Why Do We Quickly Draw Back from an Unpleasant Smell?

When an unpleasant smell makes you instinctively draw back slightly, the olfactory bulb, a brain region that first receives information from the nose, may be involved in that rapid response.

A 2021 study found early olfactory-bulb activity in response to unpleasant odors, along with differences in movement-related brain activity. In a separate experiment, it also found that the body moved slightly backward about half a second after smelling an unpleasant odor. Pleasant odors did not clearly produce a corresponding forward movement.

Where and when does the brain process an odor’s pleasantness?

Whether an odor is pleasant or unpleasant matters when deciding to approach it or move away. Human research on pleasantness has often focused on brain regions involved in relatively late processing, leaving the olfactory bulb’s early contribution less well understood.

The study aimed to examine how olfactory-bulb activity relates to subjective pleasantness and unpleasantness, and whether the timing of that processing corresponds to avoidance movements. It combined an experiment measuring brain activity with another measuring body movement while standing.

Estimating olfactory-bulb activity from 19 people’s foreheads and scalps

The first experiment involved 19 healthy adults. Six odorants, linalool, ethyl butyrate, 2-phenylethanol, 1-octen-3-ol, octanoic acid, and diethyl disulfide, were repeatedly presented in varying order. Participants attended three sessions on separate days and rated intensity and pleasantness after each presentation.

A device delivered the odors to both nostrils, with delivery beginning in coordination with inhalation. The researchers also measured the delay before the odor reached the nose and corrected the analysis timing accordingly.

Recordings used four electrodes above the eyebrows and 64 scalp electrodes. The signals and a head model were used to extract activity estimated to originate in the olfactory bulb, a method called an electrobulbogram. Unlike inserting electrodes directly into the bulb, this method includes a source-estimation step.

The analysis compared how similar odors were in their pleasantness ratings with how similar they were in their brain-activity patterns. It adjusted for odor intensity and used each participant’s actual ratings rather than relying only on predetermined pleasant and unpleasant categories.

Both rapid and somewhat later processing appeared

The brain’s electrical activity contains rhythms at different frequencies. This study focused on two bands, gamma and beta. The researchers observed coupling between the rhythms from about 250 milliseconds onward, and used these frequencies to investigate relationships with pleasantness.

Activity patterns corresponded to odor pleasantness in the gamma band at about 250–325 milliseconds after odor arrival and in the beta band at around 800 milliseconds. With 1,000 milliseconds making one second, these changes all occurred within the first sniff.

In the later beta-band activity, odors were grouped more coherently according to pleasantness, showing a more detailed correspondence with the ratings participants ultimately reported. The authors proposed a staged process involving rapid initial processing followed by later processing that incorporates information such as experience. However, this analysis could not establish which brain regions might have supplied returning signals, or the direction of information flow.

Unpleasant odors showed an even earlier activity difference

The researchers next compared each person’s two most pleasant and two most unpleasant odors. Unpleasant odors produced greater beta-band activity than pleasant odors in an early window of approximately 50–200 milliseconds. In a later window, around 690–780 milliseconds, the response was greater for pleasant odors.

The team considered whether this early response to unpleasant odors might relate to preparing a withdrawal movement. Examination of movement-related regions revealed differences between unpleasant and pleasant odors at around 300–400 milliseconds.

In other words, pleasantness-related information did not appear only at a later stage; it was already reflected in early olfactory-bulb processing. The timing of its relationship with movement-related activity then provided the basis for the next experiment.

A slight backward movement after about half a second

Body responses were measured using a floor-mounted force plate. Participants stood with their feet together, looked at a marker at eye level, and tried to avoid unnecessary movement. The device can record small forward and backward body sways that are difficult to see with the naked eye.

A pilot experiment with 21 people first examined several time points from 0.25 to 1.25 seconds after odor presentation. The researchers selected 0.5 seconds for the main analysis. They then preregistered the hypothesis and analysis and tested a new group of 44 people.

The main experiment used vanillin, carvone, a fish odor, and diethyl disulfide. Some odors were changed from the pilot study to help assess whether the result depended only on particular stimuli.

In the 44-person experiment, body movement at about 0.5 seconds again differed between pleasant and unpleasant odors. Unpleasant odors produced a significant backward movement relative to the neutral standing position. Pleasant odors did not produce significant forward movement. Approach and withdrawal therefore do not seem to occur symmetrically at the same speed for every pleasant or unpleasant odor.

The researchers also examined whether breathing differences alone could explain the finding, but found no association between body movement and respiratory airflow at that time point. The olfactory-bulb analysis likewise found no relationship between activity and the respiratory measures examined, addressing the possibility that the results simply reflected differences in sniffing.

Reading the findings as evidence relevant to rapid avoidance

Participants in the brain-activity experiment sat still, whereas those in the body-movement experiment stood in a separate study. The experiments therefore did not directly record a causal chain in which one person’s olfactory-bulb signal produced that same person’s withdrawal. The two findings are related through their compatible timing.

The study also concerned the first sniff of an unexpectedly presented odor. Whether the same processing occurs after repeated smelling, or under different conditions such as eating or hunger, remains to be investigated. Subjective unpleasantness also needs to be distinguished from actual danger. Because the moment of conscious awareness itself was not measured, the study cannot establish that movement necessarily precedes awareness.

Summary

Olfactory-bulb activity corresponded to pleasantness and unpleasantness early after odor arrival, with unpleasant odors producing greater early beta activity than pleasant odors. Later activity showed a more detailed correspondence with the final pleasantness ratings.

In a separate preregistered experiment, the body moved slightly backward about half a second after an unpleasant odor, whereas pleasant odors produced no clear forward movement. The study suggests that odor evaluation and rapid withdrawal may interact from an early stage of olfactory processing.

The work was supported by the US NIDCD, the Knut and Alice Wallenberg Foundation, and the Swedish Research Council. The authors declared no conflicts of interest.

Reference

Iravani B, Schaefer M, Wilson DA, Arshamian A, Lundström JN. (2021). The human olfactory bulb processes odor valence representation and cues motor avoidance behavior. PNAS, 118(42), e2101209118. Original paper.