It’s been generally accepted for some time now that the human sense of smell can detect 10,000 distinct odors. Yet many scientists have suggested this number was too low, even dramatically so. In an effort to come up with a more precise estimate, Dr. Andreas Keller from Rockefeller's Laboratory of Neurogenetics and Behavior, created an experiment for testing volunteers' ability to distinguish between complex mixtures of scents. Based on the results, the research team calculated our human sense of smell as being capable of detecting more than one trillion odor mixtures. "The message here is that we have more sensitivity in our sense of smell than for which we give ourselves credit,” Keller stated in a press release. “We just don't pay attention to it and don't use it in everyday life.”

A Rose By Any Other Name Would Smell As Sweet

The odors we encounter every day contain complex mixes of molecules, and for this reason the quality of an odor has multiple dimensions. The familiar scent of a rose, for example, has 275 distinct components, but only a small percentage of these dominate when we actually smell the flower. When creating mixtures for the experiment, then, Keller drew upon 128 odor molecules responsible for relatively common scents, such as orange, anise, and spearmint. Next, he mixed these in combinations of 10, 20, and 30 with different proportions of components in common. Finally, the volunteers received three vials — two containing identical mixes, one with a different mixture — and they were asked to pick out the odd scent.

While individual volunteers' performance varied greatly, on average, they could tell the difference between mixtures containing as much as 51 percent of the same components. Once the mixes shared more than half of their components, though, fewer volunteers could spot any differences between them. This was true for mixes of 10, 20, and 30 odors.

Taking this data and analyzing it, the researchers were able to calculate the total number of distinguishable mixtures.

"It turns out that the resolution of the olfactory system is not extraordinary — you need to change a fair fraction of the components before the change can be reliably detected by more than 50 percent of the subjects," said Dr. Marcelo O. Magnasco, head of the Laboratory of Mathematical Physics at Rockefeller. "However, because the number of combinations is quite literally astronomical, even after accounting for this limitation the total number of distinguishable odor combinations is quite large."

By comparison, researchers estimate the number of colors we can see and distinguish at between 2.3 and 7.5 million and the audible tones we can hear at about 340,000. Smell, then, far outperforms these other senses. Plus, the researchers said, even the one trillion estimate may be too low, due to the fact that a large number of odor molecules exist and, in the real world, they get mixed together in many different ways.

Keller hypothesizes that our ancestors appreciated their sense of smell more than we do. An upright posture means our noses are now far from the ground where most smells originate. Add to that the fact that our contemporary conveniences, such as daily showers and refrigerators, means we have effectively limited the number of odors we experience. "This could explain our attitude that smell is unimportant, compared to hearing and vision," he said. No matter how important we believe smell to be, Keller understands that studying this complex sense should reveal a lot about how our brains process complex information.

 

Source: Bushdid C, Keller A, Magnasco MO, Vosshall L. Humans Can Discriminate More Than 1 Trillion Olfactory Stimuli. Science. 2014.