A near-death experience (NDE) is the perception reported by a person who nearly died or who was clinically dead and revived. The experience is somewhat common, especially since the development of cardiac resuscitation techniques. According to a Gallup poll approximately eight million Americans claim to have had a near-death experience (Mauro, 1992). The experience often includes an out-of-body experience.

The phenomenology of an NDE usually includes physiological, psychological and transcendental categories (Parnia, Waller, Yeates & Fenwick, 2001) such as subjective impressions of being outside the physical body (an out-of-body experience), transcendence of ego and spatiotemporal boundaries, and other transcendental experiences (Lukoff, Lu & Turner, 1998; Greyson, 2003).

Typically the experience follows a distinct progression (Mauro, 1992; Morse, Conner & Tyler, 1985; Morse & Perry, 1992; van Lommel et.al, 2001) as summarized in the following points:
1.The sudden awareness that one has had a 'fatal" accident and not survived.
2.An out-of-body experience. A sensation of floating above one's body and seeing the surrounding area.
3.Pleasant feelings, calmness. A sense of overwhelming love and peace.
4.A sensation of moving upwards through a tunnel or narrow passageway.
5.Meeting deceased relatives or spiritual figures
6.Encountering a being of light, or a light (possibly a religious figure, i.e. Jesus, God, Buddha)
7.Being given a life review
8.A feeling of being returned to the body, often accompanied by a reluctance to return

A 'core' near-death experience reflects — as intensity increases according to the Rasch scale — peace, joy and harmony, followed by insight and mystical or religious experiences (Lange, Greyson & Houran, 2004). The most intense NDEs involve an awareness of things occurring in a different place or time. Among the clinical circumstances that are thought to lead to an NDE we find such factors as: cardiac arrest, shock in postpartum loss of blood or in perioperative complications, septic or anaphylactic shock, electrocution, coma, intracerebral haemorrhage or cerebral infarction, attempted suicide, near-drowning or asphyxia, apnoea, serious depression (van Lommel et.al, 2001)

A significant amount of the research on near-death experiences is co-ordinated through the field of Near-Death Studies. Among the pioneers of Near-Death Studies we find Dr. Raymond Moody, who has chronicled and studied many of these experiences in several books (Moody, 1975;1977;1999), and Dr. Kenneth Ring, co-founder and past President of the International Association for Near-Death Studies (IANDS). Major contributions to the field include the construction of a Weighted Core Experience Index (Ring, 1980) in order to measure the depth of the Near-Death experience, and the construction of the Near-Death Experience Scale (Greyson, 1983) in order to differentiate between subjects that are more or less likely to have experienced a genuine NDE. These approaches include criteria for deciding what is to be considered a classical or authentic NDE. Among the researchers associated with the field of Near-Death Studies we find such names as Bruce Greyson, Michael Sabom, Melvin Morse, PMH Atwater, Yvonne Kason and Peter Fenwick. Most of these researchers tend to emphasize the interpretative and phenomenological dimension of the experience.

Other contributors to the research on Near-death experiences come from the disciplines of medicine, psychology and psychiatry. Neuro-biological factors in the experience has been investigated by researchers within the field of medical science and psychiatry (Britton & Bootzin, 2004; Jansen, 1995; Thomas, 2004). Among the researchers and commentators who tend to emphasize a naturalistic and neurological base for the experience we find the British psychologist Susan Blackmore (1993), and founding publisher of Skeptic magazine, Michael Shermer (1998).

Among the scientific and academic journals that have published, or are regularly publishing new research on the subject of NDE's we find: Journal of Near-Death Studies, Journal of Nervous And Mental Disease, British Journal of Psychology, American Journal of Disease of Children, Resuscitation, The Lancet, Death Studies, and Journal of Advanced Nursing

According to Martens (1994), the only satisfying method to address the NDE-issue would be an international multicentric data collection within the framework for standardized reporting of cardiac arrest events. The use of cardiac arrest-criteria as a basis for NDE-research has been a common approach among the European branch of the research field (Parnia, Waller, Yeates & Fenwick, 2001; van Lommel, van Wees, Meyers & Elfferich, 2001).

Many commentators see near death experiences as an afterlife experience. This often implies the view that the NDE can not be explained by physiological or psychological causes only, and that consciousness, in some instances, might function independently of brain activity (Rivas, 2003). Some accounts include elements that, according to some theorists, are most simply explained by an out-of-body consciousness. For example, in one account, a woman accurately described a surgical instrument she had not seen previously, as well as a conversation that occurred while she was understood to be clinically dead (Sabom, 1998). In another account, from a proactive Dutch NDE study , a nurse removed the dentures of an unconscious heart attack victim, and was asked by him after his recovery to return them (van Lommel et.al, 2001).

Many people who experience an NDE tend to see it as a verification of the existence of an afterlife (Kelly, 2001). Core NDE experiencers, in particular, tend to be convinced of the reality of the experience as an intimation of the afterlife. This includes those with agnostic/atheist inclinations before the experience. Many former atheists, such as the The Rev. Howard Storm (Rodrigues, 2004)  have adopted a more spiritual view of life after their NDEs. Howard Storm's NDE might also be characterized as a distressing near-death experience. The distressing aspects of some NDE's are discussed more closely by Greyson & Bush (1992).

