## A Rebuttal to the Science Paper that Claims the Human Nose Can Detect 1 Trillion Different Odors

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Last year, a paper published in Science made waves with the stunning claim that the human nose can detect a whopping one trillion different odors.

But if you feel like your nose can’t detect a trillion smells, you may be on to something. It’s possible that none of us can.

As a Caltech researcher pointed out last fall, and a new Arizona State University paper asserts today, the data collected in last’s year odiferous study does not support this radical claim. Rather, the researchers’ interpretation of their data — and the massive figure they came to —seem to be the result of flawed mathematical logic. And that’s a big problem, because the one trillion odor estimate is already making its way into neuroscience textbooks, misinforming students, researchers, and the public.

“We disagree with several aspects of the 2014 study,” said Rick Gerkin of Arizona State University, who, along with Jason Castro of Bates College, authored a rebuttal paper that appears today in the journal eLife. “First, the assertion that humans can discriminate between at least one trillion odors is based on a fragile mathematical framework — one that’s capable of creating nearly any result with small variations in the data or the experiment design. So the result in question could be tens of orders of magnitude — a factor of one with dozens of zeros after it — larger or smaller than first reported.”

Which is to say, based on the Science paper’s own reasoning, the correct number of odors the human nose can distinguish could be as few as ten — or it could be many, many times more than there are galaxies in the observable universe.

Where They Went Wrong:

If humans can indeed detect one trillion different smells, it would (somewhat confusingly) make the human nose far and away our most sensitive sensory organ. But the rebuttal paper published today argues that the approach used to reach the one trillion figure is mathematically unsound, because the answer you get depends steeply on parameters of the experiment:

Had the experiment enlisted ∼ 100 additional subjects similar to the original ones, the same analysis would have concluded that all possible stimuli are discriminable (i.e., that each of the more than 1029 olfactory stimuli possible in their framework are mutually discriminable). By contrast, if the same experimental data were analyzed using moderately more conservative statistical criteria, it would have concluded that there are fewer than 5000 discriminable olfactory stimuli—no larger than the folk wisdom value that the new estimate purports to replace.

Precise numbers often change when more subjects are enrolled in an experiment, but as Gerkin and Castro point out, they shouldn’t change in expectation — that is, if your estimate can swing wildly in either direction when you add more data, that’s a big red flag. By re-analyzing theScience paper’s own data using the very same methods, but varying the parameters—things like the number of mixture pairs, the number of subjects, and the threshold for statistical significance—Gerkin and Castro shows that the correct number of smells the human nose can distinguish could be as few as 4,500 or as many as 10^29. Clearly, something is very wrong.

If only all of life were as easy as "take a small sample, multiply it by a large number, and then estimate!"