Brain Distinguishes Between Nose and Mouth Odors

Researchers in a study in Neuron this week present the first clear evidence that the brain processes the same odorant molecule differently if it arrives through the nose rather than the mouth.

Dana Small, assistant professor in the Department of Surgery at Yale School of Medicine and The John Pierce Laboratory, said she and her colleagues investigated whether sensing an odor “orthonasally,” or through the nose, produces similar brain response as sensing it through the mouth, or “retronasally.”

“When we sense an odor retronasally we perceive it as coming from the mouth,” said Small. “We may say that we like the ‘taste’ of a wine because of its fruity or spicy notes. However, gustation refers only to the sensations of sweet, sour, salty, savory, and bitter. The pleasant ‘taste’ to which we refer is actually a pleasant odor sensed retronasally (through the mouth) and misattributed to taste because it is perceived to be coming from the mouth.”

The researchers were interested in determining if retronasal olfaction is simply an alternate route to the same neural system that is responsible for orothonasal olfaction, or if it evokes unique neural responses. “If the latter is true,” said Small, “then we will have evidence of the existence of distinct olfactory subsystems; one specialized for sensing objects at a distance and one for sensing objects in the mouth.”

The researchers inserted small tubes into the noses of volunteers so that one tube ended at the nostrils and the other ended farther back in the nasal passage near the throat, where odors from the mouth would originate. As they introduced four odors into one tube or the other—chocolate for a food odor, lavender as a non-food odor, and two chemical odors, butanol and farnesol—they scanned the subjects’ brains using functional magnetic resonance imaging.

In support of the existence of separate olfactory subsystems, they found that retronasally sensed odors activated the part of the brain that represents the mouth, possibly reflecting the fact that retronasal but not orthonasal odors are perceived as coming from the mouth. Intriguingly, they also observed that the greatest effect of route of delivery occurred for the chocolate odor. Since only food-related odors have been previously experienced retronasally, the finding raises the possibility, the researchers said, that the route of odorant administration interacts with experience to engage unique brain regions.

The co-authors were Erica Mak of Yale and Thomas Hummel and Johannes Gerber of the University of Dresden Medical School.

Neuron 47: 593-605 (August 18, 2005)