Our nose is considered our worst sensory organ. Wrongfully so, as neuroscientists have discovered in smelling experiments. This means that we can smell and distinguish between different scents from an odor sample just by taking a breath. We recognize scents ten times faster than previously thought – just as quickly as our eyes recognize colors. Our nose is therefore much more sensitive than expected. Which scent note of an odor mixture dominates in our perception also depends on the order of the scents presented.
Of our five senses, smell is considered the slowest and least sensitive. Because unlike our eyes, which are constantly exposed to light, smells only enter our nose in bursts with each breath. How quickly we perceive scent changes therefore depends on how quickly and often we inhale. However, this assessment is also based on the fact that the sense of smell is difficult to measure. Fragrances are based on volatile chemical molecules in the air that evaporate quickly. Their composition can therefore change faster than we can perceive. This makes it difficult to determine the speed and timing with which our sensory nerves in the nose recognize different scents.
Precision apparatus enables olfactory research
A team led by Yuli Wu from the Chinese Academy of Sciences in Beijing has now removed this obstacle and examined our sense of smell in more detail. The neuroscientists developed a device that releases scents so quickly that they reach a test subject’s nose in just 18 milliseconds. This makes it possible to deliver different scents one after the other within one breath, rather than simultaneously and mixed together. Wu and colleagues used this device to test the sense of smell in 229 subjects. They each presented two different scents – including apple, onion, lemon and floral notes – in different orders and speeds. The test subjects were asked to indicate when the scent in their nose changed.
The experiment showed that a distance of just 60 milliseconds was enough for the test subjects to distinguish between two scents within the same breath. The longer the distance, the better they could recognize the scents. Even if we only breathe in for about a second, our nose notices when two scents hit our sensory nerves in quick succession. However, we do not detect the two scents equally well: The test subjects more often reported that the resulting smell smelled more like the component that was first emitted by the device and thus hit their nerves more quickly than the second scent note – at least when the distance between the two scents was between 100 and 200 milliseconds. The order of sensory perception therefore dominates how we perceive a smell.
The nose and eyes recognize stimuli equally well
Wu and colleagues conclude that our sense of smell is much more sensitive and detects individual scents about ten times faster than previously thought. “This refutes the widespread assumption that the sense of smell is our slowest,” the team writes. The speed at which the nerves in our nose recognize scents is similar to the perception of colors in our eyes, which takes between 50 and 100 milliseconds. “A breath of odors is not a long-term exposure of the chemical environment that compensates for temporal fluctuations. Rather, it includes a temporal sensitivity that is equivalent to that for color perception,” says senior author Wen Zhou from the Chinese Academy of Sciences.
How pleasant the scents smelled and how intense they were had no influence on the results in the experiments. However, the neuroscientists point out that they only used scents that differed greatly in terms of structure and odor evaluation. The team suspects that our noses may not react as sensitively to scents that are more similar. Follow-up studies should clarify whether this is the case. We will also continue to investigate how exactly scent processing occurs in the nerve cells of our olfactory system.
Source: Yuli Wu (Chinese Academy of Sciences) et al.; Nature Human Behavior, doi: 10.1038/s41562-024-01984-8