Wednesday, November 21, 2012

Olfactory White: Whale or Minnow?

A new paper out of Noam Sobel’s lab at the Weizmann Institute has been getting a lot of media play. The piece by Stephanie Pappas at LiveScience (“Olfactory White: Newly Discovered Odor Is Blend Of Smelly Compounds”) is lame, and misleadingly titled. The one from Sid Perkins at ScienceNow is better and includes this fairly restrained quote:
“Olfactory white is a neat idea, and it draws interesting parallels to white light and white noise,” says Jay Gottfried, an olfactory neuroscientist at Northwestern University’s Feinberg School of Medicine in Chicago, Illinois. The new study “definitely adds new information about how the brain interprets odors,” he notes.
Sobel’s team selected a bunch of smells that cover odor space (defined by sensory and physiochemical means) and diluted them to smell (roughly) equally intense. Then they randomly combined the odors into mixtures with varying numbers of components. By the time the mixtures reached around 30 components, they all began to smell about the same. To the extent this is remarkable it is because the 30-component mixtures had no components in common. (In contrast, a combo of 30 floral notes would be easily distinguishable from a combo of 30 resinous notes.)

Sobel et al. go to some lengths to experimentally demonstrate that these large mixtures, like large mixtures in tonal audition and color vision, converge on a perception that is, in technical psychophysical terms, “white.”


Well, the Weizmann Institute is trying to patent “a wide range of potential applications for olfactory white.” I have no doubt there are some clever applications, but my hunch is that each of them will prove to be quite narrowly focused.

Meanwhile, “olfactory white” may be rare or even nonexistent in the natural world, but I bet thousands of people smell it on a daily basis. Isn’t it the smell of any sizeable fragrance blending facility? I’ve been in a dozen of them and they all smell the same—it’s an unmistakable scent, yet not really of one thing or another. (Sure, some days a particular project will dominant the air. I’m talking about the ongoing background scent in the perfume labs and compounding rooms.)

Olfactory white: a “neat idea”? Yes. A “new smell”? Not so much.

The study discussed here is “Perceptual convergence of multi-component mixtures in olfaction implies an olfactory white,” by Tali Weissa, Kobi Snitza, Adi Yablonkaa, Rehan M. Khana, Danyel Gafsoub, Elad Schneidmana, & Noam Sobel, published online November 19, 2012 in Proceedings of the National Academy of Sciences USA.


Peter Apps said...

I am just wondering, doesn't olfactory adaptation generate "olfactory white" when it makes a previously obtrusive odour sink into the general background after a few minutes of exposure ?

Peter Apps

Avery Gilbert said...

Peter Apps:

By analogy with visual white, which is composed of wavelengths from across the visible spectrum, olfactory "white" is composed of molecules with a wide range of odor characters. And, like the color white, or like white noise, olfactory "white" is very perceptible.

Olfactory adaptation, as you correctly note, reduces the perceived intensity of only the over-exposed molecule, making it harder to detect. So adapted odor and olfactory white odor aren't really parallels.

Your question does raise a fair point about Sobel's paper: they could have done more to describe what olfactory white actually smells like.

Peter Apps said...

My hand waving model is that Sobel's mixtures are simplifications of the chemical background (what Tristam Wyatt calls the chemical cacophony)which also contains compounds from all regions of odour space. In the chemical cacophony the concentrations of the individual components have not been carefully adjusted to bring them all to about the same odour intensity, instead the sensitivity of the nose to each adjusts itself by adaptation to bring each to a similar intensity, producing a white odour background against which a change in odour (that primitively signalled predator, food or sex etc) will stand out clearly. My hand-waving visual analogy is with putting on a pair of tinted spectacles - within seconds you cannot see the tint, or going into a tungsten-lighted room from outdoor sunlight.

Best regards Peter Apps

Avery Gilbert said...

Peter Apps:

OK, now I see where you’re going with this:

If olfactory adaptation tunes down each specific smell in the “chemical cacophony” around us to the same intensity level, then we should be smelling olfactory white (according to Sobel’s functional definition).

Yes, but . . .

what olfactory adaptation does is dial down the intensity of an on-going odor to the point where one is no longer aware of it.

In other words, sensory adaptation is about zeroing out background (to make new stimuli more detectable), not tuning all available stimuli to an equal level of perceivable impact.

This edges into cognition: e.g., being told a smell is toxic keeps us from adapting as quickly as we otherwise would.

Plus, the brain actively interprets and filters signals out of cacophony, which is why we can follow a conversation in a loud cocktail party even though the sonic frequency chart is chaotic.

The intense background odor at a typical fragrance company surprises first-time visitors. Yet perfumers make nuanced judgments in it all day long, by ignoring/tuning out/adapting to the background.

In my experience, this background smell—a mix of everything on the stockroom shelves—is consistent from company to company. Thus my speculation that Sobel’s olfactory white might be what some of us say “smells like the office.”

Peter Apps said...

Thank you for the feedback.

When you say;

"what olfactory adaptation does is dial down the intensity of an on-going odor to the point where one is no longer aware of it.

In other words, sensory adaptation is about zeroing out background (to make new stimuli more detectable), not tuning all available stimuli to an equal level of perceivable impact."

I agree completely - I am suggesting that the mechanism of olfactory adaptation is the neurological complement of Weiss et al's dilution steps, and that perceptually the end results are the same; a bland, weak odour that is very difficult to describe and which is nearly impossible to differentiate from all the other background odours.

To use the office analogy; doesn't everyone's office smell the same to the people that sit in it all day, although they all smell different (for a few minutes at least) to a visitor ?

Peter Apps

Bharathi said...

If the olfactory white is due to the adaptive mechanism of the olfactory system, it also means that it cannot be used as a smell neutralizer in public washrooms. What other applications could it have?