals—has long been cited as a reason for human’s “inferior” scent of smell. New research is starting to overturn this notion. Mouse Olfactorybulb Dog Goat Macaque Human 0.31% 2% 0.18% Percent of olfactory bulb to total brain volume 0.1% 0.01%
Credits: (Graphic) C. Bickel/Science; (Data) McGann et al., Science 356 (12 May 2017)/Kavoi and Jameela, Int. J. Morphol. 29 (3): 939–946, 2011/Seidlitz et al., NeuroImage (28 April 2017)

Broca believed that a key part of having free will was not being forced to do things by odors. And he thought of smell as this almost dirty, animalistic thing that compelled behaviors—it compelled animals to have sex with each other and things like that. So he put humans in the nonsmeller category—not because they couldn't smell, but because we had free will and could decide how to respond to smells. The idea also got picked up by Sigmund Freud, who then thought of smell as an animalistic thing that had to be left behind as a person grew into a rational adult. So you had in psychology, philosophy, and anthropology all these different pathways leading to presumption that humans didn't have a good sense of smell.

Q: So what evidence should make us question these early assumptions?

A: A unique study a few years ago calculated that that we could tell a trillion different odors apart. [In another study], researchers laid out a smell trail on a field at [University of California,] Berkeley. They blindfolded undergraduate subjects and gave them earmuffs so they couldn't use any senses other than smell (see video, below). They found the students were perfectly capable of following the trail laid out in the field. The study didn’t test humans and dogs head to head—I think the dogs might win depending on the odor—but what it shows is the limits of human smell tracking haven't really been investigated.

Q: What are humans good at smelling?

A: If you look across ordinary odors that might even be very similar to each other, you'll notice that the humans will be most sensitive to one and dogs will be most sensitive to another. It’s unclear without more data. [But there’s] a neat paper showing a component of human blood odor that humans are really sensitive to.

Q: Blood? Wow! But I’ve never noticed being able to smell blood. Does that tell us anything about how smells work differently than other senses?

A: Every sense is unique and has its own quirks, bugs, and features in the human brain. One of the things that makes smell distinctive is that olfactory information is not rooted through this “switchboard” structure called the thalamus on the way to other “thinking” brain regions. It goes from the nose to the olfactory bulb and then directly from there to the olfactory [processing area] but also to places like the amygdala and the hippocampal formation, which are involved in emotion and memory. It’s a very common experience to have smells evoke strong memories. It's been speculated that that might be related to the different wiring.

Another [factor] is that the nature of smell is very synthetic. Most smells in the real world are mixtures of lots of chemicals. A cup of coffee has about 150 different chemicals … that you can smell. But you don't have 150 dimensional perceptions—you just smell coffee. You can't really think of it or label it quite as easily as you can describe the [sight of the] cup that the coffee is in or the music that's playing in the background.

Q: So if we’re not conscious of our sense of smell the way we’re conscious of our sight, why should we care about it?

A: Lots of people lose their sense of smell and they really feel it. There is increasing evidence that when you lose your sense of smell it impacts your overall psychological well-being. There are some associations with depression and a change in your senses of attachment with others. But when you go to the doctor and say, “Hey, I lost my sense of smell,” generally the physician shrugs and says, “That's too bad.” There's not really a lot that can be done about it, and there's not a great understanding of what causes it.

Q: Now that we’re having these conversations, do you think the misconception that humans can’t smell well will change?

A: It’s stuck around for 150 years. Part of it is that once things get into the textbooks, they’re really hard to get back out. I’ve seen terrible misinformation about smell in intro psychology textbooks even last semester. But I really hope that this is the beginning of the end of this myth.

Want a bigger whiff? Listen to John McGann talk about his work on Science’s weekly podcast.