In some instances there might be ambiguity between medical and spiritual facts. There exist reports about, what appears to be, connections between deceased persons and persons who have had an NDE. These visualizations happen over a long period after the NDE. Ken Mullens (1992;1995), who was clinically dead for more than 20 minutes, reported spiritual encounters in his life after his NDE. As he reported, deceased persons he communicated with were often unknown to him, but were connected to people he met at a later point. While many such reports are criticized and discredited by the scientific field they remain a mystery. Since they have no apparent medical or physical explanation they might be said to belong the interpretative and phenomenological dimension of the NDE, as investigated by the field of Near-Death studies.

In support of an after-life interpretation one might point out that neurobiological models often fail to explain NDEs that result from close brushes to death where the brain does not actually suffer trauma, such as a near-miss automobile accident. Regardless of the origin of the phenomenon, the subjective experience of NDEs is well-documented by the field of Near-Death Studies.

Other commentators see near death experiences as a purely naturalistic phenomenon. According to this perspective the cause of the NDE might be understood as a result of neurobiological mechanisms, related to such factors as anomalous temporal-lobe functioning, epilepsy, compression of the optic nerve (as a cause of the tunnel sensation), chemical changes in the brain related to serotonin and endorphins, and general changes in brain functioning (Mauro, 1992; Britton and Bootzin, 2004). The similarities amongst the experiences of the many documented cases of NDE may be understood to signify that the pathology of the brain during the dying and reviving process is more or less the same in all humans, as suggested by Russian specialist Dr. Vladimir Negovsky in his book Clinical Death As Seen by Reanimator (Pravda, 2004).

Among the studies that support a naturalistic interpretation we find the research of Blanke et.al (2002), published in Nature Magazine , who found that electrical stimulation on the brain region known as the right angular gyrus repeatedly caused out-of-body experiences to the patient; though the patient did not report seeing her own face, for example. Researchers have largely been unsuccessful in running proactive experiments to establish out-of-body consciousness. There have been numerous experiments in which a random message was placed in a hospital in a manner that it would be invisible to patients or staff yet visible to a floating being, but so far, according to Blackmore (1991), these experiments have only provided equivocal results and no clear signs of ESP.

A well-known scientific hypothesis that attempts to explain NDEs was originally suggested by Dr. Karl Jansen (1995;1997) and deals with accounts of the side-effects of the drug Ketamine. Ketamine was used as an anesthetic on U.S. soldiers during the Vietnam War; but its use was abandoned and never spread to civilian use because the soldiers complained about sensations of floating above their body and seeing bright lights. Further experiments by numerous researchers verified that intravenous injections of ketamine could reproduce all of the commonly cited features of an NDE; including a sense that the experience is "real" and that one is actually dead, separation from the body, visions of loved ones, and transcendent mystical experiences. Ketamine acts in part by blocking the NMDA receptor for the neurotransmitter glutamate. Glutamate is released in abundance when brain cells die, and if it weren't blocked, the glutamate overload would cause other brain cells to die as well. In the presence of excess glutamate, the brain releases its own NMDA receptor blocker to defend itself; and it is these blockers Dr. Jansen (amongst others) hypothesize as the cause of many NDEs.

Critics of Jansen's hypothesis point out that although some aspects of the experience may be similar, not all NDEs exactly fit the ketamine experience; and that while it might be possible to chemically simulate the experience, this does not refute the possibility that spontaneous NDEs have a spiritual component. As Dr. Jansen himself notes:
Claims that NDE's must have a single explanation (e.g. Ring, 1980), or that a scientific theory must explain all of the experiences ever given the name of NDE (e.g. Gabbard and Twemlow, 1989) are difficult to justify (Jansen, 1995).

Indeed Dr. Jansen's own shifting perspective on the conclusions to be drawn from the ketamine-NDE analogy has been notable. He started out as an unequivocal debunker of the notion that NDE's are evidence of a spiritual (or at least transnormal) realm. But with time he has developed a more agnostic hypothesis: that ketamine may in fact be one particularly powerful trigger of authentic spiritual experiences - of which near-death may be another. In each case, according to Jansen's more recent pronouncements, all we can say is that the subject gets catapulted out of ordinary 'egoic' consciousness into an altered state - we cannot comfortably rule out the possibility that the 'worlds' disclosed in these 'trips' have ontological status. Latterly, therefore, Jansens position appears closer to thinkers like Daniel Pinchbeck (2002), who has written a book on hallucinogenic shamanism, and other names like Carl Jung, Ken Wilber and Stanislav Grof, than to thinkers like Susan Blackmore or Nicholas Humphrey, who represents a sceptical position.

Related to the findings of Jansen we find the work Strassman (2001) who induced near death experiences (in addition to some different naturally-occurring altered states of being) in a clinical setting by injecting subjects with DMT, a powerful psychedelic tryptamine. Because DMT is a chemical that is produced endogenously in the human pineal gland it migh be relevant to the study of NDE's and other mystical, religious, and transpersonal experiences. Related to the findings of Jansen we also find the theory of Shawn Thomas (2004), director of Neurotransmitter.net, who has suggested that the neurotransmitter agmatine is the key substance involved in near-death experiences .

It is generally accepted that some people who reported NDEs were shown to have approached the clinical boundary between life and death. However, in support of a naturalistic interpretation of NDE's one might argue that it is not shown that the experiences themselves took place in any time other than just before the clinical death, or in the process of being revived. In altered states of consciousness such as this and during dream states or under the influence of drugs, the subjective perception of time is often dilated