We often hear that humans—esdpecially compared to other animals—have a poor sense of smell. The number 10,000 is often cited as a best guess about the number of odors the human brain can differentiate—a number far smaller than the half-million or so tones we can hear, or the millions of different colors we can differentiate.

That estimate of 10,000 odors, though, is part fact and part speculation. The factual part concerns the anatomy of the olfactory equipment in the nose and brain: A region of the nasal lining called the olfactory epithelium contains 10-20 million olfactory neurons. Spiny projections (dendrites) on these neurons extend to the epithelial surface where they end in knobs. On the knobs are tiny hairs that float in a layer of mucus. These hairs contain odor receptors, about 400 different kinds. Odor molecules fit these receptors the way keys fit locks. When an odoriferous molecule binds to its perfectly matched receptor, a nerve impulse begins traveling to the brain's olfactory center. Although signals begin in a single type of receptor, many receptors bind to more than one type of odorant, and many odorants activate more than one kind of receptor.

So how does the brain end up distinguishing large numbers of aromas using only 400 or so different kinds of receptors?

Testing smell receptors in mice, researchers in the 1990s showed that different odorants activate different combinations of receptors. The brain identifies odors by evaluating patterns of impulses. To oversimplify for the sake of example, imagine that chocolate candy stimulates receptors A, B, and C, and that simmering hot chocolate stimulates B, C, and D. The brain distinguishes the two because the combinations are different. Thus, each smell has its own unique code, corresponding to a different combination of activated receptors.

These are the facts which produced the estimate of 10,000 unique odors detectable by humans.

But now new research published this week in Science challenges that estimate, suggesting that the actual number may be far higher.

"In the scientific and lay literature," says Andreas Keller of Rockefeller University in New York City, "one often finds the statement that humans can discriminate 10,000 different smells. However, my and many other researchers' experience is that it is very difficult to find two odor mixtures that smell the same, and there are very many odor mixtures. So, it seemed clear to me that there are very many more than 10,000 different smells. Then I read in Avery Glibert's What the Nose Knows: The Science of Scent in Everyday Life, the interesting story about where this number comes from and how it made its way into the scientific literature. That inspired me to undertake this research."

Keller worked with a team led by Caroline Bushdid in France to investigate how well 28 adults could distinguish among scents created with varying numbers of the same building block odors. The researchers mixed 128 different odor molecules (representing a diverse range of smells) into groups of 10, 20, or 30 parts. Then they asked volunteers to sniff over 200 pairs of these cocktails, observing how much mixtures had to differ from each other to be discriminated. In some pairs, scent components greatly overlapped, making distinguishing the two scents quite challenging, though some participants could do so even when components overlapped by 90 percent.

Based on the results they obtained through these tests, Bushdid, Keller, and their team calculated that humans can discriminate at least one trillion scents—far more than ever before estimated.

"The idea that humans have a bad sense of smell is a myth," Keller says.

For More Information: Article #13: "Humans Can Discriminate More than 1 Trillion Olfactory Stimuli," by C. Bushdid; M.O. Magnasco; L.B. Vosshall; and A. Keller. Science. March 21, 2014.

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