EEG aligned to breathing phase tells apart three odors in 17 participants

A team reported in Proceedings of the National Academy of Sciences that odor-evoked brain activity in humans is organized by respiratory phase rather than by absolute time pnas.orgProceedings of the National Academy of Sciencespnas.orgOpen source ↗. The researchers recorded nasal respiration and electroencephalography (EEG) simultaneously in 17 participants. Peppermint, oregano and grapefruit odors were delivered synchronized to each person's breathing cycle.

Phase-locked EEG responses

Odor presentation produced distinct respiratory phase-locked event-related spectral perturbations (RPL-ERSPs) in frontal and temporal regions. They appeared during the first breath after stimulus onset. The most discriminative signatures were theta-band activity during inhalation and alpha-band activity during exhalation. According to the authors, these patterns were obscured in traditional time-locked analyses because breath duration varies naturally.

Leave-one-subject-out cross-validation gave an AUC of 0.78 for peppermint, 0.68 for oregano and 0.61 for grapefruit. The authors conclude that phase-locked features generalize across individuals.

Perception and habituation

To link the neural signals to perception without requiring participants to name the odor, the team collected free-response descriptions and scored them with a semantic-similarity approach. Recognition strength across the odors paralleled the neural decoding hierarchy pnas.orgProceedings of the National Academy of Sciencespnas.orgOpen source ↗.

Decoding performance declined over repeated presentations, which the authors attribute to selective degradation of the most informative phase-locked features. They describe RPL-ERSPs as a sensitive, task-free approach to assessing olfactory function.

The authors also provide an open-source, respiration-synchronized olfactometer design costing under $500, intended to enable replication of the method.