An odor that was clear at first may stop attracting our attention after a while. This weakening of the response to the same odor is known as olfactory habituation. Does it also involve changes in the early stages of the brain’s processing of odors?
In 2020, Kim and colleagues investigated perception and brain electrical activity in young adults after 30 seconds of exposure to the same odor. They focused in particular on very early responses, 40–200 milliseconds into inhalation. They wanted to see whether habituation-related differences appeared before the 200–800-millisecond period commonly examined in earlier research.
Three versions of the preceding odor
The participants were 14 people aged 19–29. Data from 13 were analyzed after one person was excluded because of technical problems. All had been assessed as having olfactory function within the normal range in preliminary testing.
The substances were 1-heptanol and 2-acetylpyrazine. The researchers chose two substances with different chemical structures and odor qualities to minimize the effects of habituation to one carrying over to the other. Participants first smelled a container holding an odor or distilled water as an odorless control for 30 seconds, then immediately smelled a test odor for about two seconds.
There were three conditions: the preceding and test odors were the same; they were different; or the preceding stimulus was odorless and only the test stimulus had an odor. Trials were separated by 30 seconds of rest, and the order of conditions was randomized.
The odor-intensity test, in which participants responded on a nine-point scale, and the EEG test were conducted separately. During EEG measurement, participants smelled from a container for the 30-second habituation period, after which the test odor was delivered through a device and mask. They breathed naturally, and their respiratory cycles were also recorded.
Only the same preceding odor reduced perceived intensity
Intensity ratings were lower when the same odor had been smelled beforehand than after an odorless stimulus or a different odor. Both comparisons showed statistically significant differences. By contrast, there was no clear difference between the odorless and different-odor conditions.
These results suggest that, with these two substances and presentation conditions, the decrease in perceived intensity was associated with repeating the same odor rather than simply smelling something beforehand. They do not mean that habituation never carries over between different odors in other scent combinations.
Early EEG changes were not simply a uniform reduction
EEG recordings from 64 scalp sites were averaged across trials to extract waveforms corresponding to the timing of odor inhalation. These responses are called event-related potentials. The analysis used the transition from exhalation to inhalation as its reference point and examined the most negative and most positive peaks within 40–200 milliseconds.
Differences between conditions were found in amplitude and peak timing at several recording sites. In the same-odor habituation condition, amplitudes were smaller at some sites and larger at others. Peak timing likewise did not change in the same direction everywhere.
For example, at one site near the top of the head, the positive peak occurred at an average of 107 milliseconds after an odorless stimulus and 108 milliseconds after a different odor, compared with 160 milliseconds after the same odor. At this site, the delay after the same odor was statistically significant. This does not mean that habituation uniformly slows responses throughout the brain.
An analysis comparing intensity ratings with EEG features also found associations at several recording sites. The correlation between negative-peak amplitude and intensity reached about 0.49. For positive-peak amplitude, higher intensity was associated with larger responses at some sites and smaller responses at others. The authors also regard differences between left- and right-sided patterns as clues to understanding odor habituation.
Habituation may involve early odor processing
An important feature of this study is that habituation-related EEG differences were observed before 200 milliseconds. The authors interpret this as suggesting that olfactory processing in the brain may be modulated from an early stage of receiving odor information.
Drawing on earlier work on regions such as the piriform and orbitofrontal cortices, the paper discusses whether these areas may be involved in the early changes. However, the present scalp EEG did not directly identify those regions as the sources of the changes. Nor do correlations with intensity ratings prove that a particular EEG change caused the odor to feel weaker.
The 40–200-millisecond window also refers to the response during a test conducted after 30 seconds of habituation. It does not mean that habituation is complete 0.2 seconds after smelling begins. This was not a study tracking EEG changes throughout the 30-second habituation period.
Attention, breathing, and changes in the nose remain unresolved
The authors note that paying less attention to the same odor and more attention to a new odor may also have influenced the results. Allowing natural breathing during measurement, and processing whether the preceding and subsequent odors were the same or different, may help explain differences from earlier studies.
At some sites, EEG responses differed between the different-odor and odorless conditions even though intensity ratings did not clearly differ. This means that carryover effects between the two substances cannot be considered completely eliminated. The study also did not separate changes in the responses of olfactory sensory neurons in the nose from changes within the brain.
The paper acknowledges that noise was greater relative to the signal than in earlier studies. This was a small experiment with 13 participants and two substances, examining many recording sites. Which changes occur at which sites needs further confirmation. The study did not examine perfume longevity or ways of recovering from habituation.
Summary
After smelling the same odor for 30 seconds, participants perceived the next odor as weaker, and early EEG responses during inhalation differed between conditions. These changes were not simply smaller responses: they varied in amplitude, timing, and recording site.
The authors’ findings suggest that understanding odor habituation warrants attention not only to later responses but also to processing before 200 milliseconds. When and how is our response to an odor adjusted? These results offer clues for investigating its timing.
References
Kim, K., Bae, J., Jin, Y., & Moon, C. (2020). Odor habituation can modulate very early olfactory event-related potential. Scientific Reports, 10, 18117. DOI: 10.1038/s41598-020-75263-7 / Full original article (PDF). Original article: © The Author(s) 2020, CC BY 4.0.
