The human sense of smell is no slouch, research suggests, with scientists revealing that we are much more sensitive to the sequence of smells captured by a sniff than previously thought.
Charles Darwin is one of those who decried our sense of smell, suggesting that it is “of very little service” to humans, while scientists have long thought that our sense of smell is rather sluggish.
“Intuitively, each sniff feels like taking a long-exposure shot of the chemical environment,” said Dr Wen Zhou, co-author of the research from the Chinese Academy of Sciences, adding that when an odor is detected, it may seem like one smell, rather than a perceptible mixture of smells that arrived at different times. “Sniffs are also separated in time, occurring seconds apart,” she said.
But now researchers have revealed that our sense of smell works much faster than previously thought, suggesting that we are just as sensitive to rapid changes in smells as we are to rapid changes in color.
A key challenge in investigating our sense of smell, Zhou said, is that it has been difficult to create a setup that allows different odorants to be presented in a precise sequence in time within a single sniff.
However, writing in the journal Nature Human BehaviorZhou and colleagues report how they did this by creating an apparatus in which two bottles containing different flavors were attached to a nose piece with tubes of different lengths. These tubes were equipped with miniature check valves that were opened by taking a sniff.
The setup meant that the two scents reached the nose at slightly different times during a single sniff, with an accuracy of 18 milliseconds (ms).
The team then conducted a series of experiments involving 229 participants.
In one experiment, participants were presented with an apple-like smell and a floral scent, connected to the apparatus with different lengths of tubes which meant that one scent would reach the nose approximately 120-180ms before the other. Participants were then asked to sniff the apparatus twice and report whether the order of the odors was the same or reversed.
The team found participants were correct in 597 out of 952 trials (63% of the time), with similar results when another 70 participants performed the trials with lemon-like and onion-like odors.
Further tests, including those who performed exceptionally well on these trials, revealed that participants performed better than chance even when two odors arrived at the nose only 40-80 ms apart. The team said this interval was about 10 times shorter than previously thought to be necessary for people to distinguish between two odors presented in one order and in the reverse order.
Although participants could see that the odor changed when the order of odors was changed, they found it more difficult to identify which odor actually came first. They outperformed chance on this task only for the lemon and onion-like odors, and then only when the odors reached the nose with an average time difference of 167ms. In this case, participants tended to report that the overall odor captured in a sniff was more like the first of the two odors delivered—suggesting that the order of odors shapes our perception.
“In general, distinguishing between a few temporal mixtures does not depend on accurately recognizing the order of the constituent odorants,” Zhou said. “Instead, it appears to be driven by a mechanism that operates on a much faster time scale than that involved in the sequence recognition of mixture components.”
