Near-death experiences (NDEs) have been described by many as proof of the afterlife - intense white light, hallucinations of dead loved ones talking to them, angels and so on. Now, a slightly less romantic but more scientifically sound explanation has emerged - the brain experiences a surge of activity shortly before death.
The changes, which were observed in rats may go a long way to explaining the after effects. Even after the animals' hearts stopped beating and no blood was reaching their brains, they appeared to show signs of conscious perception, said the scientists.
The study is the first to take a systematic look at the neurophysiological state of the dying brain after a cardiac arrest.
It suggests something happens at the brink of death that pushes the conscious brain to a high level of arousal, potentially triggering the visions and sensations associated with NDEs.
As many as a fifth of people who survive cardiac arrests report having had an other-worldly experience while being "clinically" dead.
Typically NDEs involve travelling through a tunnel towards an intense light, being separated from the body, encountering long-departed loved ones or angels and undergoing some kind of judgment of "life review".
Some emerge from NDEs as transformed individuals with a completely altered outlook on life, or a new belief in religion.
But many scientists believe near-death-experiences are nothing more than hallucinations induced by the effect of the brain shutting down.
The new research involved recording the electrical nerve impulses of anaesthetised rats whose hearts were artificially stopped. Within 30 seconds after suffering a cardiac arrest, all the animals displayed a short-lived surge of widespread, highly synchronised brain activity.
"We were surprised by the high levels of activity," said Dr George Mashour, one of the US researchers from the University of Michigan in Ann Arbor.
Many people see an intense light
"In fact, at near-death many known electrical signatures of consciousness exceeded levels found in the waking state, suggesting that the brain is capable of well-organised electrical activity during the early stage of clinical death."
The findings are reported in the journal Proceedings of the National Academy of Sciences.
Previously it was assumed that brain activity ceases when the heart stops.
"This study tells us that reduction of oxygen or both oxygen and glucose during cardiac arrest can stimulate brain activity that is characteristic of conscious processing," said lead scientist Dr Jimo Borjigin, also from the University of Michigan.
"It also provides the first scientific framework for the near-death experiences reported by many cardiac arrest survivors."
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British expert Dr Martin Coath, from the Cognition Institute at University of Plymouth, said: "This new research is genuinely interesting, but the conclusion that these are 'neural correlates of heightened conscious processing' isn't strongly supported, unless you take it to mean 'more of some types of activity that are associated with being awake' which is a bit of a stretch."
"As the induced cardiac arrest happens while the rat's brain is anaesthetised, the results show the response of an unconscious brain to critical lack of blood flow and oxygen. It is certainly interesting that this causes some types of activity in the brain to increase in a predictable and coordinated way well after the heart has stopped, but hardly surprising."
Dr David McGonigle, from University of Cardiff, said: "Do we know if animals experience 'consciousness'? Most philosophers and scientists are still at loggerheads over what the term refers to in humans, let alone in other species.
"While recent research now suggests that animals may indeed have the kind of autobiographical memories that humans possess, the kinds of memories that allow us to place ourselves in a certain time and place, it seems unlikely that near-death-experiences would necessarily be similar across species."
British expert Dr Martin Coath, from the Cognition Institute at University of Plymouth, said: "This new research is genuinely interesting, but the conclusion that these are 'neural correlates of heightened conscious processing' isn't strongly supported, unless you take it to mean 'more of some types of activity that are associated with being awake' which is a bit of a stretch."
"As the induced cardiac arrest happens while the rat's brain is anaesthetised, the results show the response of an unconscious brain to critical lack of blood flow and oxygen. It is certainly interesting that this causes some types of activity in the brain to increase in a predictable and coordinated way well after the heart has stopped, but hardly surprising."
Dr David McGonigle, from University of Cardiff, said: "Do we know if animals experience 'consciousness'? Most philosophers and scientists are still at loggerheads over what the term refers to in humans, let alone in other species.
"While recent research now suggests that animals may indeed have the kind of autobiographical memories that humans possess, the kinds of memories that allow us to place ourselves in a certain time and place, it seems unlikely that near-death-experiences would necessarily be similar across species."