The neural cascade of olfactory processing: A combined fMRI–EEG study

•Temporal and spatial neural correlates of olfaction were examined.•A novel MRI-compatible stimulator was created that links olfaction to inspiration.•EEG dipole modeling identified ‘early’ activations in parahippocampus and amygdala.•Both EEG and fMRI identified ‘late’ activations in orbitofrontal association cortex.•EEG may be more sensitive to transient activity in primary olfactory regions.

Olfaction is dependent on respiration for the delivery of odorants to the nasal cavity. Taking advantage of the time-locked nature of inspiration and olfactory processing, electroencephalogram dipole modeling (EEG/DT) has previously been used to identify a cascade of inspiration-triggered neural activity moving from primary limbic olfactory regions to frontal cortical areas during odor perception. In this study, we leverage the spatial resolution of functional magnetic resonance imaging (fMRI) alongside the temporal resolution of EEG to replicate and extend these findings. Brain activation identified by both modalities converged within association regions of the orbitofrontal cortex that were activated from approximately 150–300 ms after inspiration onset. EEG/DT was additionally sensitive to more transient activity in primary olfactory regions, including the parahippocampal gyrus and amygdala, occurring approximately 50 ms post-inspiration. These results provide a partial validation of the spatial profile of the olfactory cascade identified by EEG source modeling, and inform novel future directions in the investigation of human olfaction.