Rats Smell in Stereo, Study Says

John Roach
for National Geographic News
February 3, 2006
Eew, where's that smell coming from?

If you could ask a rat, it could locate the direction of the stench's source in a single sniff, scientists report in a new study.

Rats accomplish this feat by effectively smelling in stereo: Each nostril operates independently of the other, sending different signals to the brain that are then computed to determine the direction of the odor.

Stereo sniffing likely gives rats a leg up in everything from tracking down a meal to evading an enemy, said Upinder Bhalla, a neuroscientist at the National Center for Biological Sciences in Bangalore, India.

"Imagine you have a predator sneaking up on you. If you can smell in stereo, you can detect and localize it in one sniff, and you'll have a decent chance of getting away," he said in an email interview.

"If you have to look around, or take multiple sniffs to find the predator, you may get eaten."

Bhalla is a co-author of the study reported in today's issue of the journal Science.

The study, Bhalla said, provides insight into the spatial dimension of smell and the speed at which the brain operates.

"This [finding] helps to constrain and refine our ideas about information processing," he said.

Once trained, rats in the study could determine the direction of a smell in as little as 50 milliseconds, which, Bhalla said, "really is quite fast." According to a 1992 study, one sniff takes humans about 700 or more milliseconds to process.

Lab Rats

To establish that rats smell in stereo, Bhalla and colleagues trained thirsty rats to lick different water spouts after they detected the smell of banana, citrus, or rose water. The smell was piped in through a hole in a special chamber.

The rats got a drink if they licked the left spout when the odor came from the left side or the right spout if the odor came from the right.

(Watch a video clip of the experiment.)

Trained rats did this with at least 80 percent accuracy, and most needed just one sniff to correctly determine the direction of the smell.

When one nostril was stitched shut, however, the rats' success rate was reduced to just above the level of random chance.

This shows that the rats required stereo processing to correctly locate the odor source, despite the fact that their nostrils are a mere 0.12 inch (3 millimeters) apart.

The researchers performed several controls to rule out the possibility that a cue other than odor, such as airflow or sound, drove their decision about which spout to lick.

Jay Gottfried is an assistant professor of neurology at the Northwestern University Feinberg School of Medicine in Chicago, Illinois. He says, in light of all the controls and previous research on this subject, the results appear sound.

"And it fits with a growing number of studies of animals and humans suggesting there is something more to the nose than simply detecting smell or deciding what it is," he said.

"The nasal apparatus is clearly capable of localizing things too."

In a follow-up experiment, the researchers examined how neurons in the rats' brain region that processes smells react to odors coming from the left or right. They found that 90 percent of the neurons respond differently to smells on either side.

Writing in Science, Bhalla and colleagues conclude that "each sniff is a perceptually complete snapshot of the olfactory world, including both odor identity and stereo-based location."

Not Surprising

Rats have long been known for their prowess when it comes to smell, Bhalla said. In the lab, they rely on it most when faced with tasks that require them to make a decision.

"They learn faster with smell and can do more complicated things with smell," he said.

According to Gottfried, observations of rats in the natural world also suggest that smell is their dominant sense. For example, he said, think of a rat following its nose inside a house to the pantry for a morsel of food.

"This study is sort of validating this general concept and is starting to piece out the physiological ranges and controls under which these processes operate," he said.

Research published in the journal Neuron last August by a team of scientists at the University of California, Berkeley, suggests that humans may also be able to smell in stereo, though probably not with the sophistication of rats.

Bhalla says that from here, he and his colleagues hope to learn more about the processes in the brain that make this ability possible.

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