Sunday, March 23, 2014

Trillions and Trillions

The ubiquitous and unanchored claim that humans can discriminate 10,000 different odors always bothered me. It bothered me so much that I spent days in the archives tracking down its source. The story of my quest, and the exposure of “10,000 odors” as a canard dating back to 1927, serve as the opening to What the Nose Knows.

My pals and scientific colleagues Andreas Keller and Leslie Vosshall were not satisfied with a dead duck: they wanted to find the real number. And so, along with Caroline Bushdid and Marcelo O. Magnasco, they came up with a novel way to quantify the discriminatory power of the human nose. The resulting experiment, published Thursday in Science, has received a lot of (well-deserved) attention. They estimate that there are at least 1.72 x 1012 discriminable smells.

That’s eight orders of magnitude bigger than the number proposed eighty-seven years ago by Ernest C. Crocker and Lloyd F. Henderson. To be fair, Crocker and Henderson were chemists trying to create a numerical classification of odors—a cool idea in itself. Estimating the number of discriminable odors wasn’t their main goal but such a number was necessarily implied by the limits of their rather arbitrary classification scheme.

The new number was arrived at by adopting the formula used to pack multidimensional spheres into an N-dimensional space. Check it out in the supplemental online material to Bushdid, et al. It’ll blow your mind.

The study discussed here is “Humans can discriminate more than 1 trillion olfactory stimuli,” by Caroline Bushdid, Marcelo O. Magnasco, Leslie B. Vosshall & Andreas Keller, published in Science 343:1370-1372, 2014.

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