Look in a mirror, and you'll notice that most sensory organs come in pairs. Having two eyes lets us perceive depth, and two ears are useful for locating sounds. But why two nostrils? Now neuroscientists have found evidence that this expands the range of scents we can smell.
For centuries, practitioners of yoga have known that one nostril breathes in air more rapidly than the other, and that the dominant nostril switches every few hours. (What happens is that one nasal passage swells, restricting air flow.) In fact, yogis advocate inhaling through the open nostril and breathing out through the constricted one, in the belief that this will stimulate the brain and "restore vitality" to the body. The yogic tradition is silent about what nostrils do for the sense of smell.
And so we turn to Noam Sobel, a neuroscientist at Stanford University, who has been studying the topic for 4 years. A friend of his one morning suggested over breakfast that the varying flow rates might mean the nostrils detect different chemicals. Skeptical, Sobel tested the idea by asking 20 healthy adults to sniff a blend of octane and carvone through one nostril at a time. Although the volunteers didn't know it, they were always inhaling the same mixture.
But that's not what they thought at all. In today's Nature, Sobel reports that 17 of the subjects described stronger whiffs of octane (a slowly absorbed chemical) when they used the more blocked nostril, and smelled more carvone (a rapidly absorbed chemical) through the clearer nostril. "These differences are subtle," says Sobel. "It's not as if one smells roses and the other smells cherries. But I think it will be fundamental in understanding the sense of smell." With each nostril able to detect different kinds of chemicals, he says, that would expand the variety of odors that can be detected.
The "simple but smart" experiment, as neurobiologist James Bower of the California Institute of Technology calls it, suggests that human smell is more sophisticated than previously thought. "My guess is that Noam has discovered the tip of an iceberg, and that there's a lot more fine regulation of olfaction in the human nose," he says. This understanding might one day lead to the development of better "artificial noses" that could, for example, sniff out traces of explosives.