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How Hyperventilation Affects the Prefrontal Cortex

Publication date: 11 December 2014

I tried a little experiment recently, with myself as the only subject. The results were interesting and unexpected. In theory, hyperventilation or over-breathing should deactivate the brain by reducing oxygen delivery to brain cells. I wanted to see if I could measure the effect, using hemoencephalography or HEG neurofeedback equipment. In this blog post I describe my results.

My experimental protocol was a three minute forced hyperventilation, in the middle of a HEG neurofeedback training session - in other words both before and after the hyperventilation I'm trying to activate my prefrontal cortex (PFC), while recording the infra-red temperature from my forehead. It wasn't the most dramatic piece of over-breathing, but enough to make me feel rather unpleasant for the duration, and for a few minutes after.

Why should we expect the PFC to deactivate? Two effects combine to reduce oxygen delivery to brain cells. They are both related to the concentration of carbon dioxide in the blood, which drops when you over-breathe. The first is a vasoconstriction in the brain, reducing cerebral blood flow. The second is that haemoglobin holds on more tightly to its oxygen rather than offering it to the cells that need it. (I discuss the physiology of hyperventilation at greater length in my e-book, "Mind-Body Intelligence: How to manage your mind using biofeedback and mindfulness" - free chapters are available here.)

So what actually happened? I ran the protocol three times and got more or less the same pattern each time - the session graph below shows it. The vertical blue dotted lines mark the period of over-breathing. The orange trace is the HEG signal, as I say in effect an infra-red temperature measured at the forehead. In HEG neurofeedback it's assumed that changes in this signal over the short term reflect changes in metabolic (energy-consuming) activation of the PFC, which is of course right behind the forehead.

effects of hyperventilation in HEG neurofeedback

Before the hyperventilation the signal is heading nicely upwards as I practise alert focus. During the over-breathing period,  the signal levels off, drops slightly but not dramatically. The surprising result is the big drop that comes after I was over-breathing, and when I'm recovering. During this time I did feel quite a strong fatigue, for at least several minutes.

At the same time as measuring the HEG neurofeedback signal, I used a capnometer (my favourite biofeedback device) to monitor the carbon dioxide content of my breath. So I know my breathing immediately started to recover at the end of the hyperventilation, and was more or less back to normal within a minute. Do the post-hyperventilation drop in PFC activation is not a result of the breath being slow to recover, but a more complex sort of rebound effect.

What can we learn from my experiment?

Clearly it has its limitations as a piece of science, if  you can even call it that. But as far as it goes it confirms my belief that breathing is an important factor in brain performance and in mental and emotional well-being.

The interesting and unexpected finding is the "rebound" drop off in PFC activation. This reminds me a lot of clients with chronic stress problems who perhaps have low stress tolerance but experience a sort of crash after a stressful event, into fatigue that can last for days. (To be clear, my own post-hyperventilation fatigue lasted only minutes so it's not directly comparable.)

Why should such a delayed effect happen? I don't have any definitive answers but I'm reminded of a book I read ("Maximum Willpower" by Kelly McGonigle) in which the author quoted research suggesting that the brain predicts lean times ahead and conserves its resources. So for example, when the brain detects dropping blood sugar, it anticipates a minor famine and shuts down high-demand functions such as those of the PFC. (Not low blood sugar but dropping, note.)

Maybe this conservation response is wide-spread, and triggered by any stressor not just physical? Maybe in some people it is more pronounced than others? May be it's a significant aspect of depression?

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