Out-of-body experiences, a bright light at the end of the tunnel: Scientists can only speculate about what people experience when they die. What happens in the brain shortly before death, on the other hand, has now been well researched. In an interview, the neurologist Jens Dreier explains how to research the physiological processes during death and what they have in common with strokes and migraine auras.
Mr. Dreier, you are researching the brain between life and death. How do you think it feels to die?
Of course, that depends a lot on why you die. If we are not in pain, we may not even notice the transition. It might be like falling asleep. Or we still have some kind of consciousness and are temporarily in a dream-like state, which we take to be reality. That would be something of a near-death experience.
What scientific knowledge is there about the experience of death?
Our knowledge of this is based exclusively on interviews: people who only narrowly escaped death, for example because they were resuscitated, report on their experiences. However, very few have such memories, which is why the data is relatively thin. In research, there are scales that are used to determine whether something was a near-death experience or not. However, I don’t find that completely unproblematic, because the experiences can only be standardized to a limited extent. In my opinion, if someone reports on their experiences, you should first take note of it.
What are typical near-death experiences?
There are some recurring patterns, such as the feeling of being in different eras and different places at the same time. Abstract sensory impressions often also arise, for example a bright light or a narrowing of the field of vision – as if one were walking through a tunnel. Some also talk about out-of-body experiences.
Comparable sensations sometimes appear in completely different situations.
Yes, very rarely does this happen during surgical operations, for example. A patient is under anesthesia and a critical situation occurs, such as a circulatory collapse. Some subsequently report near-death experiences. But it can also happen in non-life-threatening situations.
For example?
You are sitting in the opera house and listening to an aria. Suddenly you drift off into dreams that you experience as real. Experts call this phenomenon REM intrusion. This is a sleep phase embedded in a waking phase, which you do not perceive as such. This happens particularly often with the clinical picture of narcolepsy.
Still, is this a near-death experience?
There is a set of similar experiences that are defined by scales but can occur in different contexts. Only during resuscitation do they really occur frequently. That is why one usually speaks of near-death experiences when one means these states of consciousness. The whole field has an anecdotal character, which makes it difficult to access it scientifically. But the abundance of reports suggests such experiences do exist. Incidentally, they also occur in different cultures and do not depend on specific religions. All of this points to their existence.
Would you like to learn more about the mysterious world of near-death experiences? Our PDF guide shows experience reports and findings from perception research.
Let’s take the simplest case: someone goes into cardiac arrest. A few seconds after the heart has stopped, the oxygen concentration in the brain drops. The nerve cells switch to an economy mode, which massively throttles the neuronal activity. After about seven to eight seconds, the person loses consciousness; after 30 to 40 seconds all brain activity is gone. However, the exact timing depends on the extent of residual perfusion.
Do the nerve cells die then?
no First comes a period of inactivity, when the neurons are just inhibited but alive. Once blood flow resumes, they work normally again. Experts call the condition hyperpolarization.
Can you explain that in more detail?
Nerve cells have a so-called membrane potential, they are “polarized”. The inside of the cell membrane is normally negatively charged at rest; there is a voltage of -70 millivolts to the outside. You have to imagine it like a charged battery. During a nerve impulse, the cells depolarize. As a result, the inside becomes positive for a short time, only to then repolarize again, i.e. return to the initial state. But when the oxygen supply is cut off, the following happens: the cells hyperpolarize. So they become even more negative than they already are. From this very negative state, they can no longer be excited, even though the battery is still fully charged.
What happens then?
To maintain the hyperpolarization, the cell still needs some energy. The body normally produces these from glucose and oxygen. If there is not enough of it, the diaphragm pumps that create the voltage gradient can no longer work. After a few minutes, a huge depolarization wave occurs, also known as terminal spreading depolarization, in which the nerve cells discharge one after the other, similar to a short circuit. We first detected it in humans in 2018. The light phenomena just before death could be due to this pathophysiological process.
Which parts of the brain does the wave pass through?
It usually begins at certain vulnerable points in the cerebral cortex and spreads across the entire brain at an estimated rate of three millimeters per minute. It migrates through all areas where the nerve cell bodies are located. In addition to the cerebral cortex, these include, for example, the basal ganglia, the cerebellum and even structures in the spinal cord.
Some also speak of a death wave. Does it ultimately herald brain death?
In fact, it causes massive changes inside the nerve cells: all sorts of molecules are mixed up wildly. For example, the concentration of calcium increases thousands of times. If this goes on for too long, the neurons become poisoned and die. The amazing thing, however, is that they can withstand this condition for a certain period of time. If the membrane pumps start up again and remove everything that doesn’t belong inside, the cells will survive.
When does resuscitation have to start at the latest so that the pumps start up again?
