Catherine Yang

A new Yale study sheds light on the process by which humans detect flavor, and indicates that the design of the upper human airway might have developed as an evolutionary safeguard against consumption of hazardous food and drink.

The study, which was a collaboration between researchers at the School of Medicine and the School of Engineering and Applied Science, examined the process of retronasal smell, during which food particles at the back of the oral cavity are carried through the nasal cavity by exhaled air and stimulate the olfactory receptors at the top of the nose. According to study authors, retronasal smell, an outward-flowing process that occurs when people breathe out, has not been the subject of as much research as orthonasal smell, an inward-flowing process that occurs when people sniff in, which is what prompted their inquiry. The study was published on Nov. 9 in the journal Proceedings of the National Academy of Science.

“There’s been a certain amount of analysis of turbulent currents in the nose stimulating the receptor cells at the top of the nose, but nobody has looked at the kind of flow that happens in the air in the retronasal pathway,” said Gordon Shepherd, senior author and neuroscience professor. “And so that’s what we thought it was time to do, to understand better how the airflow actually occurs.”

To accomplish this, Shepherd and his interdisciplinary team took a scan of a human airway and used it to produce a 3-D model that allowed them to simulate the flow patterns of inhalation and exhalation using water seeded with fluorescent tracer particles. After collecting their results using standard fluid mechanics analysis techniques resembling the flow of air within the respiratory tract, the researchers found that the volatile particles collected in a virtual cavity at the back of the mouth and, to their surprise, had minimal interaction with the inward-flowing orthonasal patterns, which went “right by” the cavity, Shepherd said. This was the case even at the relatively low flow rates that occur during eating and drinking. Shepherd said these findings might have evolutionary significance.

“It suggests, for the first time, that [retronasal smell] might be a protective mechanism,” Shepherd said. “When you first take stuff into your mouth, of course, for the animal, if it’s a food that you know, then that’s fine, but if it’s a food that you don’t know, then … it’s obviously of adaptive value to minimize the volatiles coming from the substance going into the lungs and just let it be breathed out through the nose.”

Shepherd noted that the evolutionary explanation was “pure speculation,” but also said that is to be expected with experimental data. He said the study points toward potential avenues for more comprehensive research, including studies that would expand the research to more subjects of diverse demographics and make a distinction between food and drink consumption.

Gary Beauchamp, the emeritus director and president of the Monell Chemical Senses Center in Philadelphia, said it would be “interesting” to see whether the respiratory pathway functions similarly in other animals. Pennsylvania State University professor Rui Ni, who was involved with the study, said he was excited about the study’s potential implications on future interdisciplinary research.

“Like every other piece of research, it stands on the shoulders of other people who’ve done somewhat similar work,” Beauchamp said. “And so there are a few other papers that’ve tried to do this … But this is much more sophisticated work, that [Shepherd] and his colleagues are doing, clearly state-of-the-art kinds of research.”

The study was funded by grants from the National Institute on Deafness and Other Communication Disorders of the National Institutes of Health and the National Science Foundation.