Humans can distinguish at least one trillion different odors, study shows

Vials of odors are among hundreds that Rockefeller University researchers used to measure volunteers' ability to distinguish between scents. Credit: Zach Veilleux / The Rockefeller University

In a world perfumed by freshly popped popcorn and exhaust fumes, where sea breezes can mingle with the scents of sweet flowers or wet paint, new research has found that humans are capable of discriminating at least one trillion different odors. Howard Hughes Medical Institute (HHMI) scientists determined that our sense of smell is prepared to recognize this vast olfactory palette after testing individuals' ability to recognize differences between complex odors mixed in the laboratory.

It has been said for decades that humans are capable of discriminating between 10,000 different odors. The number is cited in scientific literature and appears in popular magazines. "It's the generally accepted number," says HHMI investigator Leslie Vosshall, who studies olfaction at the Rockefeller University. "Our analysis shows that the human capacity for discriminating smells is much larger than anyone anticipated."

Vosshall and her colleagues published their findings March 21, 2014, in the journal Science. "I hope our paper will overturn this terrible reputation that humans have for not being good smellers," she says.

Vosshall had long been bothered by the idea that humans were limited to smelling 10,000 odors – an estimate that was made in the 1920s, and not backed by any data. "Objectively, everybody should have known that that 10,000 number had to be wrong," she says. For one thing, it didn't make sense that humans should sense far fewer smells than colors. In the human eye, Vosshall explains, three light receptors work together to see up to 10 million colors. In contrast, the typical person's nose has 400 olfactory receptors.

But no one had tested humans' olfactory capacity. "We know exactly the range of sound frequencies that people can hear, not because someone made it up, but because it was tested. We didn't just make up the fact that humans can't see infrared or ultraviolet light. Somebody took the time to test it," Vosshall says. "For smell, nobody ever took the time to test."

Vosshall and Andreas Keller, a senior scientist in her lab at Rockefeller University, knew they couldn't test people's reactions to 10,000 or more odors, but they knew they could come up with a better estimate. They devised a strategy to present their research subjects with complex mixtures of different odors, and then ask whether their subjects could tell them apart.

They used 128 different odorant molecules to concoct their mixtures. The collection included diverse molecules that individually might evoke grass, or citrus, or various chemicals. But when combined into random mixtures of 10, 20, or 30, Vosshall says, they became largely unfamiliar. "We didn't want them to be explicitly recognizable, so most of our mixtures were pretty nasty and weird," she says. "We wanted people to pay attention to 'here's this really complex thing – can I pick another complex thing as being different?'"

Andreas Keller in the lab surrounded by vials of odors he and his colleagues used to measure volunteers' ability to distinguish between scents. Credit: Zach Veilleux / The Rockefeller University

The scientists presented their volunteers with three vials of scents at a time: two matched, and one different. Volunteers were asked to identify the one scent that was different from the others. Each volunteer made 264 such comparisons.

Vosshall and her colleagues tallied how often their 26 subjects were able to correctly identify the correct outlier. From there, they extrapolated how many different scents the average person would be able to discriminate if they were presented with all the possible mixtures that could be made from their 128 odorants. "It's like the way the census works: to count the number of people who live in the United States, you don't knock on every single door, you sample and then extrapolate," she explains. "That's how I like to think of this study. We knocked on a few doors."

In this way, they estimated that the average person can discriminate between at least one trillion different odors. "I think we were all surprised at how ridiculously high even the most conservative lower estimate is," Vosshall says. "But in fact, there are many more than 128 odorants, and so the actual number will be much, much bigger."

Vosshall says she doubts individuals are exposed to a trillion smells on a daily basis. "But I like to think that it's incredibly useful to have that capacity, because the world is always changing," she says. Plants are evolving new smells. Perfume companies are making new scents. You might move to some part of the world where you've never encountered the fruits and vegetables and flowers that grow there. But your nose is ready. With a sensory system that is that complex, we are fully ready for anything," she says.

More information: "Humans Can Discriminate More than 1 Trillion Olfactory Stimuli," by C. Bushdid et al. Science, 2014.

