New research suggests the brain does much of the heavy lifting when it comes to scents, and it can recognize an odor even when one nostril is locked out of the smelling loop.
In a simple study of a complex human sense, scientists found people recognized the scent of androstenone -- a pig pheromone that smells a bit like dirty laundry -- even though they were smelling it through a nostril that had never detected it before. The brains of the subjects appeared to have been primed by exposure to the scent through their other nostrils.
"Gaining sensory capabilities is not a capability we usually associate with the adult brain. The prevailing belief has been that the brain is static," says study co-author Noam Sobel, an assistant professor of neuroscience at the University of California at Berkeley. "This is another instance that's showing the adult brain is actually quite changeable and malleable."
The results of the study appear in tomorrow's issue of Nature.
At issue is the brain's ability to detect odors after extended exposure to them, a phenomenon known as "plasticity."
For example, about 30 percent of humans can't smell the odor of androstenone, which can show up in the sweat of humans, especially men.
After extended exposure to the odor, non-smellers can actually learn to detect it, Sobel says. "One thing we want to ask ourselves is, 'Where in the olfactory system is this plasticity?' " he says. Do the odor receptors in the nose learn to detect new smells, or does the process happen in the brain itself?
In his study, Sobel and colleagues found some subjects who couldn't detect androstenone and exposed one of their nostrils to the scent while the other nostril was blocked.
Eventually, some subjects learned to detect the scent. Later, when the original nostrils were blocked and the unexposed nostrils were opened, the subjects still recognized androstenone.
"Our view is that their brain was seeing information before, but it just didn't know how to make sense out of it," Sobel says.
Charles J. Wysocki, a neuroscientist at the Monell Chemical Senses Center in Philadelphia, is skeptical of the findings. It's possible that androstenone molecules transferred from one nostril to another, corrupting the experiment, he notes.
"There are these infrequent occasions when you've smelled something, and a long time after the source is gone, the smell returns," Wysocki explains. "In part, we think the reason is that those molecules that provided the smell are very tenacious. They tend to stick to anything and everything. They get trapped in the mucous, or in the cells in the nose or in the lining of the esophagus, and over the course of time, these molecules are slowly released from where they were sequestered."
The smell of smoke is an especially sticky smell, and androstenone shows the same tenacity, Wysocki said.
One thing is clear, he says: "The brain and the nose work in concert to appreciate an odor. How do people become sensitized to odors they can't initially smell? That's still an unanswered question."
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