Debunking myths on genetics and DNA

Sunday, March 30, 2014

What do one trillion different scents smell like?


I've been really happy with the comments on my upcoming detective thriller CHIMERAS. The book will be released in two weeks, but I've already heard back from some early readers (and yes, I'm still offering free ARC's, see details here), and many have praised Track's sensitivity to smells. Apparently, it's a trait many relate to and yet you don't find so often in fiction.

Most of our memories are stored as images. So, even when we write, we tend to over-emphasize visual descriptions and forget all about our nose. From a scientific point of view, though, how does olfaction compare to other senses? Can we "see" and "hear" more than we can "smell"?
"Humans can discriminate several million different colors and almost half a million different tones, but the number of discriminable olfactory stimuli remains unknown. The lay and scientific literature typically claims that humans can discriminate 10,000 odors, but this number has never been empirically validated [1]."
In a study published last week in Science, Bushdid et al. calculated that, contrary to previous estimates, humans can discriminate at least one trillion different olfactory stimuli -- far more than colors and tones.

How did they make such an estimate?

Colors are created from light: changes in wavelength create different hues and saturations. Similarly, sounds are created from air waves and changes in frequencies create different tones. Because we can physically measure both the frequency and wavelength of waves, it is relatively easy to determine the ranges within which human eyes and ears can detect these stimuli:
"Humans can detect light with a wavelength between 390 and 700 nm and tones in the frequency range between 20 and 20,000 Hz [1]."
But while colors and tones are created by waves, olfactory stimuli are created by mixtures of numerous distinct odor molecules. Even the "simple" scent of a rose contains 275 components. The odor molecules bind to the olfactory receptor cells in the nasal cavity, sending a signal up to the olfactory nerve.

One trivia that I discovered while writing CHIMERAS is that olfactory receptors are not restricted to the nasal cavity. They are also found in sperm cells [2] where they are possibly involved in the control of sperm migration and fertilization.

To measure the resolution of the human visual or auditory system, scientists measure how close two signals need to be in frequency in order to become undistinguishable. In other words, if the signals are like hair and our ability to pick them up is a comb, how fine are the comb teeth? How far apart do two light wavelengths need to be in order for our eyes to discern them as distinct colors?

Bushdid et al. used a similar criterion to measure how good we are discerning scents. They used 128 odor molecules to make different scent mixtures of 10, 20, or 30 components. Each mixture yields a different smell, and the more components the mixtures shares the harder they are to distinguish from one another. So, similarly to what's typically done for the visual and auditory system, in order to measure the resolution of the human nose, Bushdid et al. measured how much two mixtures need to overlap in order to become indistinguishable to the human nose. Of the 26 subjects in the study:
"At least half of the tested subjects could discriminate mixture pairs that overlapped by less than 75% of their components. Some could also discriminate mixture pairs that overlapped by 75 and 90%, but none could discriminate mixture pairs with more than 90% overlap [1]."
Bushdid et al. then used mathematical extrapolations to predict that the majority of individuals can distinguish mixtures that overlap less than 51%, which amounts to over one trillion mixtures made with 30 components. This is only a lower limit since for their experiment the researchers used 128 different components while in nature you can find many more, and in mixtures of often more than just 30 components.

I wonder what they would've concluded had Track been part of the study. :-)

[1] Bushdid C, Magnasco MO, Vosshall LB, & Keller A (2014). Humans can discriminate more than 1 trillion olfactory stimuli. Science (New York, N.Y.), 343 (6177), 1370-2 PMID: 24653035

[2] Vanderhaeghen P, Schurmans S, Vassart G, & Parmentier M (1993). Olfactory receptors are displayed on dog mature sperm cells. The Journal of cell biology, 123 (6 Pt 1), 1441-52 PMID: 8253843

ResearchBlogging.org

14 comments:

  1. Absolutely fascinating, E.E. ! I've never before given a lot of thought to the actual mechanism behind someone being a "nose" for a perfume house. I've wondered, of course, why some people can detect subtle differences in a scent, and others can't. This is great stuff. :-)

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    1. Thanks, Teresa, I'm glad you enjoyed it. I found it fascinating, too. :-)

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  2. Eventually, I'd guess there will be some sort of graphic representation of odors, something on the order of a color wheel, perhaps. Very interesting methodology as is the mathematical extrapolation of the findings to 1,000,000,000,000+ detectable odors.
    It would be interesting to know if any of the subjects were smokers or worked in an environment with respiratory irritants.

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    1. Those are excellent questions, Mike. Science is very stringent on the number of words allowed, all I saw in the paper was the gender and number of subjects (26). I should check if they address any of those questions in the supplemental material.

      an odor wheel would be quite interesting! :-)

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  3. The sense of smell is so interesting. I know that a smell can bring back a memory for me a lot more strongly than an image.

    I would guess the authors probably took this into account, but since people can get desensitized to a scent quickly, I wonder if that also affects their perception of different but related scents.

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    1. that's a great point Lily ... and it's also really hard to distinguish scents when they are out of a larger context.

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  4. I enjoyed reading Patrick Suskind's Perfume. Look forward to reading your book.

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    1. thank you ! I enjoyed that book too, kinda eerie in a good way. :-)

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  5. That's some serious science.
    Good luck with your release day!

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    1. Thanks, Lynda! sorry, I haven't posted for the insecure group yet... it's still tuesday here, wasn't sure what time zone to go by ... :-)

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  6. I never knew there were so many smells. Some people aren't very good at distinguishing smells though, like my husband.
    And I format books, EE. Among other things.

    (Did you know word verification is on?)

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    1. Hi Diane, my husband's the opposite and believe me, it drives me nuts ! What do you mean word verification is on? Did auto-correct mess up some of my words? Every time I write "germline" it'll turn it into gremlins. :-)

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