Smells like emotion: brain splits duties to sniff out feelings.

The inner-brain structure known as the amygdala is getting an emotional makeover. Fingered in many studies as the brain's fear center, the amygdala actually takes charge of assessing the emotional intensity of both pleasant and unpleasant sensations, according to a new investigation.

At the least, this new view of the amygdala applies to fragrant and foul odors, say neuroscientist Adam K. Anderson of Stanford University and his colleagues. The amygdala probably operates in the same way to mark the emotional intensity for sights, sounds, tastes, and tactile sensations, the scientists speculate in the February Nature Neuroscience.

"The amygdala coordinates early processing of the physical intensity of smells and other sensory stimuli, which are then perceived as either pleasant or aversive," contends study coauthor Noam Sobel of the University of California, Berkeley. Until confirmed in further studies, this view will undoubtedly attract controversy, he adds.

The researchers used functional magnetic resonance imaging (fMRI) scanners to probe brain responses in eight women and eight men as they whiffed chemical solutions of varying aromatic intensities. Each volunteer smelled low--and high-concentration versions of a fruity, fragrant odor and a rancid, sickening odor. For comparison, participants also sampled "pure air," which contained no odors.

During scanning, volunteers pressed a computer key to indicate that they detected an odor. After scanning, they rated the intensity and pleasantness of the odors.

The fMRI scans revealed an overall increase in blood flow within the participants' amygdalae in response to intense odors whether they were refreshing or repellant. However, Anderson's team found that low-intensity odors of any kind, as well as pure air, failed to stir the structure's blood flow. Neuroscientists regard increased blood flow in a particular brain area as a marker of increased neural activity.

The researchers also found that a part of the right frontal brain previously linked to smell perception exhibited increased blood flow as volunteers smelled the pleasant odor, regardless of its intensity. This brain area also responded, to a lesser extent, to pure air.

A corresponding section of the left frontal brain showed elevated blood flow as volunteers sniffed the nasty odor at either low or high concentrations. However, pure air drew no enhanced response from this region.

These findings enter a larger debate about the nature of emotion, comments neuroscientist Stephan Hamann of Emory University in Atlanta. The new results fit with the theory that simple emotional states of feeling good or bad--and also energized or enervated--represent the foundation of more complex feelings, such as fear and happiness, in a given situation. A popular opposing theory holds that these and other emotions axe built into the human brain.

In line with the new data, Hamann reported last year that amygdala activity increases comparably when men view either disturbing images of injured bodies or arousing images of nude females. In the past, other researchers had shown volunteers disturbing images and moderately pleasant images, such as puppies, thus missing the amygdala's response to highly positive sights, Hamann says.