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Stanford Report, November 10, 1999

Smelling roses: Your nostrils may part ways


Each nostril of the human nose is tuned to smell some odors better than others, and the specialization moves back and forth from one nostril to the other, Stanford researchers reported in the Nov. 4 issue of the journal Nature.

When consciously sniffing, many people recognize that one nostril often sucks in air faster than the other, but scientists have not known what purpose this difference might serve. Now it appears that the difference in airflow through the nostrils allows one nostril to better detect the odor of some substances while the other better detects others.

The nostrils' different perceptions bear some resemblance to specialization in the other distal senses -- sight and hearing, said Dr. Noam Sobel of the Program in Neurosciences, and head of the Olfactory Research Project in the lab of psychology Associate Professor John Gabrieli. Each eye sees slightly differently, improving our depth perception, Sobel said, and each ear hears slightly differently, enabling spatial localization of sounds. "It's a reasonable assumption that the difference in nostril perception might be a basis for the fundamental properties of the sense of smell, and we are just now finding out what they are."

This illustration shows how the difference in airflow through the nostrils allows the left nostril to better perceive the peppermint smell of an airborne chemical called l-carovone and the right nostril to better perceive the anise smell of octane. The nostrils appear to shift roles during the course of a day.

Michael Leon, a professor of neurobiology and behavior at the University of California at Irvine said the Nature article "describes a wonderfully clever piece of work that answers a question that nobody had even thought to ask. That is, why do we have two nostrils?" The data suggest, he said, that the olfactory system maximizes the ability of its distributed receptor neurons to encode differentially absorbing odors.

The difference in perception of the nostrils is very subtle and probably cannot be noticed by most people most of the time, Sobel said. The world is made up of complex, smelly things, such as a rose that may consist of countless odor components. To establish that there was a difference between the nostrils, the researchers mixed equal amounts of two chemicals whose odors sorb at different rates --octane, which smells similar to anise, and l-carvone, which smells like peppermint.

People in the study were told the two were mixed in different proportions for each trial and asked to guess those proportions, using one nostril at a time. Seventeen of the 20 subjects judged the identical mixture to consist of a greater proportion of octane when they used the low airflow nostril and a greater amount of l-carvone when they used the high-airflow one. Furthermore, eight subjects were retested hours later when the high and low airflow rates switched from one of their nostrils to the other. In seven of the eight, the odorant perception also switched.

The researchers' theoretical explanation for the observations is based on the interaction of airflow and chemical sorption rates. For an odor to act on the olfactory receptors, it first must cross a mucous membrane in the nostril, called the mucosa, to reach olfactory receptors. Blood flow to the tissue surrounding the nostril varies so that when it is high, the passageway is narrowed and air flows more slowly through it. "If you have a high-sorbant odorant in a high-airflow nostril, the molecules of the odorant want to sorb rapidly," Sobel said. "But they are also flowing along fast so you get a long-range distribution, or a lot of receptors involved in the response. If you take that same odorant and bring it in at a slow flow rate, it will sorb before it gets very far, and so you'll have fewer receptors involved, making a smaller response."

With a low-sorbing odorant at a high flow rate, he said, the fact that the molecules sorb slowly probably means most of them reach the respiratory system and throat before there is much chance for the olfactory receptors to detect them. "If that same low-sorbing odorant comes in slowly, it has time to sorb and again you will have a large nasal portion involved in the response.

"The difference is not as dramatic as smelling apples with one nostril and oranges with the other," he said. "It's a difference subtle enough that we had to have a careful experiment to tease it out."

Researchers haven't established how often the nostrils switch roles, Sobel said. There are also times when both nostrils receive airflow about equally.

Sobel's co-investigators include Gabrieli and Rehan Kahn of the Psychology Department, Edith Sullivan of the Department of Psychiatry and Behavioral Sciences and Amnon Saltman of the Israeli Ministry of Environmental Protection. Saltman is a non-scientist friend of Sobel's whose speculation over the breakfast table led to the experiment.

"He was visiting me and I was telling him about another study of ours in which we discovered that each nostril detected an odorant at a different concentration level, and he said, off the top of his head, 'Maybe they just smell different things altogether.' I immediately blew it off, but I thought about it later and finally got to thinking maybe it could be, and then I figured out how it might work." SR