Does an Odor Leave an Aftereffect in the Brain? Clues from 90 Seconds of Rest

Even after leaving a scented space, an impression can seem to linger. Does that aftereffect also appear in brain activity?

A 2020 study examined the resting brain after exposure to pleasant and unpleasant odors. Differences in the coordination of activity between brain regions were found during the 90 seconds after odor presentation ended. Rather than looking only at the moment of smelling, the researchers investigated the state that followed it.

Examining the brain after an odor stops

The brain remains active even when we are not performing a task. During rest, networks such as the default mode network show coordinated activity across several regions. The study began by asking whether a preceding experience could influence this resting activity.

Heather Carlson and colleagues at the University of Geneva investigated whether emotionally pleasant or unpleasant odor experiences leave traces in subsequent resting brain activity, and whether those traces differ with pleasantness. They used fMRI to record both odor exposure and the rest periods afterward.

The central measure was functional connectivity: how closely signals from separate brain regions fluctuate together over time. It does not directly show that new neural wiring has formed or that information has flowed from one region to another.

Twenty participants experienced a pleasant-odor day and an unpleasant-odor day

The participants were 20 students aged 18–35, ten women and ten men. Each took part on two separate days, experiencing pleasant odors on one and unpleasant odors on the other. The order of the days was counterbalanced across participants.

There were five pleasant stimuli, including lavender, strawberry, and caramel, and five unpleasant stimuli. A separate group of ten people had rated them in advance to select odors with relatively consistent pleasant or unpleasant evaluations. An odorless-air condition was also included.

Each day comprised four runs: odorless, odor, odor, and odorless. In the odor runs, each stimulus was presented three times for 15 seconds, giving 45 seconds in total. Brief interruptions allowed intensity ratings, followed by final pleasantness and intensity ratings. Participants then rested for 90 seconds with their eyes closed, letting their thoughts wander freely.

After half of the rest periods, participants were asked what they had been thinking about. Options included odor-related matters, their own lives, the surrounding environment such as MRI noise, and fighting sleepiness. They also completed the PANAS-X mood questionnaire before and after each day’s experiment.

Mood and thoughts produced distinct findings

Participants rated the selected pleasant odors as pleasant and the unpleasant odors as unpleasant. The two odor groups did not differ significantly in intensity, and pleasantness ratings of the odorless condition did not differ substantially between days.

Positive-mood scores fell after the experiment on both days, with a larger decline on the unpleasant-odor day. Mean changes were −5.6 points on the unpleasant day and −3.1 on the pleasant day, a significant difference in the authors’ one-tailed test. No significant difference was found in negative-mood scores.

The broad categories of thoughts reported during rest did not vary significantly with odor condition. The findings were therefore not simply that pleasant scents improved mood or that everyone recalled unpleasant memories after an unpleasant odor.

Coordination of activity changed after smelling ended

The imaging analysis selected 27 regions associated with olfaction, emotion, memory, resting activity, and related functions. The researchers compared resting functional connectivity after odors with that after odorless air. Slow signal fluctuations were divided into two frequency bands and analyzed separately.

After unpleasant odors, connectivity increased between the precuneus, associated with resting-state networks, and regions involved in olfaction and emotion, among others. In the faster band, increases included precuneus–medial orbital gyrus and insula–amygdala connections. In the slower band, the precuneus also showed stronger coordination with several regions involved in olfactory, sensory, and behavioral regulation processes.

After pleasant odors, connectivity decreased in the faster band between the hippocampus and insula, and between the hippocampus and piriform cortex, among other pairs. In the slower band, some connections increased, including links between the precuneus or thalamus and frontal regions.

In other words, pleasant odors did not uniformly increase coordination throughout the brain while unpleasant odors decreased it. The direction of change depended on the regions paired and the frequency band examined.

Directly comparing pleasant- and unpleasant-odor rest periods revealed differences in the faster band, but none exceeding the study’s statistical threshold in the slower band. Finding different patterns relative to odorless air and establishing differences in a direct pleasant-versus-unpleasant comparison are distinct results.

Classifying pleasant and unpleasant aftereffects from connectivity patterns

The researchers also tested machine learning to combine multiple connectivity values and classify rest periods as following pleasant or unpleasant odors. They used random forests, a method that combines many branching decision rules.

Internal evaluation within the study dataset yielded a classification error of approximately 14%. Four of the five most important connections linked the left insula with regions including the prefrontal cortex, areas around the hippocampus, and the amygdala.

This suggests that post-odor coordination of activity may contain information distinguishing the pleasant and unpleasant conditions. It was not external validation in a new population, nor did it establish a technology for reading a person’s mood or thoughts.

Changes in the brain and the aftereffects people feel

The authors discuss the increased connectivity involving memory and behavioral-regulation regions after unpleasant odors in relation to processes such as learning what to avoid. They also suggest that changes after pleasant odors may involve flexibility in directing attention.

However, the study did not directly link each brain change to a specific memory, avoidance behavior, or mood change. Mood and thoughts were measured, but their level of detail and timing do not determine the psychological meaning of every neural change.

The study analyzed 90-second post-odor periods in 20 students. It did not test whether effects last for hours or improve everyday concentration, sleep, or health. Further work would need to examine individual preferences, odor familiarity, a wider range of stimuli, and the duration of these aftereffects.

Summary

Pleasant and unpleasant odors left different patterns of coordinated brain activity during a brief rest period after presentation ended. Connections involving the precuneus and insula were particularly relevant, and exploratory classification was possible using combinations of these connections.

Meanwhile, the mood questionnaire showed a larger decline in positive mood on the unpleasant-odor day, but no clear condition differences in negative mood or broad categories of thoughts. Neural aftereffects and experiences people describe in words are not the same measure. Studying scent beyond the moment of smelling is a step toward understanding their relationship.

The research received funding from Firmenich’s EMODOR project, and Firmenich supplied the stimuli. A co-author affiliated with the company also participated.

References

• Carlson, H., Leitão, J., Delplanque, S., Cayeux, I., Sander, D., & Vuilleumier, P. (2020). Sustained effects of pleasant and unpleasant smells on resting state brain activity. Cortex, 132, 386–403. DOI and original article. University-hosted full text.