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cabhanlistis
not rated yet Mar 20, 2014
This is really hard for me to believe. Does the brain even have such a capacity to report on such subtle differences?
JVK
1 / 5 (2) Mar 20, 2014
Detecting Fat Content of Food from a Distance: Olfactory-Based Fat Discrimination in Humans http://dx.doi.org....0085977

The Scent of Disease: Human Body Odor Contains an Early Chemosensory Cue of Sickness
http://pss.sagepu...abstract

If we did not have the same abilities as other animals, there could be no such thing as
Nutrient-dependent/pheromone-controlled adaptive evolution: a model
http://www.socioa...53/27989

Instead, this should have become known -- "Olfaction and odor receptors provide a clear evolutionary trail that can be followed from unicellular organisms to insects to humans (Keller et al., 2007; Kohl, 2007; Villarreal, 2009; Vosshall, Wong, & Axel, 2000)." -- when I published that paper in 2012
http://www.socioa...38/20758
russell_russell
1 / 5 (1) Mar 20, 2014
What you see depends on the spectra of EM radiation which humans have labeled visible light.

There are no lower or upper bounds to this spectra - you decide which wavelengths makes sense at which stage science advances. The more science advances the smaller the visible spectra becomes for us in comparison to total spectra.

What you hear depends on the spectra of mechanical vibration which humans have labeled sound.

There are no lower or upper bounds to this spectra - you decide which wavelengths makes sense at which stage science advances. The more science advances the smaller the acoustic spectra becomes for us in comparison to the total.

Eventually you will determine what type of gas spectra humans evolved to smell and label odor.

And you will discover the bounds that exist for this spectra as well have no lower or upper bounds - the limits always extended from the advances in science.

The number trillion viewed from this perspective looks, sounds and smells insignificant.

tadchem
5 / 5 (1) Mar 21, 2014
Discriminating odors is not the same thing as identifying odors. We can easily discriminate millions of colors, as well, but we can scarcely name a few dozen of them.
One can easily discriminate between smoke from pine, oak, ash, mesquite, hickory - just to name a few woods commonly burned.
Identifying them requires experience and training.
A trained nose can tell how fresh the wood was, whether paper or fuel (and which fuel) was used to ignite it, etc.
On the other hand, my Irish nose and palate cannot tell vinegar from Reisling without the label.
russell_russell
not rated yet Mar 22, 2014
"Identifying them requires experience and training. - tadc"
(Identification rooted in the senses and not rooted from science)

and

"On the other hand, my Irish nose and palate cannot tell vinegar from Reisling without the label. - tadc"
(The "label" is a measure from science instead of from the "measure" our senses provide.)

"Color" has infinite spectra..."but we can scarcely name a few dozen of them"

The senses were once the first units of measure for us.

Later on science came and introduced a scientific concept of measure. (Arbitrary precision)
Discrimination and identification became independent from the language of labels.
Experience, training and the language of labels became superseded and obsolete.

We automate all processes (including our senses). We still take for granted and mistake our senses for a measure. Science rejects this measure.


JVK
not rated yet Mar 22, 2014
Eventually you will determine what type of gas spectra humans evolved to smell and label odor.


This has nothing to do with evolution. The complexity of this system clearly attests to the fact that ecological variation leads to experience-dependent ecological adaptations via the de novo creation of olfactory receptor genes. Attempts to place this evidence into the context of evolutionary theory are as misguided as ever.

We automate all processes (including our senses). We still take for granted and mistake our senses for a measure. Science rejects this measure.


What does that mean? No experimental evidence suggests anything other than the fact that our response to odors is innate-- just like it is in every other species -- and that our response to other sensory input, which is associated with the odors, exemplifies classically conditioned responses just like Pavlov's dog did.

http://www.socioa...53/27989
russell_russell
not rated yet Mar 22, 2014
Sand dunes produce sound which humans perceive as pitch.
Software drives hardware to replicate sound indistinguishable from the pitch humans perceive from sand dunes.

Is our response to software driven hardware innate? --just like software driven hardware is in every other species?

Is software driven hardware ecological variation?
Does software driven hardware evolve under evolutionary theory?