Brain Activity in Theta Range Decodes Objective Scent Characteristics, Study Finds

A groundbreaking study published in The Journal of Neuroscience in November 2025 has illuminated the distinct temporal patterns in brain activity that separate the objective processing of olfactory information from subjective emotional responses. This significant finding holds substantial promise for developing objective diagnostic and therapeutic approaches for olfactory disorders.

Led by Masako Okamoto of the University of Tokyo, the research team utilized high-density electroencephalography (EEG) to monitor the electrical activity in volunteers' brains. Participants engaged in tasks requiring odor detection, discrimination, and identification. The central discovery revolves around brain activity within the theta frequency band, specifically around 4 Hz. This activity, initiating just 80 milliseconds after inhalation and peaking around 370 milliseconds, appears to encode the fundamental physical and chemical properties of odorant molecules.

Professor Okamoto noted that during this initial phase, the brain is primarily focused on mapping objective molecular characteristics to facilitate behavioral discrimination of smells. The accuracy with which researchers could decode information based on this early theta signaling directly correlated with each participant's individual proficiency in distinguishing various aromas. Furthermore, decoding accuracy was notably higher in trials where participants correctly identified the odors, underscoring the critical role this initial neural signal plays in olfactory behavior.

In contrast, activity in the slower delta frequency range, approximately 1–3 Hz, emerged later in the process, beginning around 720 milliseconds post-inhalation. This later delta activity was exclusively linked to the subjective experience of odor pleasantness. Significantly, this slower delta activity showed no correlation with an individual's skill in objective odor discrimination but was associated with stronger emotional reactions to scents in daily life.

The research team, which included Mugihiro Kato and Kazushige Tohara, employed a specially engineered apparatus for the precise delivery of ten distinct odors. This setup allowed for the capture of high temporal resolution data detailing when and where these odors were processed within the brain. These differentiated patterns of neural activity represent a crucial juncture in understanding how the brain translates chemical input into conscious sensory and emotional experiences. The value of this discovery lies in the potential to create objective assessment tools for olfactory impairments, moving beyond reliance on subjective self-reports.

Currently, the application of EEG for objective odor assessment remains more common in scientific research settings than in routine clinical practice, largely due to the associated high costs.

In essence, this early theta-band encoding establishes the neural groundwork for scent-based behavior by effectively separating the physicochemical reality of a molecule from its affective evaluation. This distinction provides a clear roadmap for future neurological investigations into the sense of smell.