Intense, pleasant, and sometimes profound experiences that people report when they have "come back" from states close to death.
Tales of near-death experiences (NDEs) are not unusual. Out-of-body experiences, the sensation of moving through a tunnel toward a light, the review of the events of one's life, and pleasurable glimpses of other worlds are relatively consistent features of people's "near death" reports. In fact, research suggests that almost one fifth of Americans report having almost died, and a large proportion of them have recounted experiences like the ones mentioned above. The reported events are very vivid, seem completely real, and can sometimes transform people's lives. How to explain these experiences is the subject of debate. Throughout history people have interpreted them as journeys toward the divine. The out-of-body experience was the soul or spirit leaving the body, the tunnel was the passageway, the life review was the time of judgement, and the light at the end of the tunnel was heaven (or the equivalent).
It appears that, rather than any spiritual journey or other world phenomenon, NDEs may be best understood by examining human physiology, neurochemistry, and psychology. At this time, there is strong research evidence to indicate that many of the symptoms of NDEs may be caused by anoxia, or a lack of oxygen to the brain. In the human visual system, for example, neurons (brain cells) deprived of oxygen will start to fire out of control. Since the majority of the cells in our visual cortex (the portion of the brain where visual information is processed) respond to stimulation in the central visual field, the result is a white spot in the center with fewer cells firing out of control in the periphery. As oxygen deprivation continues, the white spot grows and the sensation of moving through a tunnel toward a white light is produced. Similarly, it is possible that the life review process is a result of depriving the temporal lobes of oxygen. When the temporal lobes of the brain, an area largely involved in memory production, are deprived of oxygen, neurotransmitters are released and massive electrical activity ensues. In laboratory research, when people's temporal lobes are stimulated with electrodes, many subjects experience the reliving of memories, out of body experiences, and even the sensation of moving through a tunnel toward a light. Oxygen deprivation can also affect the limbic system, which contains the seat of emotions in the brain. The intensely pleasurable feelings of love and well-being that accompany moving toward the light may therefore be a consequence of increased activity in the limbic system.
According to some people, the similarities among people's accounts of NDEs provide powerful evidence for the existence of an afterlife. These similarities however, can also be interpreted as evidence in support of the involvement of human physiology, neurochemistry, and psychology. The visual cortex, temporal lobes, and limbic system are structurally and functionally common to everyone. Consequently neurological activity associated with stress or oxygen deprivation may be similar across many different individuals.
Blackmore, S. "Near-death experiences: In or out of the body?" Skeptical Inquirer, 16, (1991): 34-45.