By studying blindfolded college students who crawled through grass to sniff out a chocolate-scented trail, scientists say they’ve found evidence of a human smelling ability that experts thought was impossible.
The study indicates the human brain compares information it gets from each nostril to get clues about where a smell is coming from. And it suggests dogs, mice and other mammals do the same thing, contrary to what most scientists have thought.
People compare signals from each ear to locate the source of a noise. But the prevailing notion has been that mammals can’t follow the same strategy for smells, because their nostrils are too close together to get distinct signals.
“We debunked that,” said Noam Sobel of the University of California, Berkeley, who reported the new results Sunday with graduate student Jess Porter and others on the Web site of the journal Nature Neuroscience. The work will appear in the journal’s January issue.
The report isn’t the first to suggest the two-nostril idea. But Sobel and colleagues have now “opened the doors for full consideration of it,” said an expert familiar with the work, neuroscientist Charles Wysocki of the Monell Chemical Senses Center in Philadelphia.
The new paper reports five experiments. One tracked tiny particles used in theatrical fog to show that each human nostril really does sample a distinct region in space.
But most of the paper focuses on what a group of undergraduate psychology students could do on a patch of lawn on the Berkeley campus.
30-foot trail of chocolate
One outdoor experiment was designed to see if people could use just their noses to follow a 30-foot-long trail of chocolate scent, which traced a dogleg course through the grass.
The trail was created with scented twine. But the 32 participants were blindfolded and equipped with thick gloves, kneepads and elbow pads to make sure they couldn’t see or feel it. They also wore earmuffs.
Before they began, they were shown a video of proper scent-tracking form, which requires putting the nose on the ground. “People don’t really want to do that,” Porter said.
Two-thirds of the participants succeeded in following the scent. But when they tried it again with their noses plugged, nobody could do it.
Two nostrils better than one
Another experiment found that people got better with practice. Yet another experiment, with 14 participants, found that the volunteers did better if they used two nostrils than if one nostril was taped shut. They succeeded 66 percent of the time with two nostrils, versus 36 percent with one nostril.
But did that really mean their brains were benefiting from two independent signals? Maybe both signals are the same, but the olfactory system just works better if it gets input from both nostrils. Or maybe the real explanation is that people take in less odor with one nostril than with two, giving a weaker signal.
To sort that out, researchers retested four of the participants who’d gone through the practice sessions. This time, the subjects wore devices over their nostrils that controlled the airflow into their noses.
One version of the device was basically an extension of a normal nose, with two holes to sniff through, each supplying air to one nostril. The other version had only one hole. It took in the same amount of air as the first version, but simulated the effect of having only one big nostril.
The participants were less successful and slower when they had the equivalent of one nostril. That supports the idea that people benefit from having two, researchers said.