Study Suggests Our Sense of Smell Is More敏捷than We Think

When we inhale, all the chemicals in the surrounding air enter our nose. We thus detect what we call an odor – more or less intense, more or less pleasant. An inspiration usually lasts between one and three seconds. Until now, it was thought that this time frame limits the speed at which we can perceive odors. Our sense of smell was thus considered to be a rather “slow” sense. A Chinese study reveals that this is not the case at all.

Immersed in a complex and dynamic chemical environment, the human olfactory apparatus appears to have low temporal resolution. At least, compared to that of sight or hearing, which operate on the scale of tens of milliseconds. Thus, humans would be unable to distinguish two odors released one after the other if the interval between these odors reaches 0.6 seconds. Additionally, each breath through your nose typically lasts one to three seconds. And the next inspiration only comes after a delay of a few seconds.

Intuitively, each inspiration gives the impression of taking a long exposure photo of the chemical environment said Dr Wen Zhou of the Institute of Psychology of the Chinese Academy of Sciences. The chemical changes that occur during a single breath therefore seem to be combined into a single odor. The human sense of smell is therefore considered “blind to change,” explain Zhou and his colleagues in Natural Human Behavior. However, electrophysiological examinations of odor-induced responses in rodents have suggested that the temporal dynamics of the mammalian olfactory system may be more rapid than previously believed.

A device specially designed to emit two odors just milliseconds apart

A team led by Dr. Wen Zhou recently set out to reassess the potential of the human sense of smell. However, it is not easy to present different smells at different times in a single breath. The researchers therefore designed a device specifically for this task. Triggered by inspiration, the device controls the emission of odors with a precision of 18 milliseconds.

Nasal inspiratory pressure ranges from 1 to 14 kPa. “ We reasoned that this pressure could be harnessed to activate valved olfactory channels. The time required for odorous molecules to reach the nose would then vary depending on the flow rate and the distance between the valve of this canal and the nose (for a canal of fixed section) », Explain the researchers. The first factor (flow) is directly linked to pressure (i.e. the intensity of inspiration) and can be measured. It was possible to vary the second (the distance) by adjusting the length of the pipe.

odor emission device diagram

The device included two odor channels, controlled by check valves with a length difference ΔL, here connected to a miniature vacuum pump. Two miniature photo-ionization detectors (PIDs) were used to measure vapor phase odor concentrations in the two channels. The curves were almost identical over time with ΔL = 0. Credits: Wu et al., Nature Human Behavior (2024)

The device included a miniature vacuum pump, connected to two Teflon tubes via a Y-shaped structure. One tube was 10 cm long, the other was longer. At their ends was a check valve in contact with a bottle of odor. The probes of two miniature photo-ionization detectors (PIDs) were inserted into the two tubes. They were used to measure odor concentrations in the vapor phase.

Using this device, the team could create mixtures of odors, emitted one after the other during a single inhalation. They then tested the sense of smell of 229 participants during five experiments. The objective was to check if they could distinguish these odors and if their order had an impact on olfactory perception.

Odors separated by only 60 milliseconds!

The researchers first used pentyl pentanoate and isobutyl phenylacetate. The first has an apple smell, the other has a sweet floral smell. “ They are different in structure and odor, but have similar diffusivity in air and are easily soluble in mucus », specify the scientists. The odors were comparable in terms of intensity, valence (the pleasure of breathing them) and nasal pungency.

Participants first had to familiarize themselves with each of the odors. They then had to inhale several times into the device, indicating whether the order of the odors presented was the same or not. Their answers were correct in 63% of cases (597 trials out of 952). The finest noses managed to differentiate between the two temporal mixtures when the delay between the two odors was between 40 to 80 milliseconds (60 ms on average). That's about a third of the time it takes to blink. And it's ten times shorter than we thought, which indicates that our sense of smell is not so slow after all!

For comparison, the frequency at which flashing green and red lights appear continuous is around 10 to 20 Hz (i.e. a resolution of 50 to 100 ms), points out the press release from the Chinese Academy of Sciences. We would thus be as sensitive to rapid changes in odor as to rapid changes in color.

This ability did not depend on the intensity of the odor, its valence, its pungency, nor the total amount of odor molecules in an inhalation. The researchers note, however, that participants' ability to distinguish mixtures improved as the delays between compounds lengthened.

To test whether the observed fine temporal sensitivity extended to other odorants, the team introduced two new compounds. They used citral, an aldehyde that smells like fresh lemon, and dimethyl trisulfide, which smells like onions. They obtained similar results in terms of “order of odor” recognition.

Evidence for a “time code” for odors

While they could distinguish a change in the order of odors presented, participants had more difficulty reliably identifying which odor occurred first. Experimenting with lemon and onion scents (easier to distinguish) provided the best results. The team also noticed that participants tended to report that the overall odor picked up during an inhalation was more similar to the first of the two odors emitted, suggesting that the order of the odors shapes our perception.

Overall, discrimination between a pair of temporal mixtures does not depend on precise recognition of the order of the constituent odorants. Instead, it appears to result from a mechanism that operates on a much faster time scale than that involved in the serial recognition of mixture components. “, explains Zhou.

These results provide empirical evidence for the existence – hitherto assumed – of a “temporal code” of odor identity. “ In the same way that timing affects the perception of notes in a melody, the timing of the individual components of a complex olfactory mixture that reaches the nose may be crucial to our perception of the olfactory world. “wrote Dr. Dmitry Rinberg, professor in the Department of Neuroscience and Physiology at NYU Langone Health in New York, in an editorial accompanying the study.

By enabling precise control of odor diffusion that aligns with the natural dynamics of sniffing, this research opens new avenues for studying the temporal properties of smell. “ NWe have shown in human adults that sniffing odors is not a long exposure to the chemical environment that averages temporal variations. On the contrary, it contains fine temporal information », conclude the researchers.

These results could guide the design of electronic noses reproducing the human sense of smell and olfactory virtual reality systems.

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