That depends on various factors, such as temperature and age. Suppose we have an otherwise healthy young human at room temperature. It takes an estimated five minutes from cardiac arrest to the onset of nerve cell death. After about three minutes, the huge wave sets in motion. But as soon as someone resuscitates, i.e. presses the heart properly, the body and brain are slightly supplied with blood. Then the nerve cells last much longer.
How did you discover the wave just before death? To do this, you had to record the brain activity at the precise moment when a patient died.
That was random. The real motivation for our research was to help people with a certain type of cerebral hemorrhage. The depolarization waves occur not only when dying, but also in strokes. With my team at the Charité in Berlin, I examine so-called subarachnoid hemorrhages. They develop when a bulging part of a brain vessel ruptures. The bleeding often stops temporarily. Then neurosurgeons or neuroradiologists have the opportunity to close the sac securely so that the risk of further bleeding is averted. However, the patients are unfortunately not out of the woods yet. Because the coagulated blood is now on the surface of the brain and often triggers strokes about a week later due to insufficient blood flow.
And you wanted to track them down with your measurements?
Exactly. The patients are usually in a coma in the intensive care unit, where they can only be examined neurologically to a very limited extent. This is why these delayed strokes usually go unnoticed. As with the dying process, however, the depolarization waves also occur here. If you record the neuronal activity with electrodes placed on the surface of the brain, you can tell from the waves if the person is suffering a stroke. In this way, we can intervene therapeutically in good time.
Does that mean the same thing happens in the brain when you have a stroke as when you die?
The spreading depolarization follows principles similar to those just before death. An important difference is that the lack of energy in stroke is local, but global in dying.
Similar to near-death experiences, the depolarization wave also occurs in situations that are not life-threatening. Which are they?
An example is the migraine aura. Here, scientists were able to use functional magnetic resonance imaging, among other things, to observe how the wave spreads in the brain. There was an amusing background to a 2001 study: a team in Boston had an employee who developed a visual migraine aura every time he played basketball. He then always had to go directly from the sports field to the scanner. During the MRI scan, he was able to report in which part of the field of vision he perceived the aura.
It is said that the discharge wave before death is huge. Is it that big when you have a migraine?
Yes, it is much greater than any epileptic seizure – both in the migraine aura and just before death! In the case of the former, however, it almost never leaves any consequential damage.
The dying brain runs out of energy, so the membrane pumps can no longer maintain the voltage gradient and spreading depolarization occurs. But why does this occur with migraines?
As already mentioned, the same thing that happens in a stroke – when a blood vessel is blocked – as in death, only locally. This is due to the lack of energy. Some of the migraine auras could have the same cause: a small blood clot occludes a vessel and triggers the wave. However, it is so tiny that it dissolves on its own and therefore does not cause any damage.
A persistent foramen ovale (PFO), i.e. a small hole between the two atria, is more frequently associated with strokes in adults. Studies now suggest that migraines with aura are also more common in people with PFO. That would support your theory as to the cause.
Yes, that’s right. The hole creates a kind of short circuit: instead of going into the lungs, low-oxygen venous blood flows directly to the other side of the heart and from there, along with any clots, it gets to the brain. Sometimes we can even detect very small strokes after a migraine aura. Just recently we had such a case: A colleague of mine was sure that the symptoms of a patient had to be a migraine aura. To be on the safe side, we did an MRI and saw three dots in the brain, so three tiny strokes. The only cause we found was a patent foramen ovale.
So could migraines with aura be treated by plugging the hole in the heart?
They actually tried that. But it doesn’t seem to work for everyone affected, because there are many other triggers for the wave, most of which are not yet understood. For example, certain congenital disturbances in nerve cell or astrocyte metabolism are associated with this form of migraine, which has nothing to do with vascular problems. Incidentally, the process of spreading depolarization can even occur without any typical blood circulation: in grasshoppers and cockroaches, for example. He is phylogenetically very old.
Some believe the dying discharge wave may be the physiological equivalent of the visual manifestations of near-death living. Do we all have a big migraine aura before we die?
I would be careful there. A colleague of mine at the University of Copenhagen used internet-based surveys to investigate whether people with migraine auras were more likely to have near-death experiences. So there really is an association. However, these are only vague indications. My group from the Charité is currently conducting another study on the subject together with the Copenhagen team, this time on patients in the headache clinics. Unfortunately, the results are not yet available. But what might also help elucidate the nature of near-death experiences are certain drugs.
Why this?
There are substances that trigger exactly such experiences. The two most important are ketamine and dimethyltryptamine (DMT for short). The interesting thing is that they inhibit the depolarization waves. In emergency situations, the body may release similar substances to prevent or delay spreading depolarization. The near-death experiences could be due to the effect of the “internal drugs” and not to the wave itself. Only the bright light would be an indication that parts of the brain are already affected by the wave. But that is of course pure speculation.
The original of this post “Researcher explains: This is what happens in our brain when we die” comes from Spektrum.de.
