Nostril rivalry

When the two sides of the nose smell conflicting scents, the brain doesn’t combine them

Nostrils usually get along great. But when they smell conflicting scents, those nose holes become deadly rivals.

When one nostril smells something different from the other, the brain chooses between the two scents instead of combining them, researchers report online August 20 in Current Biology. The authors argue that their study is the first to demonstrate this phenomenon, which they call two-nostril, or binaral, rivalry.

Studying the rivalry between the nares may help scientists understand how the brain processes smells, says study coauthor Denise Chen of Rice University in Houston.

“It’s an interesting article,” says neuroscientist Jay Gottfried of Northwestern University in Evanston, Ill. “It shows something that has not been appreciated much before.”

Scientists have known about rivalry between the eyes and between the ears for years. When a subject’s right eye views an image that is incompatible with the image that the left eye views, the subject reports seeing the images alternating rather than superimposed upon each other. Similarly, in two-ear rivalry, when each ear hears a different tone, the brain switches back and forth between them.

To test whether the same phenomenon exists for smell, Chen and Rice University colleague Wen Zhou exposed12 volunteers to two different scents, one in each nostril. One nostril was connected by a tube to a bottle of phenylethyl alcohol, which smells like rose petals. The other was connected to a bottle of n-butanol, which pongs of marker pen. During each whiff, the volunteers breathed in both scents.

Volunteers indicated whether they smelled rose, scented marker, neither or a combination. After each volunteer’s 20 trials, spaced out to keep the noses sharp, the researchers found that smell perception alternated, more or less, between rose and markers.

As in binocular studies, subjects usually reported sensing the least pleasant of the two options — in this case the marker — first.

Gottfried says that he would like to see these experiments replicated with other scents. Other studies with the same setup haven’t seen this result, so the findings may be specific to these smells. “This issue needs to be examined more carefully,” he says.

Chen says that previous tests may have overlooked this phenomenon because they were investigating other questions.

The new phenomenon offers an opportunity to study how the brain perceives smells separately from how the nose perceives them, Chen says. “Rivalry only exists in the mind of the perceiver,” she says. This phenomenon “offers a wonderful paradigm to study a window into consciousness and awareness.”

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