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Stanford Medicine, published quarterly by Stanford University Medical Center, aims to keep readers informed about the education, research, clinical care and other goings on at the Medical Center.

smelling

roses

By KATHLEEN O'TOOLE

Your nostrils may part ways

IF YOU SNIFF ATTENTIVELY, YOU'LL LIKELY NOTICE THAT USUALLY ONE NOSTRIL SUCKS IN AIR FASTER THAN THE OTHER. SCIENTISTS HAVE RECOGNIZED THIS IS OFTEN THE CASE BUT HAVE NOT KNOWN WHAT PURPOSE THIS DIFFERENCE MIGHT SERVE.

Now a Stanford research group proposes a purpose: The difference in airflow through the nostrils seems to allow 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, says Noam Sobel, MD, who heads the Olfactory Research Project in the lab of John Gabrieli, PhD, an associate professor of psychology at Stanford.

Each eye sees slightly differently, improving our depth perception, Sobel says, 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," says Sobel, a member of the group that reported this finding in the Nov. 4, 1999, issue of Nature.

The difference in perception of the nostrils is very subtle and probably cannot be noticed by most people most of the time, Sobel says. The world is made up of complex, smelly things, such as a rose that may consist of countless odor components. To establish that there is a difference between the nostrils, the researchers mixed equal amounts of two chemicals whose molecules are attracted, or "sorbed," to the nostril tissue 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 rates of airflow 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 says. "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 says, 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 says. "It's a difference subtle enough that we had to have a careful experiment to tease it out."

Sobel got the idea for the study while eating breakfast with a non-scientist friend, Amnon Saltman of the Israeli Ministry of Environmental Protection. "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." ­ KATHLEEN O'TOOLE