Your Brain Detects Subtle Changes in Odors with Just One Sniff

Human sniffs last between one and three seconds. During that time, chemicals enter the nose and allow us to perceive the smells around us. But whether humans can perceive odor changes shorter than the length of one sniff has been an open question.

A new device that allows for precise odor control shows that people perceive much more detail within a single sniff than previously thought. They can perceive odor changes in fractions of a second, researchers report October 14 in Nature Human Behaviour.

“Intuitively, each sniff feels like taking a long-exposure shot of the chemical environment,” says Wen Zhou, a psychology researcher at the Chinese Academy of Sciences in Beijing. “We may detect an odor, and that odor feels like a whole, without discernible temporal structure.”

But the new device Zhou and her team designed tells a different story. It detects the beginning of a sniff using changes in nasal pressure and then triggers the release of two odors, with one slightly farther from the nose than the other.

More than 200 participants were exposed to the sequences of two chemical odors in a single sniff. The scientists controlled the time difference between the odor release with millisecond precision. Some of the odors smelled like apples or flowers, while others smelled like onions or lemon. Participants reported which order of odors, the sniff sequence, they perceived.

Test subjects were able to discern different sequences with above-chance accuracy, even when the odors were delivered only 60 milliseconds apart. The results suggest that our sense of smell has a speed similar to that of color perception.

Next, Zhou wants to investigate what’s going on from sniff to sniff, especially given that our noses can detect more than a trillion odors. “Sniffs are separated in time,” she says. How the brain processes the temporal information within and between sniffs is still an open question.