(Image: Mark Cornelison)

Neurologist Kevin Nelson explains how the brain slips into a strange state of hybrid consciousness during a near-death experience

How common are near-death experiences (NDEs)?
A 1997 survey reported that 18 million Americans had had one. When my team surveyed people who have had them, we found that some occurred during cardiac arrest but the vast majority were during fainting. Thirty-seven per cent of all Americans will have fainted at one point in their life, so I suspect NDEs are common.

In your book The Spiritual Doorway in the Brain you talk about borderlands of consciousness. What are they and how do they relate to NDEs?
We have three states of consciousness: awake, non-REM sleep and REM sleep. But there aren't absolute dividing lines between them - they can blend with one another, most commonly REM and waking. Twenty to 25 per cent of people at some point experience some kind of blending, a borderland of consciousness. What I have discovered is that the switch in the brainstem that regulates these three states functions differently in people who have had NDEs. These people are more likely to get stuck between the REM state and waking. So it looks like some people are prone to having these kinds of experiences. Interestingly, it tends to run in families.

Does that mean NDEs are a kind of lucid dream?
Lucid dreams are among the closest things we know of to an NDE. They are very similar. Brainwave measurements show that lucid dreaming is a conscious state between REM and waking. During REM consciousness, the dorso-lateral prefrontal cortex is turned off. As that's the executive, rational part of the brain, this explains why dreams are so bizarre. But if the dorso-lateral cortex turns on inside a dream, you become aware that you are dreaming. It is like waking up in your dream. When the body is in crisis during an NDE and the brain is slipping from consciousness to unconsciousness, it can get momentarily stuck in a borderland between REM and waking, just like a lucid dream.

But unlike dreams, NDEs tend to feature some specific images, such as seeing a tunnel with a light at the end.
The tunnel actually has nothing to do with the NDE - it's to do with what's happening to your vision. During fainting, for instance, there's a blackout because the eye isn't getting enough blood, so the eye begins to shut down even though the brain is still going. As it shuts down first from the sides and then into the centre, it's like looking through a tunnel.

The light that people tend to see has a few sources. To start with, the eye might only be capable of seeing smudges of light because of the tunnelling and lack of blood flow. Then, as the brain enters REM consciousness, the visual system becomes strongly activated - that's the rapid eye movement that defines REM consciousness. When the visual system is activated, you get light.

People often report having out- of-body experiences during NDEs.
These come about because the temporoparietal region of the brain is turned off, so the brain is no longer able to map the body's position in space. A Swiss researcher named Olaf Blanke was able to use electrodes to turn the temporoparietal region of a woman's brain on and off, making her feel like she was floating up out of her body and then returning. It was like flipping a light switch.

REM consciousness turns the temporoparietal region off, so if you are semi-conscious in a borderland between waking and REM, you can easily have an out- of-body experience. These are extremely common during lucid dreams, narcolepsy, fainting and sleep paralysis - all borderland states. I have never had one, though. I wish I could!

You often hear people claim that these experiences happened during minutes when they were declared clinically dead. How could that be?
This is an incredible misconception that has arisen because people use the term "clinical death" when they really mean cardiac arrest. When your heart stops and you lose blood flow, you don't lose consciousness for another 10 seconds and brain damage doesn't occur until 30 minutes after blood flow is reduced by 90 per cent or more. So when experiencing an NDE, you are not dead.

People like to say that these experiences are proof that consciousness can exist outside the brain, like a soul that lives after death. I hope that is true, but it is a matter of faith; there is no evidence for that. People who claim otherwise are using false science to engender false hope and I think that is misleading and ultimately cruel.

Do your findings undermine religious belief?
There's no conflict. I'm interested in how the brain works during spiritual experience, I'll leave the "why" to others. I'm a "big tent" guy. I think a dispassionate, non-judgemental view is important.

Kevin Nelson is a neurophysiologist at the University of Kentucky. His book The Spiritual Doorway in the Brain is published in December in the US by Dutton, and will be published in the UK as The God Impulse by Simon and Schuster in March