What the Nose Knows - Avery Gilbert [2]
Back in 1927, Crocker and another Little chemist named Lloyd F. Henderson were struggling for an objective way to classify odors. They settled on a method in which an odor was rated by how strongly (on a scale of 0 to 8) it resembled each of four elementary odor sensations. Given the mathematics of their rating system, it was theoretically possible to discriminate 94 or 6,561 different odors. The math is watertight, but the outcome is highly dependent on the initial assumptions. Had Crocker and Henderson used, say, five elementary sensations and a 0-to-10 rating scale, the estimate would have been 115 or 161,051 different odors. (Harvard psychologist Edwin Boring was a fan of the new system, but he believed the rating scale should have fewer steps. He did some calculations and decided that the number of distinguishable smells was somewhere between 2,016 and 4,410.) Discussing this work years later, Ernest Crocker generously rounded up the estimate to 10,000 odors. His colleagues took the number and ran with it.
In the end, it appears that no one has ever attempted to count how many smells there are in the world. Estimates of odor diversity lead either to a dead end or to Ernest C. Crocker. The comfortable, often-cited figure of 10,000 smells is, from a scientific perspective, utterly worthless.
WHY DOES IT MATTER exactly how many smells there are? Suppose we want to build a device that can reproduce every possible odor. (This is a popular fantasy. As a kid you may have scratch-and-sniffed your way through Mickey Mouse and the Marvelous Smell Machine.) A pair of industrial engineers once looked into how many distinct odors it would take to create a lifelike smellscape in virtual reality. They settled on a figure of 400,000. (This number has no more basis in fact than 10,000 or 30,000; its ultimate source is an obscure Japanese technical publication). Four hundred thousand is a staggeringly large number, but it sounds reasonable to engineers who use 16.7 million colors per pixel in visual displays for VR goggles. The trouble is that an engineer’s solution doesn’t always correspond to how the brain solves the problem.
The human eye detects tiny differences in color; across the visible spectrum we are capable of millions of such discriminations. Yet when it comes to naming categories of color, there is nearly universal agreement that only a half-dozen are needed to cover the range of human perception. People in all cultures get by very well with white, black, red, green, yellow, and blue. (Esoteric hues such as ecru and mauve occur mainly in clothing catalogs.) The physical spectrum of visible light is continuous; the stripes of the rainbow are created in our head. We give color names to these few categories.
This simplification of sensory input is a general feature of the brain, a phenomenon psychologists call categorical perception. In hearing, categorical perception helps us carve the continuous dimension of pitch into the individual notes of the musical scale, or the sonic blur of vowel sounds into a distinct a or e. Perhaps we shouldn’t obsess about the number of odors. Our question should be, How many natural odor categories are there, and how do our nose and brain simplify the world?
The Art of the Achievable
I grew up in Davis, California, amid the smells of agriculture. Our house, when we first moved there in 1962, was near vast tomato fields; walking through them brought up the sharp, funky smell of the vines. The approach of a new school year was signaled by the heavy, stewed smell of tomatoes being cooked into ketchup at the Hunt-Wesson