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
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).