Anxiety disorders are humanity's most common mental disorders, affecting some 359 million people globally. Controlled breathing can help many people regulate their anxiety, as research has shown, although it remains unclear exactly how this works.In a new mouse study published in PNAS, researchers identify a novel "nose-to-brain circuit" that may explain one way that nasal breathing can influence anxiety.Humans have long appreciated the emotional effects of breathing, as manifested in deep-rooted cultural traditions such as pranayama, Taoist breathing practices, and various forms of meditation.A group practices nadi shodhana pranayama (alternate nostril breathing) on the International Day of Yoga in Kolkata, India, in 2017. (Biswarup Ganguly/Wikimedia Commons/CC BY 3.0)Modern science also increasingly supports the link between breathing patterns and brain regulation, with new evidence suggesting neural patterns respond even to subtle variations from one breath to the next.Studies have shown that slow, regular breathing can have "profound modulatory effects on emotional states" and is clinically effective against anxiety disorders, the authors of the new study write.We know our automatic breathing rhythm begins in the brainstem, and that our deliberate modulation of that rhythm employs a frontal cortico-brainstem 'top-down' pathway, so called because it lets us consciously override an autonomic process like breathing.We don't know much, however, about the specific neural mechanisms that allow either type of breathing to influence anxiety. That's where the new study comes in, seeking to help unlock the full potential for breathing exercises as a simple, noninvasive anxiety treatment.Previous research offered clues. Breathing can affect mouse emotions regardless of how it originates, including via top-down circuit manipulation (to mimic conscious breathing) and brainstem manipulation."This implies the existence of a nonconscious body-brain interaction for breathing-emotion regulation, in addition to volitional control," the authors write.Slow nasal breathing seems to tame anxiety more effectively than slow mouth breathing, hinting at something unique about the nose in emotional modulation. Nasal breathing rhythm also has been shown to synchronize brain oscillations in both humans and rodents.Inside the nasal cavity, special cells called olfactory sensory neurons (OSNs) may play a key role. While OSNs detect odorants and send resulting signals to the brain, research shows they're also mechanosensitive to airflow inside the nose.