The human sense of smell is nothing to turn one’s nose up at, research suggests, with scientists revealing we are far more sensitive to the order of odours captured by a sniff than previously thought.
Charles Darwin is among those who have cast aspersions on our sense of smell, suggesting it to be “of extremely slight service” to humans, while scientists have long thought our olfactory abilities 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 a smell is detected it can seem like one scent, rather than a discernible mixture of odours that arrived at different times. “Sniffs are also separated in time, occurring seconds apart from one another,” she said.
But now researchers have revealed our sense of smell operates much faster than previously thought, suggesting we are as sensitive to rapid changes in odours as we are to rapid changes in colour.
A key challenge to probing our sense of smell, said Zhou, is that it has been difficult to create a setup that enables different smelly substances to be presented in a precise sequence in time within a single sniff.
However, writing in the journal Nature Human Behaviour, Zhou and colleagues report how they did just that by creating an apparatus in which two bottles containing different scents were hooked up to a nosepiece using tubes of different lengths. These tubes were fitted with miniature check valves that were opened by the act of taking a sniff.
The setup meant the two scents reached the nose at slightly different times during the course of a single sniff, with a precision of 18 milliseconds (ms).
The team then carried out a series of experiments involving 229 participants.
In one experiment, participants were presented with an apple-like odour and a floral scent, hooked up to the apparatus with different lengths of tubing that meant one scent would reach the nose about 120-180ms before the other. Participants were then asked to sniff the apparatus twice and report whether the order of the odours was the same or had been reversed.
The team found participants were correct in 597 out of 952 trials (63% of the time), with similar results when another 70 participants carried out the trials with lemon-like and onion-like odours.
Further tests, involving those who fared particularly well on these trials, revealed participants did better than chance even when two odours arrived at the nose a mere 40-80ms apart. The team said this interval was about 10 times shorter than previously thought to be necessary for humans to discriminate between two odours presented in one order and in the reverse order.
However, while participants could tell the smell had changed when the order of odours was switched, they found it harder to identify which odour actually came first. They fared better than chance on this task only for the lemon and onion-like smells, and then only when the odours reached the nose with an average time difference of 167ms. In this case, participants tended to report the overall smell captured in a sniff was more like the first of the two odours delivered – suggesting the order of scents shapes our perception.
“Overall, the discrimination between a pair of temporal mixtures is not dependent on accurately recognising the order of the constituent odorants,” said Zhou. “Instead, it seems to be driven by a mechanism that operates on a much faster timescale than that involved in the serial recognition of mixture components.”
