Introduction: Increasing evidence points out to a fundamental role of nasal breathing in higher-order cognitive functions in humans. We herein investigate psychophysiological effects and state of consciousness alterations induced by active mechanical stimulation of the olfactory epithelium during slow nasal breathing. Methods: Sixteen healthy meditation experts (mean of 1700 hours of practice) performed 15 min of Samavritti Pranayama at 2.5 b/min through the nose (Slow Nasal Breathing – SNB). Similar frequency breathing performed through the mouth (Slow Mouth Breathing – SMB) was the control condition. Respiratory frequency, heart rate variability and high-density EEG, as well as psychometric tests assessing 1) state of consciousness, 2) level of anxiety, and 3) perceived emotions, were acquired both before (baseline) and after the experimental sessions. Putative effects of SNB were assessed by comparing values related to the post-SNB with those collected during the baseline period (paired t-test). Effects of the olfactory epithelium active stimulation were assessed by comparing, for each variable of interest, values related to the post-SNB period to those collected during the post-SMB period (independent samples t-test). Results: Compared to baseline, SNB elicited a widespread increase of theta and alpha EEG bands power densities, and more localized increases in gamma, beta, and delta bands. Increases in EEG connectivity were detected in theta, delta, and beta broadly over the scalp, whereas decreases of connectivity were detected in gamma band. SNB elicited an altered state of consciousness characterized by: 1) an overall perception of altered experience, 2) distortion of time duration, 3) reduced anxiety, and 4) increased joy, comfort, and somatic awareness. Compared to post-SMB, the post-SNB period shows increases in delta, theta, and beta power in prefrontal areas, paralleled by increases of connectivity in midline areas when considering theta band. Widespread decreases in connectivity were found when considering delta, alpha, and gamma bands. Conclusions: The results herein presented shed further light on the psychophysiological effects of breathing techniques and point to a fundamental role of olfactory epithelium stimulation in inducing altered states of consciousness during slow breathing.
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