A mysterious surge of brain activity observed in dying rats may explain reports of near-death-experiences, say researchers.

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 near-death-experiences.

As many as a fifth of people who survive cardiac arrests report having had an other-worldly experience while being "clinically" dead.

Typically, these involve travelling through a tunnel towards an intense light, being separated from the body or encountering long-departed loved ones or angels.

Many scientists believe near-death-experiences are nothing more than hallucinations induced by the effect of the brain shutting down.

The 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 Michigan University.