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Olfactory White: When Smells Converge
When multiple sound frequencies are presented at the same volume, you hear a uniform sound called “white noise.” When you see multiple wavelengths of light at the same intensity, you see white light. But, have you ever thought about white smell?
Researchers in the laboratory of Dr. Noam Sobel in Israel have been on a mission to identify exactly how specific odorant molecular structure results in perception of specific smells by the brain. This is no easy task, as each “smell” that one perceives can consist of a multitude of odorant molecules of varying complexity. In order to better understand the human perception of smell, Dr. Sobel’s laboratory has created an olfactory perception termed “olfactory white.” Olfactory white consists of a mixture of smell molecules with evenly distributed intensity.
This creation of “olfactory white” is a fascinating accomplishment, because not every smell mixture converges into a uniform “white” perception. For example, coffee, rose, and wine all have extremely complex and distinct scents that can be easily identified from each other by the human nose. According to the authors, this difference in perception occurs because the smell molecules that make up coffee, rose and wine are not presented at the same intensity, nor do they “span stimulus space,” meaning that they do not span a spectrum of different odorant molecules (Khan et al., 2007).
In their recently published study, various mixtures of distinct odorant molecules were presented to 208 adult study participants. Several different mixtures were made and presented to study participants, each mixture using a diverse span of odorant molecules at a consistent intensity. The participants were asked to rate perceptual similarities between different odors presented to them. When mixtures of 30 or more different molecules were given, the smells were indistinguishable to the participants. This happened even when the smell mixtures had no individual molecular components in common.
This study concludes that a perception of odor can be based on properties other than molecular identification of its components, including diversity of odorant molecules and odorant molecule intensity.
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Khan RM, Luk CH, Flinker A, Aggarwal A, Lapid H, Haddad R, Sobel N. (2007). Predicting odor pleasantness from odorant structure: pleasantness as a reflection of the physical world. J Neurosci. Sep 12; 27(37): 10015-23.
Mori K, Takahashi YK, Igarashi KM, Yamaguchi M. (2006). Maps of Odorant Molecular Features in the Mammalian Olfactory Bulb. Physiol Rev April; 86 (2) 409-433.
Weiss T, Snitz K, Yablonka A, Khan RM, Gafsou D, Schneidman E, Sobel N. (2012). Perceptual convergence of multi-component mixtures in olfaction implies an olfactory white. Proc Natl Acad Sci U S A. Nov 19. [Epub ahead of print].
To explore olfactory space further, visit the Sobel Lab website: