Bloodhound detectives can track down missing persons or criminals, the adorable Beagle Brigade will bust you for smuggling mangoes across international borders, and rats have been put to work sniffing out landmines and tuberculosis. The human nose, with our few hundred scent receptors, gets no respect. But now, a new study shows that our olfactory system can distinguish more than a trillion different odors. That is way more refined than researchers thought.
Humans have been known to differentiate up to 7.5 million different colors and 340,000 different audible tones -- yet rough calculations from nearly a century ago suggest that we can only distinguish about 10,000 unique scents. That estimate, however, has never been validated with experiments because the resolution of the olfactory system has been harder to define than wavelengths of light for the visual system and frequency bands for the auditory system. “Everyone in the field had the general sense that this number was ludicrously small,” study coauthor Leslie Vosshall of Rockefeller University says in a press release. So, they decided it was time to put the number to a real scientific test. “These findings should give the whole human race a confidence boost,” she adds.
To understand how many discrete scents our nose can detect, Vosshall, Andreas Keller and their Rockefeller colleagues asked a couple dozen or so adults to distinguish among scents. Most smells are complex mixtures of odor molecules (the characteristic smell of a rose, for example, is made of 275 of these building-block molecules). Once inhaled through the nose, they bind to receptors that relay messages to the brain (ahhh, coffee).
The team created a couple hundred test scents by mixing 128 different odor molecules -- responsible for scents like orange, anise and spearmint -- into combinations of 10, 20 or 30 components with varying proportions. To figure out how much mixtures had to differ before they could be discriminated, the team gave volunteers three vials: two of them contained identical mixes. They were asked to pick the odd one out.
On average, they could tell the difference between mixtures that contained as much as 51 percent of the same components. But once the mixes shared more than half of their components, fewer volunteers could tell the difference. Though, individual performance varied. Some participants with very high olfactory resolutions could distinguish different cocktails even when their components overlapped by 90 percent.
Using these results, the team calculated that humans can discriminate at least one trillion olfactory stimuli -- way more than what we can detect with our eyes or ears. In fact, even 1 trillion may be understating it because there are many, many more odor molecules in the real world that can be mixed in many more ways. “The message here is that we have more sensitivity in our sense of smell than for which we give ourselves credit. We just don’t pay attention to it and don’t use it in everyday life,” Keller adds.
The work was published in Science last week.
Image: Rockefeller University