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Whiffs of Perception: Sniffing activates the mind's nose

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12:20pm, November 12, 2003
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A rose by any other name smells as sweet, even when you only conjure up its fragrance in your mind. That's because people use their noses to sniff imaginary as well as real aromas, and the mere act of sniffing scentless air kick-starts odor perception, a new study finds.

Behaviors, such as sniffing, that are used to acquire sensations do themselves activate brain representations of those sensations, concludes a research team led by Moustafa Bensafi of the University of California, Berkeley.

"Sniffing is not just a way to pick up smells, it's a part of olfactory perception," says Berkeley psychologist Noam Sobel, a coauthor of the new study.

Sobel suspects that all the senses inform perception in this way. For instance, other researchers have found that people move their eyes in much the same way whether they're visualizing an object in its absence or actually seeing it. Moreover, tests have shown that it's difficult to generate a mental image if one's eyes are prevented from moving.

Bensafi's team hooked up 30 college students to a machine that measured nasal airflow as they imagined pleasant and unpleasant sights, sounds, and smells. Visual images included a sunset and a scar. Imagined sounds ranged from rainfall to a person crying. Imagined smells encompassed roses and rotten eggs.

Volunteers spontaneously sniffed only when imagining smells, the researchers report in the November Nature Neuroscience. Imaginary pleasant odors evoked larger sniffs than imaginary unpleasant odors did.

In a second set of trials, the scientists measured nasal airflow in 10 additional participants, who completed four tasks in random order–smelling odors, imagining odors, looking at objects, and visualizing objects that weren't present.

Sniffing occurred only as individuals smelled or imagined odors. Volunteers drew in larger volumes of air for real smells than for imagined ones. As observed for imagined smells, pleasant real smells evoked larger sniffs than unpleasant ones did.

A final workout for the mind's nose involved 20 more volunteers. In these tests, each volunteer reported imagining smells more vividly after having been encouraged to sniff air than while wearing a nose clip that prevented sniffing.

These results underscore the influential view in neuroscience that the brain creates and saves information, such as smells, in the form of neural activity patterns, comments psychologist Stephen M. Kosslyn of Harvard University in an editorial published with the new research.

Acts such as sniffing trigger odor-specific activity patterns in the brain, Kosslyn contends.

Walter J. Freeman of the University of California, Berkeley, who investigates odor perception (SN: 10/19/02, p. 252: Available to subscribers at Spreading Consciousness), interprets the data differently. Sniffing triggers action-oriented brain activity related to specific smells and the contexts in which they occur, Freeman holds. These flexible neural responses, unique to each individual, reflect the shifting significance of particular smells rather than stored information about smells, he theorizes.

Bensafi is now directing a brain-imaging study to see whether odor imagery provokes responses in the neural gateway for smell perception or only in so-called higher olfactory tissue.

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