Smelling an odor and then, after a delay, recognizing it as the one encountered earlier: performance on this task varied depending on whether people breathed through their nose or mouth during the rest period after learning. In a 2018 study, participants who rested for an hour breathing through their nose distinguished previously learned odors from new ones more accurately than when they breathed through their mouth.
The timing of the breathing manipulation is important. The researchers examined not just the moment when odors were taken in, but the subsequent period of consolidation, when learned information becomes established in memory.
Separating odor learning from the following hour
The main experiment involved 24 healthy men and women aged 19–25. Each person completed two sessions two days apart, breathing through the nose in one and through the mouth in the other. The initial condition was randomly assigned, with the order counterbalanced across participants.
In each session, blindfolded participants smelled and learned 12 odors one at a time. These comprised six familiar odors, such as strawberry, and six unfamiliar ones, such as 1-butanol. Different sets were used on the two experimental days.
Participants then rested quietly, awake, in a chair in another room for an hour. Only during this interval was the breathing route altered, either by taping the mouth closed or applying a nose clip. Participants did not read or talk, and an experimenter remained in the room to monitor the procedure. This was a controlled manipulation for research purposes.
After the rest period, the 12 learned odors were presented together with 12 new ones, and participants indicated whether each had been smelled before or was new. In both conditions, odors were smelled through the nose during learning and the recognition test. The comparison therefore focused on breathing during the intervening rest period.
Distinguishing old and new odors was easier after nasal breathing
The researchers used d′, pronounced d-prime, a measure that combines the rate of correctly identifying learned odors as previously smelled with the rate of incorrectly identifying new odors as previously smelled. Larger values indicate better discrimination between the two; this is not a percentage-correct score.
For familiar odors, mean d′ was 1.63 after nasal breathing and 1.27 after mouth breathing. For unfamiliar odors, the corresponding values were 1.52 and 1.05. Overall performance was higher after nasal breathing, and the effect of breathing condition was statistically significant (p = .002). There was no interaction indicating that the effect differed with odor familiarity.
Both correct responses to learned odors and incorrect responses to new odors contributed to the difference between conditions. By contrast, response bias, such as a tendency to label everything as previously smelled, did not differ significantly. The result can thus be read as a difference in odor-memory discrimination rather than simply a change in answering style.
Checking differences in sniffing and attention
The researchers also recorded nasal airflow during learning and testing. Measures related to inhaled volume, duration, and other sniff characteristics did not differ significantly between conditions. Accounting for odor pleasantness, intensity, familiarity, and ease of naming likewise did not change the overall memory findings.
Another possibility was that maintaining mouth breathing distracted participants. A separate group of 24 people therefore rested for 10 minutes under each breathing condition and then, while continuing that breathing pattern, completed a sustained-response task involving numbers and a task requiring them to name colors without being distracted by word meanings. Neither sustained nor selective attention differed significantly, and Bayesian analyses also supported the absence of a difference.
This check weakens the explanation that attention alone accounted for the memory difference. However, it was not a control that reproduced the full hour of the main experiment. Although discomfort from the devices did not differ significantly in the main experiment, the evidence was insufficient to firmly establish that the two conditions were equivalent. The authors likewise did not claim to have completely ruled out stress during rest or unmeasured aspects of attention.
The mechanism connecting breathing and memory remains a question
The study was motivated by earlier research linking nasal airflow to rhythmic activity in the olfactory bulb, odor-processing regions, and the hippocampus, which is involved in memory. The authors considered the possibility that nasal breathing supports processes that reactivate and consolidate memories during rest.
What this study measured was performance on a memory task, not brain rhythms or memory reactivation itself. Other possible pathways, including mentally imagining odors, remain candidates. Separate testing would be needed to extend an experiment in a small group of young adults resting awake immediately after odor learning to long-term learning outcomes or effects during sleep. The study also does not provide a basis for trying mouth taping at home.
Summary
In the experiment with 24 people, an hour of rest with nasal breathing after learning odors produced better discrimination between learned and new odors than rest with mouth breathing. There were no clear differences in sniffing during learning or testing or in response bias, and separate attention tasks likewise showed no differences between breathing conditions.
This study broadens the time frame over which we consider the nose’s role. Besides serving as the entry point for odors, the nose may contribute to the process of retaining an experience as a memory after smelling has ended. Testing this role through brain activity is a next step.
The research was supported by the Swedish Research Council, the Netherlands Organization for Scientific Research, the Ammodo KNAW Award, and the Knut and Alice Wallenberg Foundation. The authors declared no competing financial interests.
Reference
Arshamian, A., Iravani, B., Majid, A., & Lundström, J. N. (2018). Respiration Modulates Olfactory Memory Consolidation in Humans. The Journal of Neuroscience, 38(48), 10286–10294. doi:10.1523/JNEUROSCI.3360-17.2018 (Open full text)
