Does the air quality in your area make you a 'mouth breather'?

With Expert Insight From: Samuel Hunley, Ph.D.

If the air quality in your area or your activities make you into a "mouth breather", be careful - that may impact brain activity and decrease cognition.

As athletes are well aware, how we breathe affects every aspect of our physical performance. A runner in Los Angeles knows that his or her times will be significantly slower on days when smog warnings have been issued. A football team traveling to play the Denver Broncos has to take extra precautions to prepare for playing in the thin air of the Mile High City. An allergy sufferer in Atlanta needs to stock up on antihistamines before a spring tennis match if they expect to survive the deluge of pollen that afflicts the city. Simply put, if you can't breathe normally, you won't perform at your best.

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But do our cognitive processes change in accordance with our breathing? A growing body of research suggests that this may indeed be the case. In a recent study, Dr. Christina Zelano and her colleagues at Northwestern University found that, even under normal circumstances, our patterns of neural activation shift in sync with our breathing1. In fact, this shifting neural activation is actually associated with distinct behavioral changes as well.

Breathing in the Brain

Work with hedgehogs and rats has long shown that neural activation in brain regions associated with olfaction (i.e., the sense of smell) syncs up with breathing2-3 but only when the breathing occurs through the nose4. This relationship makes sense. Breathing through the nose necessarily involves the sense of smell. Consequently, brain regions associated with smell should activate when an animal is breathing in through the nose. Such coordinated activity could play an important role in processing olfactory information5-6. Crucially, evidence suggests that this synced activation extends beyond olfactory regions of the brain. For instance, the hippocampus, a brain region heavily involved in memory and emotional processing7-8 also seems to activate in sync with nasal breathing9-10. Given this region's important role in cognition, it could be that these patterns of activation are associated with distinct behavioral differences.

With these findings in mind, Dr. Zelano and her colleagues investigated this phenomenon in humans1. Specifically, they examined whether this breathing-synced activation occurred in humans and whether such patterns involved non-olfactory regions, specifically the amygdala and hippocampus, both important in memory and emotional processing7-8. They also examined whether breathing patterns were associated with differences in performance. They wanted to know: do cognitive abilities, particularly as they relate to emotional processing and memory, shift in accordance with this changing neural activation?

Epilepsy Patients Help Shed Light on Neural Effects

To study such specific patterns of activation, you need an extreme level of precision that is not available with popular neuroimaging paradigms such as functional magnetic resonance imaging (fMRI) and electroencephalography (EEG). Instead, researchers must record activation directly from the brain using a technique known as intracranial EEG (iEEG). Whereas typical EEG involves recording from electrodes placed comfortably on the scalp, iEEG requires that electrodes are implanted directly on the brain. Consequently, this technique can be used with humans only under extreme circumstances.

For their study, Dr. Zelano and her colleagues were able to recruit severe epilepsy patients who were undergoing procedures to locate the source of their seizures1. These patients have electrodes implanted in their brain in order to monitor for seizure-related activity. Thus, the researchers were able to find a subset of these patients who were willing to also participate in their study and had the electrodes placed near the amygdala and hippocampus. In a session during the patients' hospital stay, the researchers asked them to breathe either exclusively through the nose or the mouth and then simply monitored their brain activity. They found that, much like in rats, patterns of neural activation in human olfactory regions appeared to sync-up with normal breathing patterns but only when the patients breathed through their nose. Furthermore, they found that this synced activity was not limited to olfactory regions. Instead, they found that activation in the amygdala and hippocampus also appeared to sync with breathing but, again, only when patients were breathing in through their nose.

Why might this be the case, though? One possible explanation is that activation from olfactory regions propagates, or spreads, to the amygdala and hippocampus. These three regions are already tightly connected10-11. Thus, when breathing activates olfactory regions, this activation could spread through these connections, causing increased activation in these non-olfactory regions.

