Our brain decides what we're smelling before it knows if we like it

Your brain does not treat every scent in the same way. It first sorts out what the odor is, then works out how pleasant or unpleasant it feels.

Researchers in Tokyo used EEG to track brain activity while volunteers sniffed a panel of everyday smells. In a new study, they found that one brain rhythm helped people tell odors apart, while a slower pattern tracked smell pleasantness.

The work was led by Masako Okamoto, associate professor at the University of Tokyo (UT).

Professor Okamoto’s research focuses on the olfactory system, the network of brain areas that turns airborne chemicals into vivid smell experiences.

How smells are processed in the brain

Every odor you notice begins as a mix of molecules drifting into your nose. Sensory cells detect those molecules and send electrical signals into brain regions that specialize in smell.

Scientists can follow these signals by placing electrodes on the scalp and recording electroencephalography, a technique that measures tiny voltage changes over time.

This lets them see brain rhythms that rise and fall several times per second as people breathe in odors.

One key rhythm for smell is the theta band, a brain wave that cycles about 4 to 7 times per second.

A recent review of human olfactory recordings highlights how these slow waves organize activity across the smell pathway and link breathing to perception.

Tracking early signals

In the new project, volunteers wore a dense cap with many electrodes while they smelled nine different odorants, including food related and floral scents.

The researchers also measured each person’s ability to detect weak odors, tell similar smells apart, and correctly name specific scents.

The team then examined early activity in the theta band recorded over frontal and central scalp regions. They asked whether the detailed pattern of this activity reflected the physical chemistry of each odorant molecule.

Their analyses showed that theta activity from about 80 to 640 milliseconds after odor onset carried information about structural features of the odors.

People whose brains encoded these features more precisely achieved higher scores on a demanding odor discrimination test.

A separate experiment used a two choice task where participants judged which of two odors was present on each trial. On trials with correct answers, decoding based on early theta patterns was more accurate than on trials with wrong answers.

How the brain identifies a scent

This link between early theta coding and behavior fits with earlier work on how odor information changes over time in the brain.

The research suggested that signals in the first few hundred milliseconds mostly represent physical stimulus features rather than subjective qualities.

Other groups have recorded directly from the piriform cortex, a primary smell area tucked near the base of the brain.

In patients with epilepsy, intracranial recordings showed that piriform theta activity could reveal odor identity within half a second after a sniff.

Distinguishing one odor from another

The new findings extend that story by showing similar timing in healthy volunteers, using only scalp electrodes instead of implanted sensors.

The research also links the precision of early theta coding directly to how well a person can distinguish one odor from another in careful tests.

“In the very early stage after odor onset, the brain primarily encodes objective molecular features of odors to support odor discrimination at the behavioral level,” said Professor Okamoto.

Her team also examined how later brain rhythms relate to the emotional tone of each smell.

How the brain judges smell

Later in each trial, a different pattern appeared in much slower brain waves. Several hundred milliseconds after the theta response, activity in a very slow band began to mirror how pleasant or unpleasant participants rated each odor.

This slow wave pattern did not track the physical chemistry of the odorants. Instead, it reflected each participant’s emotional reactions to smells, based on questionnaires about how strongly they notice and enjoy odors.

Other researchers studying odor pleasantness have found broad changes in how frontal brain regions talk to each other during smell.

One EEG experiment reported that pleasant and unpleasant fragrances produce different interaction patterns among regions in the olfactory cortex and nearby emotional areas.

There is also growing evidence that the human olfactory bulb sends quick signals about odor valence, or emotional value, to higher cortical regions.

The results suggest that such deeper signals may feed into slower cortical waves that track how good or bad a smell feels.

Broader implications of the study

Taken together, the results point to a clear division of labor in the smell system. Early theta rhythms emphasize objective molecular information, while later slow waves emphasize each person’s personal feelings about that scent.

That separation could help the brain handle smell under pressure. With rapid access to chemical details, people can quickly decide whether a new scent signals food, safety, or potential danger.

Professor Okamoto and colleagues suggest that these early rhythms could serve as a sensitive measure of smell health.

If early theta coding weakens, it might hint at olfactory loss related to viral infection, head injury, or neurodegenerative disease. The rhythms related to pleasantness might also prove useful in rehabilitation.

Training that pairs scent exposure with paying attention to the emotions those scents evoke may reshape slow-wave brain activity – potentially helping people regain the ability to enjoy smells after illness.

The study is published in The Journal of Neuroscience.

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