While psychologists and medical experts have long pondered such light-at-the-end-of-the-tunnel events, rigorous investigation of what they are and how they are triggered have fallen by the scientific wayside in recent years.

But new findings from a team from Slovenia suggest Joy just needed a big breath of fresh air, or three, and have triggered renewed interest in the phenomenon.

Specifically, physician Zalika Klemenc-Ketis of the University of Maribor and her colleagues discovered cardiac arrest patients who report a near-death experience are significantly likelier to have higher blood carbon dioxide levels than those who don't.

Carbon dioxide is a waste product of respiration and is carried in red blood cells. It assists regulation of blood supply around the body and levels usually fall during a cardiac arrest.

The study doesn't show whether the higher levels of carbon dioxide among the group of patients who have had a near-death experience were caused by the cardiac arrest or pre-existing.

"As these associations with carbon dioxide and potassium have not been reported before, our study adds new and important information to the field of NDE phenomena," Klemenc-Ketis claims, adding that many cardiac arrest patients experience them after successful resuscitation.

Writing this month in the journal Critical Care, her group claims that 11 of 52 patients (21 per cent) reported a near-death experience after cardiac arrest, but finds no relation between the experience and age, sex, religious beliefs, fear of death, level of education, drugs administered during resuscitation or time to recovery. They also suggest blood potassium levels may play a part in provoking near-death experiences, which occur more frequently in people with previous experiences of them.

"One definition describes NDEs as deep psychological experiences with feelings of transcendence or mystical encounter that typically occur in persons close to death or in situations of intense physical or emotional danger," Klemenc-Ketis says,

"Our study suggests that some physiological factors or processes might be important in provoking NDEs. On the other hand, the experiences induced by some neurophysiological processes mostly consist of fragmented and random memories and confused experiences, unlike the real NDEs that are clear, highly structured and easily recalled." Klemenc-Ketis suggests multiple physiological factors are involved.

"Clearly, the presence of NDEs pushes the current knowledge of human consciousness and mind-brain relation to the edge of our understanding."

Susan Blackmore, a freelance writer, lecturer, broadcaster and visiting professor at Britain's Plymouth University, has investigated near-death experiences extensively, discussing the phenomenon in her 1993 book Dying to Live. One theory she canvassed is that low levels of oxygen are behind them.

She believes the experience is essentially due to disinhibition in parts of the brain. "The inhibitory neurons in the brain are very sensitive to oxygen and electrolyte changes," says Blackmore. "I believe this can disinhibit the brain and lead to the feelings of freedom commonly associated with NDEs." Blackmore applauds the Slovenian authors of the new study, agreeing that carbon dioxide levels could be another piece of the puzzle.

But Blackmore says that while near-death experiences are life-changing events for anyone who undergoes one, they aren't evidence of life after death, spirits or souls, as one camp argues while dismissing the hard science camp.

"I take the middle ground. I would not want to belittle anyone's experience of an NDE because it's a fascinating, complex and intense thing to happen. Some may consider it spiritual. But there is no evidence that it is."

Sam Parnia is an expert in pulmonary and critical care medicine and splits his time between Weill Cornell Medical Centre, in New York and the University of Southampton, in Britain. "These findings are interesting, although it is a small study," he says.

Parnia says one of the biggest problems with resuscitation is when patients are over-ventilated, leading to what is termed "death by ventilation". In such circumstances, he says, too much carbon dioxide is "blown off" by the ventilator, leading to the clamping down of blood vessels that normally deliver blood containing oxygen and nutrients to the brain.

Clearly, over-ventilation is bad for patients. One sign that patients are ventilated correctly in intensive care is appropriate levels of carbon dioxide in their blood. Parnia suspects such patients have better recall of near-death experiences because they've been resuscitated to a higher standard.

"The observations from this study may indicate that those patients who had improved quality of resuscitation and had their hearts restarted after cardiac arrest had better brain recovery and hence better recall and less amnesic effects of brain injury, which seems to be what limits people's ability to recall their NDE. It does not mean that carbon dioxide itself caused that experience," Parnia adds.

"It is entirely possible that carbon dioxide could cause at least some of the various symptoms of NDE," says Krissy Wilson, a lecturer in psychology at the University of Tasmania and member of Australian Skeptics, a federation of groups across the country that investigate paranormal and pseudo-scientific claims from a responsible scientific viewpoint . According to Wilson, who has studied near-death experiences extensively, it's unlikely that better recall is behind them. "Clearly physiological phenomena play a significant role in reports of NDEs, but it's important to highlight both psychological and cultural factors to help explain these alleged experiences," she says.

"All reports of NDEs are based on anecdotal evidence, but human memory is notoriously unreliable and susceptible to all kinds of bias.

"It's quite possible that reports of NDEs are merely a result of false memories based on religious or paranormal beliefs and our desire to impart a spiritual or supernatural explanation to a dramatic and often life-changing event."

Wilson also notes that many supposed features of an NDE can be experienced without being near death, although that's seldom reported. For instance, ketamine is a powerful hallucinogenic that produces very similar effects.

"Regarding cultural factors, most NDEs often report recognised Christian symbolism, images of Christ, for example. Does this mean a Muslim will see images of a Muslim God or that a Hindu would see Hindi religious figures? One also wonders what an atheist would see or experience," says Wilson.

Parnia is running the AWARE study of the Human Consciousness Project in British and US hospitals. It involves placing targets or objects visible only from the ceiling in hospital rooms. When a resuscitated patient reports an NDE, including floating above the body, researchers check to see if they saw the object while experiencing this NDE. The team is reviewing data from more than 700 cases and will report next year.

According to Wilson, it will not be surprising if, after a near-death experience, study patients report seeing objects usually found in hospital rooms. "If, however, the items chosen are purely random and in no way associated with medical procedures and patients accurately describe them after an alleged NDE, then that would be very interesting indeed."

Blackmore is ambivalent about the project. She supported it when it was proposed and is happy such studies are taking place. However, she is convinced the patients won't see these objects during any near-death experience. In fact, Blackmore ceased her studies of the paranormal, including NDEs, in 2000, although she continues to cover the wider subject of consciousness. "I'm so sure that this study will turn up nothing unusual that, if it ends up doing so, I will return to studying the paranormal," she says.

Scepticism doesn't deter Parnia. "Recent discoveries from resuscitation science have wider implications, not only for our understanding of life and death but also the ability to bring people back to life after they have died, following cardiac arrest," he claims. "[NDEs] also bring greater understanding of the mental processes that occur after people have entered the process of death." Perhaps he should ask Harry Joy.