Friday, October 7, 2022

Perceptual Cartography

This piece of news from from the Monell Chemical Senses Center, and it's supposed to be about how we can predict whether a molecule will have an odor or not. For example, the molecule for water, H2O, does not have a smell. Neither does a molecule of iron. Water smells, like all different kinds of things, and so does metal, but it's because of the fact that you can't find a place on Earth where life doesn't live, and life smells. So that includes how we deposit our skin flora on loose change and handrails, where select families of microbes proliferate in the microscopic textures and pores of the metal. Or how water can have any number of living (or dying) things in it, many of which can smell. And that's all because living things are walking, breathing chemical reactor factories that transforms molecules like their life depends on it, and these transformations give us single molecules of let's say isovaleric acid or cineole or delta-decalactone that offgas from the collective biosphere. 

But if we were to take any molecule, chosen at random, we don't have a good checklist to guess whether it will smell or not. And there's plenty of molecules still out there for us to discover, so how do we know where to start? That's where this study comes in. And for that purpose, it seems like it could be pretty helpful.

That's not what makes this article interesting to us here at Limbic Signal however. Instead, let's look at how this research tries to map out the information space of olfaction.

Information space is a hard concept to wrap your head around, because you're a meatbag bounded by the three dimensions of conventional reality (unless you're a robot reading this, of course). The entry point to discussions about perceptual information space start with vision. Is it light or dark, on a scale of one to ten, one being white and ten being black. That's one dimension where the space is a single line, and our "color" then exists somewhere along that line. 

Next, where does it fall on the rainbow, a spiraling spectrum that gradually changes from red to orange to yellow to green etc, each of which can be measured by it' radiating energy. Conveniently, the end can be shifted back to the beginning, so that after blue turns to violet, it keeps going, and we think we're approaching the end, but then violet starts to look like red, and we're back to the beginning again.

This gives us two dimensions, the light-dark and the rainbow (it's called hue but I'll call it rainbow here since it's a more familiar term. Color then has a light-dark number and a rainbow number, like an X-axis and a Y-axis, and any particular color exists at the intersection of these axes. This intersection is the information space of color. (There's typically an extra dimension used, called brightness or saturation, but let's just stop here.)

This information space allows us to group together similar colors, and to create different organizations of color combinations, based on color theory for example. And we can use charts that visualize this information space, so that we can communicate with each other about colors. Because as humans, we like to communicate about things. Some might say our ability to communicate, especially using verbal language, is what makes us human. 

But smells have a problem. There's no way to organize them, except to split them into "good" or "bad" categories, and since each of us has slightly and sometimes dramatically different opinions about whether a smell is good or bad, well, that distinction isn't very helpful.

Hot smells and cold smells? That doesn't mean anything. Wet and dry? (There's something here actually, related to chemical reactions that can and cannot take place underwater, because our sense of smell has been with us since we were fish, and now we're not, but we still hold some of that history.) It just doesn't work. Fruity, cheesy and burnt? Ok great, now what about the rest? And do they fit on a spectrum like the wavelengths of visible electromagnetic radiation? No they don't. 

Odor space is hard because instead of two dimensions like color (three really but who's counting), there are as many dimensions for smells as there are smells (and that's a lot). And that's just not very helpful if what we're trying to do is collapse the information, to make it easier to look at, think about, talk about...

And that's why this new research from Monell is so interesting, because it is one of the first comprehensive maps of odor space. It's huge, but it's useful because we've now managed to cross-off an even bigger world of molecules that simply can't be smelled.

What Makes a Molecule Smell?
Jul 2022, Monell Chemical Senses Center

Smell scientist Richard Gerkin produced a visualization of the odor space showing the universe of possible molecules and the regions of that universe where odorous molecules live.

A smell needs to be:
  • Volatile enough evaporate from the surface of its source
  • Not too volatile to still pass through the mucus layer coating the olfactory epithelium
  • Hydrophobic enough to escape the mucus layer and enter the binding pockets of olfactory receptors

via Monell Chemical Senses Center: Transport features predict if a molecule is odorous. Emily J. Mayhew et al. PNAS. April 4, 2022. 119 (15) e2116576119

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