Summary: New research reveals that the human brain separates the objective identity of an odor from its subjective emotional quality, processing each at different times. Shortly after an odor is inhaled, fast neural activity encodes molecular features that support precise odor discrimination. Only later does a distinct neural signal reflect how pleasant or unpleasant the scent feels to the individual.
These findings describe a temporal hierarchy in olfactory processing: an early stage that captures low-level chemical properties of odorants and a later stage that represents subjective affective evaluations. The temporal separation of these signals offers potential for new diagnostic markers and therapeutic approaches for olfactory disorders and may inform how smell influences behavior and memory.
Key Facts:
- Two-stage processing: Early neural activity decodes odor identity and physical features; later activity encodes pleasantness and affective value.
- Behavioral relevance: Stronger and clearer early signals predict better odor discrimination accuracy on behavioral tests.
- Clinical potential: Distinct frequency bands linked to different olfactory functions might become biomarkers for diagnosing or tracking treatment of smell impairments.
Source: SfN
In a new Journal of Neuroscience paper, Masako Okamoto and colleagues from the University of Tokyo examined the timing and functional roles of brain activity during human odor perception.
The team measured electroencephalography (EEG) while volunteers inhaled a varied panel of odors. In addition to brain recordings, participants completed standardized tests that measured odor detection, discrimination, and identification, and they filled out questionnaires about their everyday olfactory experiences and affective reactivity.
Using time- and frequency-resolved decoding and representational similarity analysis, the researchers identified two separable patterns of neural activity. Early theta-band activity—beginning roughly 80 milliseconds after odor onset and peaking near 370 milliseconds—carried information about low-level physicochemical properties of odor molecules. This early signal was objective and closely tied to the accuracy with which participants could discriminate similar odors.
In contrast, delta-band activity associated with perceived pleasantness emerged later, from about 720 milliseconds onward. The fidelity of this later signal correlated with participants’ self-reported sensitivity to odor pleasantness in daily life rather than with discrimination performance. In other words, early theta-band coding supports precise perceptual discrimination, while later delta-band activity reflects subjective affective responses.
The study further showed that in a separate odor discrimination task, trials that participants answered correctly exhibited higher early-theta decoding accuracy than incorrect trials. This trial-by-trial relationship underscores the functional importance of the early theta signal for real-time odor-guided behavior.
Okamoto summarizes the findings: in the immediate period after an odor appears, the brain primarily represents objective molecular features to support discrimination, and only afterward begins to encode subjective perceptual attributes like pleasantness. This temporal ordering suggests distinct stages of processing that serve different behavioral and perceptual goals.
Because the two signals are separable in time and frequency, they may provide measurable neural markers for assessing olfactory function. Such markers could help identify specific deficits in olfactory processing—distinguishing problems with basic odor detection and discrimination from altered affective responses to smells—and could guide targeted therapies or training programs aimed at restoring or enhancing olfactory abilities.
Key Questions Answered:
A: It generates an early, objective neural signal—primarily in the theta frequency band—that encodes low-level molecular features and supports odor detection and discrimination.
A: Pleasantness is represented later, in a different frequency range (delta band), reflecting subjective affective evaluation rather than immediate perceptual discrimination.
A: Identifying distinct temporal and frequency-specific neural signatures offers potential biomarkers for diagnosing different types of olfactory dysfunction and for monitoring responses to interventions.
Editorial Notes:
– This article was written by a Neuroscience News editor.
– The Journal of Neuroscience paper was reviewed in full.
– Additional context was provided by editorial staff.
About this olfaction and sensory neuroscience research news
Author: SfN Media
Source: SfN
Contact: SfN Media – SfN
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Original Research: Closed access.
“Behavioral Relevance of Early Neural Coding of Low-Level Odor Features in Humans” by Masako Okamoto et al., Journal of Neuroscience.
Abstract
Behavioral Relevance of Early Neural Coding of Low-Level Odor Features in Humans
Understanding how the brain transforms peripheral sensory inputs into higher-level representations, and how these representations contribute to perception and behavior, is a central question in sensory neuroscience. In human olfaction, however, the temporal progression of neural odor codes and their behavioral significance have been incompletely characterized, particularly at very early stages.
To determine which odor features define early neural responses and how those responses relate to olfactory function, the researchers recorded EEG from male and female participants as they inhaled a diverse set of odorants. The same participants completed standardized tests of odor detection, discrimination, and identification, in addition to questionnaires assessing odor-related affective reactivity and awareness.
Time- and frequency-resolved decoding and representational similarity analysis revealed that early theta activity (beginning around 80 ms and peaking near 370 ms) encodes low-level physicochemical properties of odor molecules. Importantly, the fidelity of this early-theta coding correlated selectively with individual differences in odor discrimination ability, but not with other olfactory measures.
By contrast, delta-band representations linked to pleasantness emerged later (from approximately 720 ms) and were associated with trait-level affective responses to odors as measured by questionnaires. Follow-up EEG recordings during an active discrimination task showed higher early-theta decoding accuracy on correct trials compared with incorrect ones, indicating that theta-band coding tracks trial-by-trial perceptual performance.
Overall, these results demonstrate that early theta-band representations of low-level odor features are functionally relevant to odor-guided behavior and that later delta-band activity reflects subjective affective evaluation. This temporal dissociation highlights distinct roles for neural activity at different frequencies in human olfactory processing.