A perennial topic on this weblog is the categorization of
smells. Today I'm looking at a study from 2011
that looks at common features that group smells together. One of the common denominators
is hedonics, or pleasantness vs non-pleasantness.
It always makes me pause to think about this, because it
seems that people can never really agree on what makes a smell good or bad, and
yet the hedonic dimension is the only one that keeps coming back as the primary
distinction between odors. I guess that's just the law of large numbers
at work, a law which is against natural human cognition.
If you include enough people in your study, the differences
between us cancel out and you're left with a fuzzy but recognizable picture of
a smell map, which is seen above.
The other common denominator (it’s not a denominator if
there’s two, right?) is a dimension the researchers call natural/chemical.
This map is organized as follows: Whereas the pleasantness
of an odor can be predicted on the number of carbon atoms per molecule (related
to how fast it evaporates), the natural/chemical dimension is predicted by the
polarity of the molecules, or how attracted they are to water.
Why? Not so sure. Mention is made to the difference in the olfactory
receptors themselves - some are from when we were fish and some are from when
we became land animals, so the two may have a different relationship with water
(polarity).
For example, odorants are dispersed more slowly in the
water. Also, smellable molecules to fish don’t have to be volatile organic
compounds, because for a fish, the air itself is already a liquid. So fish
detect water soluble molecules whereas humans detect airborne molecules.
Actually, now that I look at the ‘natural’ part of the map,
I realize that none of those things exist underwater, right? Burnt? Nope.
Moldy? Although mold is always associated with moisture, it doesn’t grow underwater.
And Earthy? Kind of the opposite of water.
Natural - Burnt, Smoky, Nutty, Woody,
Resinous, Musty, Earthy, Moldy, Almond, Popcorn, Peanut Butter, Oily, Fatty,
Warm, Dry, Powdery
Chemical - Etherish, Anaesthetic, Chemical,
Medicinal, Disinfectant, Carbolic, Sharp, Pungent, Acid, Gasoline, Solvent,
Cook, Cooling, Cleaning Fluid, Paint, Camphor
Good - Fragrant, Sweet, Perfumery, Floral,
Light, Aromatic, Cool, Cooling, Fruity, Citrus, Rose
Bad - Sharp, Pungent, Acid, Heavy, Musty,
Earthy, Moldy, Burnt, Smoky, Oily, Fatty, Sour, Vinegar
-image source: link
Notes:
In search of the
structure of human olfactory space. A. A. Koulakov, B. E. Kolterman, A.
G. Enikolopov, D. Rinberg. Front. Syst. Neurosci. 5, 65 (2011).
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