Next time you stop to smell the roses -- or any other blooms -- you might think twice: once for each nostril. Stanford researchers have discovered that in humans, each nostril smells things differently. People have long recognized that air sometimes flows faster through one nostril than the other (which side is which switches back and forth), but scientists didn't know why. Now it appears that the variation in airflow enables each side to better sniff out some substances. Differences in olfactory perception are very subtle, even imperceptible to most people, says Noam Sobel, of the Olfactory Research Project. His team's findings were published in the November 4 issue of the journal Nature.
Sobel and his colleagues tested the theory by mixing equal amounts of two chemicals whose odors are absorbed in the nose at different rates. Volunteers were told the two were mixed in different proportions for each trial and asked to guess those proportions, covering one nostril and then the other. When using their low-airflow nostrils, 17 of the 20 subjects thought the mixture contained more of a substance that smells like anise. If they used the high-airflow side, the subjects thought an identical mixture smelled more like peppermint. When volunteers were retested hours later, after the high and low airflow had switched nostrils, most of the subjects' perceptions also switched. A similar specialization can be found in eyes, where it affects depth perception, and ears, where it helps listeners locate sounds.
"The difference is not as dramatic as smelling apples with one nostril and oranges with the other," Sobel says. "It's a difference subtle enough that we had to have a careful experiment to tease it out."
