Sniff And Smell Are Equally Important In The Brain's Perception Of Odor

In the Oct. 19 online issue of Nature: Neuroscience the researchers report that the sniff people take when trying to imagine an odor closely resembles the sniff they would have taken if the odor were really there. For example, when imagining the smell of bus fumes, people take a timid sniff, but when imagining the smell of a rose, they take a vigorous sniff.

If people are prevented from sniffing, the vividness of the image is significantly reduced. From these findings, the scientists conclude that the brain recreates the components of real sensation, such as sniffing, in order to create mental images.

This research "teaches us how the primary olfactory cortex works," said UC Berkeley researcher Moustafa Bensafi. "It has long been known that the act of sniffing, without any odor present, induces neural activity in the primary olfactory cortex, the region of the brain involved in odor perception." The researchers believe their results suggest that this sniff-induced neural activity is, in fact, a fundamental component of the olfactory perception, a part of the brain's representation of smell.

"As far as the brain is concerned," Bensafi concluded, "a sniff and a smell are equally important components in the perception of odor."

The research was conducted by Bensafi, a postdoctoral research fellow, and assistant professor Noam Sobel, both of the UC Berkeley Helen Wills Neuroscience Institute and the UC Berkeley Department of Psychology.

Bensafi and collaborators in the Sobel lab, together with a visiting researcher from McGill University in Montreal, first studied how 30 UC Berkeley undergraduates reacted to visual messages.

"The participants were seated in a chair in front of a monitor where visual messages instructed them to imagine different sounds, sights or smells, once every 30 seconds," Bensafi explained. "For example, the message on the monitor would read: 'Imagine the smell of strawberries.' During this time, and most importantly, unbeknownst to the participants, the researchers measured airflow in the participants' noses."

The results clearly pointed to a pronounced sniff that was spontaneously generated when participants were trying to imagine a smell, but not when trying to imagine a sound or sight. And, their sniffs were more vigorous during imagination of pleasant versus unpleasant odors.

In their paper, "Olfactomotor Activity During Imagery Mimics That During Perception," the authors explain how they conducted several additional control experiments to validate their results. In one of these experiments, the authors blocked sniffing by placing a nasal clip on participants' noses, and measured the effect this had on the reported quality of mental imagery.

"Consistent with a functional role for sniffing in olfactory imagery, overall vividness was reduced during the sniff-blocked condition for olfactory, but not visual, imagery," Bensafi said.

The results of these experiments have led the researchers to a series of conclusions. "On the first level, the finding is interesting because it quantifies what seems obvious, but was never before measured or validated," Sobel said. "But more important, our findings point to a fundamental concept in how the brain may create internal representations of external events."

"The current findings suggest that one strategy used by the brain in this task is to 'recreate' or 'play back' sensory-motor events, such as sniffing or eye movements, that would normally occur in perception," Sobel said. "To perceive an odor we normally sniff, and to help us imagine an odor, or mentally recreate it, we similarly sniff."