Breathing Changes Cognition

The researchers next investigated how these patterns of activation relate to behavior. To answer this question, the researchers recruited healthy participants to complete behavioral tasks while the researchers monitored their breathing. In one task, participants were shown pictures of fearful or surprised faces, and they were asked to indicate which emotion was depicted. One set of participants were asked to breathe through only their nose while the other set breathed through only their mouth. The researchers found that participants were slightly faster at recognizing fearful faces when they were inhaling versus when they were exhaling but only when breathing through the nose. In another task, participants were presented with pictures of objects (e.g., buildings, fruits, musical instruments) and asked to remember them. Again, one set of participants breathed through only their nose while the other set breathed through only their mouth. After a brief delay, participants were presented with the old images as well as new images and asked to indicate which objects they had seen before. The researchers found that participants were slightly better at remembering images when they had first seen those images while inhaling versus when they were exhaling but, again, only when they were breathing through the nose. The researchers posit that both of these effects could be due to the enhanced activation in the amygdala and hippocampus when inhaling through the nose.

Important Implications for Our Understanding of Brain and Behavior

While these results are intriguing, what do they mean? First and foremost, these results do not suggest that you should breathe in every time you want to enhance emotional processing or remember something better. The effects reported here were meaningful, but small, and found under very specific, passive breathing conditions. It is unlikely that forcing yourself to breathe in when trying to remember an important phone number or date will have any meaningful effect on memory.

These findings are important because they give us significant insight into how the brain works and how seemingly unconnected behaviors (e.g., breathing and memory) can be related. Such work could also shed light on other important findings. For instance, we know that human breathing patterns shift in response to emotional imagery and fear-inducing circumstances12. It could be that this shift in breathing serves to enhance emotional processing and memory under strenuous circumstances. Much more work is needed to examine this possibility. The current study highlights, though, the way we breathe may play an important role in how we think.

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1. Zelano, C., Jiang, H., Zhou, G., Arora, N., Schuele, S., Rosenow, J., & Gottfried, J. A. (2016). Nasal Respiration Entrains Human Limbic Oscillations and Modulates Cognitive Function. Journal of Neuroscience, 36, 12448-12467.

2. Adrian, E. D. (1942). Olfactory reactions in the brain of the hedgehog. The Journal of physiology, 100, 459.

3. Kay, L. M., & Freeman, W. J. (1998). Bidirectional processing in the olfactory-limbic axis during olfactory behavior. Behavioral neuroscience, 112, 541.

4. Fontanini, A., Spano, P., & Bower, J. M. (2003). Ketamine-xylazine-induced slow (< 1.5 Hz) oscillations in the rat piriform (olfactory) cortex are functionally correlated with respiration. The Journal of neuroscience, 23, 7993-8001.

5. Laurent, G., Stopfer, M., Friedrich, R. W., Rabinovich, M. I., Volkovskii, A., & Abarbanel, H. D. (2001). Odor encoding as an active, dynamical process: experiments, computation, and theory. Annual review of neuroscience, 24, 263-297.

6. Martin, C., & Ravel, N. (2015). Beta and gamma oscillatory activities associated with olfactory memory tasks: different rhythms for different functional networks?. Olfactory memory networks: from emotional learning to social behaviors.

7. Phelps, E. A. (2004). Human emotion and memory: interactions of the amygdala and hippocampal complex. Current opinion in neurobiology, 14, 198-202.

8. Richardson, M. P., Strange, B. A., & Dolan, R. J. (2004). Encoding of emotional memories depends on amygdala and hippocampus and their interactions. Nature neuroscience, 7, 278-285.

9. Chi, V. N., Müller, C., Wolfenstetter, T., Yanovsky, Y., Draguhn, A., Tort, A. B., & Brankačk, J. (2016). Hippocampal respiration-driven rhythm distinct from theta oscillations in awake mice. The Journal of Neuroscience, 36, 162-177.

10. Yanovsky, Y., Ciatipis, M., Draguhn, A., Tort, A. B., & Brankačk, J. (2014). Slow oscillations in the mouse hippocampus entrained by nasal respiration. The Journal of Neuroscience, 34, 5949-5964.

11. Carmichael, S. T., Clugnet, M. C., & Price, J. L. (1994). Central olfactory connections in the macaque monkey. Journal of Comparative Neurology, 346, 403-434.

12. Boiten, F. A. (1998). The effects of emotional behaviour on components of the respiratory cycle. Biological psychology, 49, 29-51.

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