UC Berkeley Scientists Uncover the Brain's Role in Recalling Smells

A rose by any other name might smell as sweet, but a recent discovery by UC Berkeley scientists may help better understand the whole process.

A recent study led by UC Berkeley psychology professor Noam Sobel and graduate student Joel Mainland showed that exposing a nostril to the chemical androstenone can increase sensitivity to the chemical in both nostrils.

This finding may imply that the sense of smell has its root deeper in the brain than previously thought.

Although many adults cannot discern certain chemical odors, they can learn how to detect these unique smells through repeated exposure.

Sensitivity to androstenone, a sex pheromone produced by male boars, varies widely from one individual to another.

"You actually have a group of people who can smell androstenone and it smells horrible and can detect minute concentrations of it," Mainland said. "Then you have this really broad distribution of people who can detect it. Some of them can detect it pretty well, some of them not so well, and some of them not at all."

Prior to this finding, it was thought that smell-related learning took place in the nose.

Conventional wisdom held that nasal neural receptors first detect an odor, then succeed in amplifying the signal to the brain the next time a whiff is sensed.

Based on the new findings, androstenone detection appears to emanate from deep inside the brain where the nostrils can share information about previous smells, Sobel said.

The discovery represents "solid research," said Ohio State University food science professor Jeannie Delwiche.

Charles Wysocki, a neuroscientist at the Pennsylvania-based Monell Chemical Senses Center, who first discovered that androstenone detection can be learned, urged caution in interpreting Sobel's results.

"There are rare exceptions where if you smell something and you are removed from the source, a long time afterwards you can still smell it," Wysocki said. "Androstenone is one of those very sticky molecules. It is very hard to get rid of androstenone when anything has been exposed to it."

A nostril plugged during androstenone exposure might still perceive the chemical's presence after the experiment concluded, he added.

"Once the nostril was unplugged, the androstenone that was sticking to the tissues inside the exposed nostril was free to have access to the previously plugged nostril," Wysocki said.

Sobel's team partially addressed such concerns by using a control group in the experiment in which both nostrils were plugged, Mainland said.

A small amount of androstenone could bypass the barrier, but this amount did not lead to an improved ability to detect the chemical.

In order to identify the part of the brain involved in olfactory learning, Sobel's group is currently using brain imaging techniques, taking brain scans at various stages in the learning process.

The images would reveal no consistent change if the brain is not involved in the process.

"We want to find out if misleading feedback can eliminate the learning effect," Mainland said. "Is this something that's inherent, where if you're just exposed to it you will learn to detect androstenone and it will smell bad no matter what, or is it something that through trial and error you learn to detect?"

The study was published in the Oct. 24 issue of the British journal Nature.

Tags: