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It was to have been a simple procedure to clear a blockage in an artery of her heart. No need for a general anesthetic, just something to make her drowsy. But when the job was done and Florence Cohen began to feel her senses sharpening, events took a turn for the odd.

Standing next to her bed in Sydney's St. Vincent's Hospital, her cardiologist asked her how she was feeling.

"I feel great," Florence said.

Noting, most likely, changes to her vital signs or coloring, he repeated the question.

"Terrible," came the reply.

A moment later, Florence went into cardiac arrest. She recalls, she says, the sounds of bells and sirens, being in a lift, and someone thumping her chest. But these memories are vague compared to her recollection of a subsequent scene. She was lying in surgery, surrounded by medical staff, yet she was not in her body but up near the ceiling, watching the activity below. She noted that her body on the bed wore a green gown with a split in the middle, and that she was otherwise completely covered. "I was calling out, 'Don't cut me. I'm still awake.' " She saw an incandescent light in the shape of a cone. "Then, boom," she says: her floating self rocketed to the cone's tip and . . . nothing.

That was 20 years ago. "I found it spooky," she says now. "I don't like to talk about it. It was the whitest light you could imagine. It wasn't a dream. It's still very, very vivid."

Florence had what's known as a near-death experience. But as strange as it was, it didn't contain all the elements of a classic NDE. As well as the bright light and out-of-body experience, other people, while clinically dead, see a tunnel, deceased relatives and divine figures. They may be guided by one of these spirits through a life review in which, some report, they feel again every emotion the past events aroused. Though they believe themselves to be dead, this cascade of feelings typically occurs against a prevailing sense of euphoria. At some point, they're told it's not their time and they return to the confinement of their body — most often through the top of the head. There's nothing hazy about the experience. On the contrary, it's reported as seeming more real than real life, whatever that means. Most NDEs change those who have them, dampening or obliterating any fear of death.

The conflict in science over NDEs centers not on whether they happen but on what they are. It's accepted, based on various studies, that between 4% and 18% of people who are resuscitated after cardiac arrest have an NDE. Researchers tend to fall into one of two camps. The first argues that an NDE is a purely physiological phenomenon that occurs within an oxygen-starved brain. "There's nothing mysterious about NDEs," says Mark Mahowald, director of the Minnesota Regional Sleep Disorders Center. "Many people want it to be a religious, paranormal or supernatural phenomenon. The fact that NDEs can be explained scientifically detracts from the mystique."

The second camp is as adamant that no theory based purely on the workings of the brain can account for all elements of an NDE, and that we should consider the mind-bending possibility that consciousness can exist independent of a functioning brain, or at least that consciousness is more complex than we suppose. Though NDEs are driven in part by neurochemistry and psychology, says Auckland psychiatrist Karl Jansen, it has "underlying mechanisms in more mysterious realms that cannot currently be described."

These are the best of times in the NDE field, with research gathering pace and new insights emerging. University of Virginia psychiatrist Bruce Greyson reported recently on a tantalizing investigation into whether the observations people claim to make during an NDE (details of their resuscitation, the color of a nurse's shoes) are in fact accurate. Meanwhile, University of Kentucky neurophysiologist Kevin Nelson theorizes that NDEs are what can happen when a particular sleep state intrudes on the imperiled brain. "I wouldn't say it's definitive," says Nelson. "But it's an intriguing hypothesis that answers a great deal."

Entertaining the idea of mind-body duality invites the scorn of those who regard any attempt to dabble beyond the boundaries of conventional science as a waste of time. But that's the point, say others: NDEs don't fit into our current understanding of the brain. They shouldn't happen, yet they do. The task is to build new models in which they do fit.

At last year's first International Medical Conference on Near-Death Experience, held in Martigues, France, eight participants describing themselves as "a group of dedicated physicians and researchers working in different scientific fields" released a statement. They said that while the NDE is mediated by chemical changes in the brain, "its extremely rich and complex content cannot be reduced to a mere illusion." It is of the "utmost importance," they argued, "that scientists wishing to understand the nature of human consciousness conduct research without prejudice."

So what's so baffling about NDEs? We know that when a person's heart stops, the decline in brain function caused by a cut in blood supply is steep. Simultaneous recording of heart rate and brain output shows that within 11 to 20 secs. of the heart failing, the brain waves go flat. A flat electroencephalogram (EEG) recording doesn't suggest mere impairment. It points to the brain having shut down. Longtime NDE researcher Pim van Lommel, a retired Dutch cardiologist, has likened the brain in this state to a "computer with its power source unplugged and its circuits detached. It couldn't hallucinate. It couldn't do anything at all."

Yet it's in this period, between switch-off and resuscitation, that many researchers believe NDEs occur. "Many near-death experiencers describe heightened perceptions and clear thought processes, and form memories, at a time when the brain is incapable of coordinated activity," says Greyson, director of the University of Virginia's Division of Perceptual Studies. "Our current neurophysiological models can explain NDEs only if one ignores much of the empirical data."

Of the thousands of NDEs reported, none has done more to convince some researchers that the phenomenon's explanation must lie outside the square than the case of Pam Reynolds, an American who underwent brain surgery for an aneurysm in 1991. Preparation for Reynolds' operation included taping her eyes shut, blocking her ears and monitoring her EEG to ensure her brain was functioning at only the most basic level. Yet after coming around, Reynolds described not only a full-blown NDE but the bone saw that had been used to cut her skull.

For many years, says cardiologist van Lommel, he was in the first camp on NDEs, sure their basis was entirely material. His interest having been pricked in the mid-'70s by the first book about NDEs, Life After Life by American doctor Raymond Moody, van Lommel in 1988 began a study that would encompass 344 survivors of cardiac arrest in 10 Dutch hospitals. Van Lommel and his co-authors wrote in The Lancet in 2001 that 18% of subjects reported some recollection of the time of clinical death, and 7% an experience that qualified as a deep NDE.

The Dutch team found little about the NDErs that distinguished them as a group from those for whom clinical death was a blackout. Factors such as psychological profile, medications, religion and previous knowledge about NDEs all appeared to be irrelevant. To this day, Van Lommel can't explain why some people have NDEs and most don't. But the fact the experience isn't universal undermines, to his mind, a purely physiological explanation: if lack of oxygen were the cause of NDEs, then all survivors of cardiac arrest should have one.

A few years ago, Van Lommel retired from cardiology to concentrate on NDE research. "I'm lecturing all over the world," he says. "I know all the skeptical questions and I love to answer them." In trying to account for NDEs, he's challenged ideas residing in the bedrock of science, including that consciousness and memories are localized in the brain. As astounding as it may be, he argues, the implication of NDEs is that consciousness can be experienced in some alternative dimension without our body-linked concepts of time and space. "In my view, the brain is not producing consciousness, but it enables us to experience our consciousness," he says. He compares the brain to a television, which receives programs by decoding information from electromagnetic waves. Likewise, he says, "the brain decodes from only a part of our enhanced consciousness, which we experience as waking consciousness. But our enhanced consciousness is different, and this is what is experienced during an NDE."

The idea that the brain can be retuned to alternative states resonates with psychiatrist Jansen, who's written prolifically on how an NDE (or something closely resembling it) can be induced by an anesthetic drug, ketamine. That NDEs can be induced led him at first to suspect that the spontaneous type was similarly hallucinogenic. Now he's not so sure. Perhaps ketamine and brain stress simply make certain states more accessible. "All our realities are alternative realities," says Jansen. "Nobody sees the world in quite the same way as any other person."

Jansen once wrote: "It's good to have an open mind, but not so open that your brain falls out." For many scientists, this scenario might account for the sort of speculation just summarized. While most researchers concede that there's a lot about NDEs we don't know, they reject the push to replace tried-and-tested paradigms with new (largely untestable) ones in an attempt to fill the gaps.

Outside of cardiac arrest and the injection of ketamine, NDE-type phenomena can occur in many circumstances, including fainting spells, serious disease and in the seconds before potentially catastrophic accidents, like falling off a cliff. While that doesn't suck the mystery from the phenomenon, it does suggest that NDEs are a flawed pointer to what might await us in death as opposed to the process of dying or a really hairy moment.

Another, possibly key, point is that NDEs vary across cultures. In a soon-to-be-published review of the literature, a team of Australian researchers reports, for example, that Chinese NDEs are dominated by feelings of bodily estrangement without all the pleasant stuff, and that the Japanese see caves rather than tunnels. For co-author Mahendra Perera, a Melbourne psychiatrist, these differences don't prove that NDEs are hallucinations, only that their "final expression is colored by culture, language and learning."

Science is trying to solidify the brain-based theory of NDEs, which goes something like this: Survival is our most powerful instinct. When the heart stops and oxygen is cut, the brain goes into all-out defense. Torrents of neurotransmitters are randomly generated, releasing countless fragmentary images and feelings from the memory-storing temporal lobes. Perhaps the life review is the brain frantically scanning its memory banks for a way out of this crisis. The images of a bright light and tunnel could be due to impairment at the rear and sides of the brain respectively, while the euphoria may be a neurochemical anti-panic mechanism triggered by extreme danger.

As for perhaps the strangest element of NDEs, the out-of-body experience, studies led by Swiss neuroscientist Olaf Blanke have shed light on what may be going on there. In 2002, Blanke and others reported how they were able to induce OBEs in an epilepsy patient by stimulating the brain's temporoparietal junction (TPJ), thought to play a role in self-perception. In emergencies where blood supply is cut, says Blanke, "the effects are occurring first at the TPJ, which is a classical watershed area of the brain." It's probable, he concludes, that stress in the TPJ causes the dissociation of NDEs — a dissociation that's entirely illusory.

That's a point the University of Virginia's Greyson wanted to settle. Are NDErs up there on the ceiling or aren't they? In 2004, he began a study that he hoped would provide the answer. At the university's electrophysiology clinic, surgeons implant cardioverter-defibrillators in patients at high risk of sudden death. In the process, cardiac arrest is induced. Greyson arranged for a laptop computer, displaying a series of images, to be stationed near the ceiling, where only an elevated being could see the screen. As ingenious as it was, the investigation flopped. Greyson and his team reported last December that while cardiac arrest had been induced in 52 patients, none reported leaving his or her body.

Considering the incidence of NDEs, the result surprised Greyson. "But we can still learn from that failure," he says. "Unexpected findings like those tell us we don't understand NDEs as well as we thought, and that increases my enthusiasm for studying them."

What science has lacked until recently is an overarching theory that might explain why NDEs seem so coherent. In two articles published in Neurology, the second in March, a team of University of Kentucky researchers led by Nelson proposed that NDEs occur in a dream-like state brought on when crisis in the brain trips a predisposition to a type of sleep disorder. It's an hypothesis that's quickly gathered heavyweight support: "I think Dr. Nelson's REM-intrusion theory to explain NDEs is the actual physiologic explanation," says Minnesota sleep expert Mahowald.

His what theory? REM (rapid eye movement) sleep is the relatively active brain state in which most dreaming is thought to occur. REM intrusion is a disorder in which the sleeping person's mind wakes up before his body does. He feels awake, yet the muscle paralysis of REM can remain; he may also hallucinate until mind and body get back in sync. "Lay people think you're either awake or asleep," says Nelson, "but you needn't go directly from one to the other."

Some years ago, while studying first-hand accounts of NDEs, Nelson read the story of a woman whom medical staff had written off as dead and whose attempts to protest were thwarted by paralysis. Paralysis? As happens in REM intrusion! The seed of a new theory — that there was a link between REM and NDEs — grew in Nelson's mind.

He tested it by comparing the frequency of REM intrusion in 55 people who'd had NDEs with 55 controls. The results were striking: 60% of the first group reported some history of REM intrusion; 24% of the second. Nelson postulates that both REM intrusion and NDE involve a glitch in the arousal system that causes some people to experience blended states of consciousness. He stresses that he doesn't consider NDEs to be dreams, rather that the NDEr "engages through the REM mechanism regions of the brain that are also engaged during dreaming" — regions that infuse both dreams and NDEs with emotion, memories and images.

Nelson's theory goes some way toward explaining how NDEs can seem to occur when the brain is down. The sleep/wake switch is in the brainstem, which helps control the body's most basic functions and stays active for longer than the higher brain in cardiac arrest. "It's likely that the transition to brain death is, in fact, gradual," says Mahowald, "and NDEs occur during this transition." As for people reporting accurately on events that went on around them while they were apparently unconscious, Nelson says "they may be seemingly out of it but still processing in a very aberrant way."

Nelson's theory has been picked apart by two veterans of the field who could be said to favor a more spiritual view of NDEs. In a recent issue of the Journal of Near-Death Studies, Americans Jeffrey Long and Janice Miner Holden argue that since 40% of NDErs in Nelson's study denied ever having had an episode of REM intrusion, the idea that it underlies NDEs "seems questionable at best."

Happy to concede that "the brain deals with crisis in ways we don't fully understand," Nelson is keen to test his theory some more. He won't go into details, but it's believed he wants to monitor REM activity in subjects he would expect to have NDE-like symptoms in certain conditions.

Other researchers have their own ideas about how to solve the puzzle. Neuroscientist Blanke calls for "more work with imaging to investigate the brain functioning of large numbers of people who've had an NDE." Says Jansen, who'll soon release work comparing accounts of spontaneous NDEs with ketamine-induced ones: "We're moving on an exciting path. But nobody knows if we've made huge progress or just a little."

On balance, it's almost certain that NDEs happen in the theater of one's mind, and that in the absence of resuscitation, it's the brain's final sound and light show, followed by oblivion. Nonetheless, there's still no definitive explanation. There mightn't be a ghost in the machine. But it's a machine whose complexities remain well beyond our grasp.

• Daniel Williams