Yale researchers have found that odors sensed through eating are weaker than those sensed through inhalation.

The study, which was published in the journal NeuroImage, set out to compare smells that take an oral route to the brain with those that take a nasal route. The paper explained that odorants were delivered to rats through both routes, and brain activity of the olfactory bulb — the region of the brain responsible for smell — was measured and compared between the two pathways. The researchers found that the nasal route gave rise to higher overall brain activity when compared to the oral route.

“In recent years, we realized that sense of smell is actually a dual sense,” said Gordon Shepherd, a professor emeritus at the Department of Neuroscience and an author on the paper. “In most cases, we smell things in the air by breathing in. But we’ve recently realized that there is a second system of smell from smelling food or beverage, which we smell by breathing out.”

According to Justus Verhagen, professor of neuroscience, the two different pathways that odors can take to the brain are the orthonasal and retronasal routes. The orthonasal route, or the normal way people smell, is when odors enter into the nasal cavity during inhalation. The retronasal route is where food aromas enter the nasal cavity from the mouth during exhalation while eating.

Verhagen explained that not much is known about how retronasal food aroma is sensed and neurally processed, in part because of the technological challenge of studying the olfactory bulb in humans. As of now, optical imagery can only access about a third of the bulb.

“Human fMRI cannot investigate the olfactory bulb in any detail, due to its smallish size (2 peas) and location near the sinuses and between the eyes,” Verhagen wrote in an email to the News. “We hence were lacking insight into the food odor processing that occurs throughout the entire olfactory bulb.”

According to Verhagen, Fahmeed Hyder, professor of diagnostic radiology and biomedical engineering, specializes in high-field functional magnetic resonance imagery, or fMRI, which detects brain activity by measuring changes in blood flow.

By using fMRI in combination with optical imaging, the research team was able to study the differences in the two smelling routes in rats.

“The study was simple in one aspect,” Hyder, who is the senior author of the study, said. “It aimed to find out if there is a difference between how we smell when we smell from inside our nostrils versus inside our oral cavity.”

In order to compare the two pathways, odorants were delivered to the brains of the rats through both the orthonasal and retronasal routes, the study explained. The olfactory bulb response was then measured using fMRI.

According to Shepherd, the experiment was also complicated because the type of route the smell took depended on the animals’ breathing. Shepherd said that a calcium monitor was used to tell whether the rats were inhaling or exhaling, while fMRI subsequently measured the patterns in the brain during the inhales and exhales.

“Both imaging methods [fMRI and optical imaging] are consistent in showing smaller responses to the retronasal odors,” Verhagen said. “Furthermore, the whole bulb fMRI shows differences in activation across the landscape of the entire bulb.”

According to the study, the concentration of the retronasal stimuli had to be double that of the orthonasal stimuli in order to observe a similar response level in certain regions of the olfactory bulb. The same odor concentration showed larger activity during orthonasal versus retronasal stimulation.

Hyder explained that while the overall activity was stronger for orthonasal stimuli, the imaging showed that the orthonasal pathway was dominant in some parts of the bulb, while retronasal pathway was dominant in others.

“This implies that if there is a loss of this sensation from one pathway, the other pathway could be taught to develop more,” Hyder said. “The brain is quite plastic, but the olfactory bulb especially is a lot more plastic.”

According to Verhagen, the study does not have clinical implications. The team will continue to explore the neurobiology of smell in the olfactory bulb, Verhagen wrote.

According to the National Center for Biotechnology Information, retronasal smell is processed by a brain region responsible for taste processing while orthonasal smell is not.

Kaitlin Flores | kaitlin.flores@yale.edu