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Everyone dies, but what actually transpires during that process is a deep mystery that scientists are only beginning to seriously investigate. Increasingly, near-death experiences, or NDEs, are part of that growing field.
An incredible 5 to 10 percent of the general population reports memories of an NDE. Oftentimes, people’s recollections are similar: perceiving separation from the body and viewing it from above, passing through a tunnel and seeing a light, encountering deceased loved ones or compassionate entities, and being overcome by ineffable wisdom and a feeling of profound peacefulness. Many people describe these memories in crisp detail and say that they felt “more real than real.”
How a person’s faltering consciousness produces such fantastical experiences is unknown. But scientists have been piecing together hypotheses, constructed from interviews with survivors, studies in animals, and experiments in which people were given certain psychedelic drugs. Now one of the preeminent research groups investigating NDEs has published what it describes as the first comprehensive neuroscientific model for the phenomena.
“We found a very robust explanation for the generation of such a rich experience while a person is really in crisis,” says Charlotte Martial, a neuroscientist at the University of Liège in Belgium and co-lead author of the findings, published this week in Nature Reviews Neurology.
Martial and her colleagues’ model lays out a step-by-step hypothesis for the conditions that give rise to NDEs. They also propose an evolutionary theory for why these experiences occur.
To create the model, the authors undertook an exhaustive review of all the studies they could find on NDEs, which ranged from neuroscientific investigations to spiritual perspectives. They also included research on ecstatic seizures, psychedelics and the dying brain. Additional clues came from studies that showed that individuals who have certain predispositions are more likely to experience NDEs. This includes prolific daydreamers, as well as those with a propensity for rapid eye movement (REM) sleep intrusion, which occurs when REM sleep bleeds into wakefulness or non-REM sleep.
NDEs themselves are triggered by a precipitating event, such as a cardiac arrest, that causes a cascade of physiological stress. The authors propose that certain networks of neurons go into overdrive to produce high levels of specific neurotransmitters in the drastically altered brain environment. The researchers investigated several of those systems and hypothesized ways that they may contribute to distinct mental experiences as the person approaches death.
The fact that people can clearly remember NDEs, the authors write, is likely because of the activity of three main neurotransmitters: acetylcholine, which is involved in memory, learning and attention; noradrenaline, which plays a key role in the fight-or-flight response, as well as attention, focus and memory; and glutamate, another learning and memory aid that is also responsible for orchestrating overall brain function by instructing neurons to communicate with one another.
Beyond the triad of chemical culprits, the researchers linked the calm, peaceful feeling that characterizes many NDEs with activation of 5-HT1A receptors by serotonin, as well as with transient rises in endorphins—the body’s natural pain relievers and mood enhancers—and GABA, a neurotransmitter that reduces neurons’ activity. For the vivid hallucinations that oftentimes accompany NDEs, the team pointed to serotonin’s hyperactivation of 5-HT2A receptors. Dopamine likewise contributes to the altered visual experience and lends a sense of realism.
The new model calls into question a prior hypothesis that a yet-undiscovered, naturally occurring chemical in the brain plays a role in inducing NDEs by blocking the same receptors that the synthetic drug ketamine binds to. Those receptors, however, are essential for memory formation, so if they were blocked, people should not be able to recall NDEs with such clarity, says Nicolas Lejeune, senior author of the new study and a neurorehabilitation clinician and researcher at the University Hospital of Liège. “Instead of assuming the existence of an unknown neurochemical, we propose that NDEs arise from disruptions that naturally occur in response to life-threatening events,” he says.
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