How Breathing Tunes Cortical Dynamics: New Vistas on the Role of Olfactory Bulb in the Modulation of Consciousness, Emotions, and Cognitive Functions

Breathing, beyond its vital respiratory function, exerts a pervasive influence on brain activity. Respiration-related oscillations are rhythmic brain activities phase-locked with the respiratory cycle that originate from the mechanical activation of olfactory sensory neurons and propagate through the olfactory bulb to distributed cortical regions. These oscillations entrain large-scale neuronal networks suggesting that the olfactory bulb, via nasal breathing, may act as a global integrator of neural activity. The present thesis investigates this mechanism combining electrophysiological and psychometric measures. Using high-density EEG, slow odorless air stimulation selectively targeting olfactory epithelium versus trigeminal nerve was applied to fifteen healthy participants. Since the nasal cavities are also innervated by the trigeminal nerve, it was necessary to clarify its potential contribution to the generation of respiration-related oscillations and thus disentangle whether these oscillations originate specifically from the activation of olfactory sensory neurons. Results show that olfactory, but not trigeminal, stimulation induced widespread changes in cortical oscillatory activity and functional connectivity, indicating a respiration-driven reorganization of large-scale brain dynamics. These neurophysiological changes were accompanied by reduced anxiety and the experience of a non-ordinary state of consciousness. Complementary systematic and clinical studies demonstrated that impaired nasal airflow, as in medical condition characterized by nasal obstructions, is associated with cognitive deficits, mood disturbances, and poor sleep quality, suggesting that when nasal airlow stimulation cannot occur, the respiration-related oscillations may be disrupted. Future investigations should aim to elucidate how respiration-related oscillations interact with large-scale cortical networks across physiological and pathological conditions, and whether olfactory epithelium stimulation can be applied as a non-invasive neuromodulatory tool to restore cortical dynamics in clinical populations.