Sniffing In Stereo

I wrote this 27th April 2005

The Pyramid

We can hear in stereo, or rather hear a sound image in two parts. This allows the awareness of direction. We see images in two parts allowing depth of field to be judged. Without this benefit, we could not assess distance. We have two ears and two eyes.

Do we detect odours in stereo?

We have two nostrils. How does the brain interpret different smells. A chiral molecule is in essence much like the left and right hands: mirror images, but otherwise identical. Two such molecules may have a very different odour, though otherwise be identical. The olfactory organs can distinguish such a difference to a very high degree.

Abstract from Science

Rats Smell in Stereo

Raghav Rajan, James P. Clement, and Upinder S. Bhalla
Science 3 February 2006
311: 666-670 [DOI: 10.1126/science.1122096] (in Reports)

• It has been hypothesized that rats and other mammals can use stereo cues to localize odor sources, but there is limited behavioral evidence to support this hypothesis. We found that rats trained on an odor-localization task can localize odors accurately in one or two sniffs. Bilateral sampling was essential for accurate odor localization, with inter nasal intensity and timing differences as directional cues. If the stimulus arrived at the correct point of the respiration cycle, inter nasal timing differences as short as 50 milliseconds sufficed. Neuronal recordings show that bulbar neurons responded differentially to stimuli from the left and stimuli from the right.

Rats could locate the source of a smell with one or two sniffs. Rats can home in on smells using a method similar to the stereo processing of sounds, scientists say. Indian experts found 90% of neurons in the olfactory bulb of the rodent's brain respond differently to smells coming from the left and right. Details of the study appear in the latest issue of the journal Science.

The rats needed only one or two sniffs to accurately locate the source of an odour, the University of Agricultural Science, Bangalore, team reports. For rats, "each sniff is a perceptually complete snapshot of the olfactory world, including both odour identity and stereo-based location," Raghav Rajan and colleagues write in Science. The researchers got lab rats to stick their noses in holes in their cages to pick up odours sprayed from either the left or the right. If the smell came from the left, the rats would receive a water reward by licking the spout on the left. The same would apply if the smell came from the right. The rats were able to accurately determine which direction the odour came from in as little as 50 milliseconds.

The researchers say the rat's stereo sense of smell gives it an evolutionary advantage: allowing it to locate food or predators more quickly and precisely.