Weizmann Institute: Noses are impartial in ‘rating’ smells

Smells can be rated objectively based on an “organizing principle” for the way we experience them.

nose  / smelling_311 (photo credit: Thinkstock/Imagebank)
nose / smelling_311
(photo credit: Thinkstock/Imagebank)
Whether a certain odor is pleasant, annoying or disgusting to the sniffer is not a matter of personal taste, according to Weizmann Institute of Science researchers, who say smells can be rated objectively based on an “organizing principle” for the way we experience them.
A team headed by Prof. Noam Sobel of the Rehovot institute’s neurobiology department published on Sunday an article in the prestigious journal Nature Neuroscience suggesting odors can be rated on a scale of pleasantness.
The findings reveal a correlation between the response of certain nerves to particular scents and the pleasantness of those scents. Based on this correlation, the researchers could tell by measuring the nerve responses whether a subject found a smell pleasant or unpleasant.
Our various sensory organs have evolved patterns of organization that reflect the type of input they receive, wrote Sobel. Thus the receptors in the retina, in the back of the eye, are arranged spatially for efficiently mapping out visual coordinates. The structure of the inner ear, on the other hand, is set up according to a tonal scale. But the organizational principle for our sense of smell has remained a mystery: Scientists have not even been sure if there is a scale that determines the organization of our smell organ, much less how the arrangement of smell receptors on the membranes in our nasal passages might reflect such a scale.
Sobel’s team set out to search for the principle of organization for smell. Hints that the answer could be tied to pleasantness had been seen in research labs around the world, including that of Sobel, who had previously found a connection between the chemical structure of an odor molecule and its place on a pleasantness scale. The Rehovot neurobiologists thought that smell receptors in the nose, of which there are some 400 subtypes, could be arranged on the nasal membrane according to this scale. This hypothesis goes against the conventional view, which claims that the various smell receptors are mixed, distributed evenly, but randomly, around the membrane.
The researchers inserted electrodes into the nasal passages of volunteers and measured the nerves‚ responses to different smells in various sites. Each measurement actually captured the response of thousands of smell receptors, which are densely packed on the membrane. The scientists found the strength of the nerve signal varies from place to place on the membrane. It appeared that the receptors are not evenly distributed. Instead, they are grouped into distinct sites, each engaging most strongly with a particular type of scent.
Additional research showed the intensity of a reaction was linked to the odor’s place on the pleasantness scale. A site where the nerves reacted strongly to a certain agreeable scent also showed strong reactions to other pleasing smells and vice versa: The nerves in an area with a high response to an unpleasant odor reacted similarly to other disagreeable smells. The implication is that a pleasantness scale is, indeed, an organizing principle for our smell organ.