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Nosing Into the Science of Scent

Receiving odors via the nose or mouth alters' brain's perception, study shows

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FRIDAY, Aug. 19, 2005 (HealthDay News) -- Stopping to smell the roses may bring distinct results -- neurologically speaking -- if their scent is drawn in through the mouth vs. the nose.

Researchers reporting in this week's Neuron say the brain appears to perceive the same odor differently, based on its route of arrival. The study, by U.S. and German researchers, is the first to offer clear evidence of this "dual" sense of smell.

The team, from Yale University and the University of Dresden, found that the smell of chocolate activated different regions in volunteers' brains, depending on whether its odor was first introduced into the olfactory system through the nose or the mouth.

"Although several studies have examined brain responses to retronasal (through the mouth) olfactory stimulation, none have directly compared orthonasal (through the nose) and retronasal stimulation in the same subjects, or considered the possibility that the effects of route of stimulation depend on the way that odors are typically sensed," the study authors wrote.

They point out that most food odors are experienced via the nose and mouth simultaneously, whereas nonfood odors are typically ingested via the nose. "Therefore, it is possible that the route of stimulation may have different effects for food vs. nonfood odors," they conclude.

The researchers tested four odors -- chocolate, lavender and two odorant chemicals, butanol and farnesol. The chemicals were used to test a theory that the olfactory system distinguishes odor molecules according to whether they're water-soluble (butanol) or oily (farnesol).

Neither of the two odor chemicals caused significantly different brain responses when delivered through the nose or the mouth. That indicates that the specific properties of odor molecules don't account for differences in neurological response, the study authors concluded.

However, chocolate did activate different brain regions, depending on whether it was delivered through the nose or mouth. Lavender also activated different brain regions, but to a lesser degree.

"The effect of route of delivery was greatest for the chocolate odor," the researchers wrote. That suggests the olfactory experience of being presented with -- and eating -- food affects the brain in a way that's different than that occurring with non-food odors, they said.

"Because the current study tested only one food, future experiments are needed to determine whether other food odors produce the same differential brain activations," they noted.

More information

The Nemours Foundation has more about smell.

SOURCE: Cell Press, news release, Aug 17, 2005


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