When we inhale, airborne chemicals enter our nose, creating the "odor" we detect. These chemicals are then expelled when we exhale. Each breath lasts 3–5 seconds, which seems to limit how quickly we can perceive odors. Chemical changes that occur within a single breath appear to be combined into one odor. Because of this, our sense of smell, or olfaction, is often considered a slow sense.

Now, however, researchers led by Dr. ZHOU Wen from the Institute of Psychology of the Chinese Academy of Sciences have challenged this view. Their new study, published in Nature Human Behaviour, shows that human olfactory perception can detect fine chemical changes within the duration of a single sniff.

Dr. ZHOU's team developed a unique sniff-triggered device that controls odor delivery with a precision of 18 milliseconds—about the duration of a frame on a regular LCD display (60 Hz). Using this device, the team created temporal odor mixtures, presenting two odors one after the other with precisely measured delays. They tested 229 participants across five experiments to see if they could distinguish these mixtures.

The researchers found that when two odor compounds, A and B, were presented in different orders (A before B and B before A), participants could tell the difference when the delay between the compounds was just 60 milliseconds—about a third of the time it takes to blink. For comparison, the frequency at which flickering green and red lights appear continuous is around 10–20 Hz (50–100 ms resolution).

Participants’ ability to distinguish the odors improved with longer delays between the compounds and did not depend on knowing the correct order. They could distinguish "A before B" from "B before A" by smell, even if they couldn't identify the order. This ability was not influenced by factors like odor intensity, pleasantness, pungency, or the total amount of odorant molecules in a sniff.

These findings support the existence of a temporal code for odor identity. By providing precise control over odor delivery that aligns with natural sniffing dynamics, this research opens new avenues for studying the temporal aspects of olfactory perception and developing olfactory displays.

"A sniff of odors is not a long exposure shot of the chemical environment that averages out temporal variations. Rather, it incorporates a temporal sensitivity on par with that for color perception," said Dr. ZHOU, the study's corresponding author.

This study was supported by the Ministry of Science and Technology of China, the Chinese Academy of Sciences, the National Natural Science Foundation of China, and the China Postdoctoral Science Foundation.

Figure 1. Airborne chemical compounds are drawn into the nose with each sniff we take. Our olfactory apparatus resolves their fine dynamics within each sniff, forming a temporal code that gives rise to our varying odor perceptions over time. (Image by Mr. WU Yuli & Dr. ZHOU Wen)