The human sense of smell is nothing to sneeze at, research suggests, with scientists revealing that we are much more sensitive to the order of odors picked up by a sniff than previously thought.
Charles Darwin is among those who have slandered our sense of smell, suggesting that it is “of very little use” to humans, while scientists have long thought that our olfactory abilities are quite slow.
“Intuitively, each sniff feels like a prolonged exposure to 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 appear to be a single odor. , rather than a discernible mix of smells that arrived at different times. “The sniffs are also separated in time and occur within seconds of each other,” he 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 odors as we are to rapid changes in color.
A key challenge in probing our sense of smell, Zhou said, is that it has been difficult to create a setup that allows different smelly substances to be presented in a precise sequence in time within a single inhalation.
However, an article in the journal Nature Human Behavior Zhou and his colleagues report how they did it by creating an apparatus in which two bottles containing different aromas were connected to a revolver using tubes of different lengths. These tubes were equipped with miniature check valves that were opened by the act of sniffing them.
The setup meant that the two scents hit the nose at slightly different times over the course of a single inhalation, with a precision of 18 milliseconds (ms).
The team then conducted a series of experiments with 229 participants.
In one experiment, participants were presented with an apple smell and a floral aroma, connected to the apparatus with tubes of different lengths, which meant that one aroma would reach the nose between 120 and 180 ms before the other. Participants were then asked to smell the device twice and report whether the order of the smells was the same or reversed.
The team found that participants were correct on 597 of 952 trials (63% of the time), with similar results when 70 other participants performed the trials with lemon and onion odors.
Additional testing, involving those who performed particularly well on these tests, revealed that participants did better than chance even when two odors reached the nose just 40-80 ms apart. The team said this interval was about 10 times shorter than previously thought to be necessary for humans to discriminate between two odors presented in one order and in reverse order.
However, while participants were able to tell that the odor had changed when the order of the odors was changed, they found it more difficult to identify which odor actually appeared first. They did better than chance on this task only for the lemon and onion odors, and only when the odors reached the nose with an average time difference of 167 ms. In this case, participants tended to report that the basic odor captured in an inhalation was more similar to the first of the two emitted odors, suggesting that the order of odors shapes our perception.
“In general, discrimination between a pair of 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 serial recognition of mixture components.”
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