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White Light, White Noise, White Smell?

Sun, 25th Nov 2012

Ben Valsler

Part of the show 3D Printing Replacement Cartilage

Just as you can see white light, and hear white noise, it seems we are also susceptible to a white smell – or “olfactory white”.

“White” isn’t a distinct frequency of light, rather it’s the way we perceive light that is made up of a number of different wavelengths.  Two different mixtures, even if they don’t contain any of the same wavelengths, will still appear “white” to us if they contain enough different frequencies.  Similarly, white noise contains a very wide range of different wavelengths of sound.  The concept of “whiteness” seems to extend from having an equal intensity across a broad stimulus space.  Now, according to research published in the journal PNAS, it seems the same is true for smells.

Talio Weiss, from the Weizmann Institute of Science, Israel and colleagues, selected 86 molecules that should span the full olfactory range, and diluted them so that they each had the same intensity.  They then made a range of different mixtures – some spanning the full range, others more clustered – and asked volunteers to describe the smell.

The Human NoseThey discovered that the more components two mixtures have, the more similar they smell, even if the two mixtures actually have no shared components.  When the mixtures contained 30 components or more, they all smell alike – the olfactory equivalent of white noise.

The authors suggest that this is related to the way that human olfaction works – it’s thought of as a synthetic, rather than an analytical sense.  In other words, it considers the combination of stimuli, rather than each one individually.  This can be seen in the pattern of brain activity – the activity for a mixed smell is not simply the combination of activity for all its components.

Furthermore, we’re unlikely to encounter olfactory white in the wild.  The smell of a rose, for example, is made up of many different components, but one, phenylethyl alcohol, is far more intense than the others.  Coffee or wine are other good examples, with a range of components, but lacking uniformity and over a limited olfactory range.

The concepts of white noise and white light have been of huge benefit to understanding the neurobiology of hearing and vision.  The authors argue that white smell may now help do the same for olfaction.



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Fig. 1A shows a 2D graph with axes "PC1" and "PC2".

Can anyone with some knowledge of olfactory research explain the "perceptual space" for smells. It's something which I've never seen referred-to before, and it intrigues me.

I have a purely personal "graphical" visualisation of smells which is a simply a 1D representation (though I'd be hard-pressed to place anything on a specific part of the axis) - I mostly visualise smells as "spiky" where there's some one or more kinds of dominant sharp component, or "broad" which gives a smell more "body" rather than being "thin" (which it would be if there were nothing between the spikes). Does this make any sense to anyone? techmind, Tue, 11th Dec 2012

Seems we need some more research in this field!
I postulate that the human nose and sensory system has more in common with the ear than with colour-perception - a fair greater number of "channels" and hence required "aromatic primaries" to reproduce an arbitrary smell.
For colour, you only need three primaries (X,Y,Z or loosely red/green/blue) to simulate an arbitrary colour. techmind, Wed, 12th Dec 2012

There are after-images, are there after-smells ? RD, Wed, 12th Dec 2012

There are after-sounds too... some forms of tinnitus! techmind, Wed, 12th Dec 2012

There are definitely after smells.  I used to smoke and two or three paces past a smoker on the street I get a distinctive smell/urge; its not the smoke directly, and I don't get it in a room with a smoker.  I think its the chemicals of smoke rarefied and purified on the breeze and my body realizing what it just smelled and that it could quite go for it!  About ten years now and counting - so not a problem, but it is the only time I am tempted and sober. imatfaal, Fri, 14th Dec 2012

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