Pleasant smells: a privileged gateway to soothing autonomic responses and improving brain-body rhythm coupling

Aromatherapy commonly uses odors to improve well-being through their evocation of positive emotions. Although knowledge in this area is often very empirical, the olfactory stimulus has different properties which, taken together, could explain why it can relax. First, olfactory sense have a direct access to the limbic system, without thalamic relay processing, which confers it a strong emotional valence. Second, when appreciated, odors can slow down breathing and cardiac rates. Third, when slow and deep, breathing can entrain brain activity, due to the mechano-sensitivity of olfactory receptors to airflows. We hypothesized that, thanks to these properties, pleasant odors could enhance the subjective feeling of relaxation, slow down body rhythms, and facilitate entrainment of brain activity by respiration. Comparing the effects of a personally pleasant odor to a personally pleasant music on psychological, physiological and neuronal responses, we showed a tendency for both odors and music to enhance subjective relaxation. However, only pleasant odors were able to 1) decrease heart rate while increasing its variability, and 2) decrease respiratory rate while enhancing the respiratory drive of brain activities, regardless of the music tempo. Overall, we demonstrated that the positive emotion evoked by a personally pleasant smell is sufficient to evoke an olfactomotor response, which, by slowing breathing, synchronizes respiration, fluctuations of heart rate and brain activity.