They come in pairs, just like the ears and eyes, yet far less is known about how our nostrils send sweet and stinky scents alike to the brain. In fact, until now, no one had noticed that the process often involves a duel of sorts.

"Human olfaction is not well understood," says Denise Chen, a psychology professor at Rice University in Houston and co-author of a study on the rivalry of nostrils published in Current Biology. "Yet it offers a unique window into consciousness and awareness."

When the human nose sniffs, odors travels up the nostrils and into the brain. This happens separately on the left and right sides of an impervious wall in the nose called the nasal septum, Chen explains. So what happens when each nostril is exposed to a different scent? Does the brain interpret it as a mixture, or does it select between them?

Chen and her colleagues enlisted 12 volunteers to find the answer. During each of 20 trials, two odors were presented under subjects' noses: "a rose in one nostril, a marker pen in the other," Chen says. Subjects were then prompted to simultaneously inhale both aromas—with a single sniff—and then rate the resulting smell on a scale; the left and right extremes represented dominance of each odor, respectively, and the middle meant a mix.

Chen was surprised to find that every single subject experienced an alternating pattern of "smelling predominantly just rose or predominantly just marker," she says. The brain could apparently only attend to one fragrance at a time, which turns out to be the same phenomenon previously observed in vision: If you present to each eye a different image, instead of seeing a blend of the two, a person sees an unpredictable alternating pattern comprising the separate images. It's a sort of "tug-of-war" between the two stimuli, Chen says.

The next step was figuring out just where this phenomenon was occurring. After odors travel up the left and right nostrils, the olfactory information is picked up by nerve endings in each that eventually meet in the brain, thereby raising the question: Is the battleground in the nasal passageway or the brain?

To learn more, the researchers "fatigued" a single nostril of each subject—exposing it to the rose smell for two minutes. "Just like when walking into a department store, at first the fragrances hit you. Then, very quickly, it becomes faint," Chen says. But not only did the exposed nostril adapt to the aroma, when researchers presented the rose scent to the other nostril following the initial two-minute exposure, that one also smelled the rose much more weakly. This suggests the battle of the bouquets may be waged in the brain, according to Chen: "Information from one nostril gets transferred to the other."

The same brain-level stimulus suppression showed up when the researchers exposed all 12 noses to a mixture of marker and rose odors—the same concoction to each nostril. Instead of smelling a mixture, most subjects experienced the same illusion of alternating scents.

Evidence for a mechanism in the nostrils, however, also emerged. In a separate experiment, after subjects had taken whiffs of rose and marker (again, one in each nostril), the researchers swapped the scents. Now, the suppressed odor regained its strength: the nostril that had become accustomed to the marker could now smell the rose, and vice versa.

"These studies offer a rare opportunity to separate perception and stimulation," Chen says. She hopes her team's research will generate future studies to understand what is underlying this phenomenon. "If we could understand the mechanisms that process smells, it would benefit science and, over the long run, contribute to assessing and curing olfactory disorders," she says.

The sense of smell is a "very youthful sense," Chen adds. "I used to have an incredible sense, but now it pales when I compare myself with my students." And with deterioration in smell, taste is also lost. "[The sense of smell] affects quality of life. It's an often ignored, but serious issue."