I was asked a great question recently about how different nostrils affect different parts of the brain given all the air just ends up meeting in the same place. I’ve collated this info to explain more for anyone who likes to geek out on such stuff 🤓
There is about four decades worth of research looking at yogic alternate and unilateral breathing techniques, e.g:
A lot of the research on alternate nostril breathing is limited by the choice of comparison. Ideally studies would have compared alternate nostril breathing with other breathing options matched for respiratory rate, ratio and nasal breathing, and would have accounted for nostril patency (openness) in their analysis. Few have done this. The unilateral breathing research is a little more robust.
Recently the AUT Bio Design Lab using a type of continuous airway pressure (CPAP) machine to control breathing to examine what happens in the brain, or at least the cortex by using electroencephalography (EEG): “EEG signatures change during unilateral Yogi nasal breathing”
The same team recently repeated this study using fMRI, but the data are yet to be analysed. The affects observed were robust in terms of each condition leading to clear differences in EEG signal. What these shifts ultimately mean is still open to interpretation.
In terms of using specific breathing techniques with patients/clients, as with any technique, responses will vary, and no work appears to have been done to assess who might respond well and who wouldn’t or why. My own experience is that while alternate nostril breathing is focusing and calming for the majority, trying to breathe through a very obstructed nostril can increase agitation, so the technique is best preceded by a nose unblocking exercise.
Although the mechanisms remain unclear, while we have only one pharynx and trachea, we have two nostrils, two sets of sinuses, and two sets of turbinates that undergo engorgement through vasodilation and deflate through vasoconstriction and thereby determine which nostril is more open. This leads to the nasal cycle:
Here is a great Virtual Reality-style video looking at the upper airways:
One possible mechanism by which unilateral and alternate nostril breathing affect the brain, is by influencing the release of the gas nitric oxide from the sinuses another might be how it influences the nasal cycle (this article is only normal nasal cycles without application of specific breathing techniques).
Just as we can voluntary control the rate and depth, and biomechanics of our breathing, and subsequently influence some of our biochemistry, e.g. blood pH, carbon dioxide partial pressure, and oxygen saturation, we can influence our nasal cycle by unilateral breathing and/or applying pressure to the fifth intercostal space near the armpit: Article 1, Article 2
Another mechanism that may link nasal patency and the brain is via the trigeminal nerves influenced by carbon dioxide or perhaps stimulation of the olfactory nerves (this article covers the topic well but is a little dated).
It’s safe to say that a lot more research needs to be done to be confident about any mechanism, or interaction of mechanisms, but the weight of the evidence is on the side of brain activity influencing and being influenced by the nasal cycle and nostril patency.