Some people may retain awareness after they have technically died, according to an unusual study published on Wednesday into hospital patients who went into deep cardiac arrest.
Fifteen hospitals in Britain, Austria and the United States pooled information about more than 2,000 cardiac arrest patients.
The research, published in the European journal Resuscitation, sought to assess "near-death experiences" and other phenomena recounted by people hauled back from clinical death -- when heart and brain activity have stopped.
Of the 2,060 patients, 330 were resuscitated, of whom 101 completed in-depth, two-stage interviews afterwards.
Thirty-nine percent described a perception of awareness before their heart was restarted but did not have a explicit recall of events.
"This suggests that more people may have mental activity initially but then lose their memories after recovery, either due to the effects of brain injury or sedative drugs on memory recall," said Sam Parnia, who led the study at Britain's University of Southampton.
Forty-six percent said they had feelings of fear, violence or persecution, deja-vu, or images of relatives, animals and plants.
Only nine percent reported the better-known near-death experience, such as the sensation of detachment from the body.
Two percent said they could explicitly recall "seeing" and "hearing" events while they were technically dead.
In one such case, the patient was able to recall two beeps from a machine that made a noise at three-minute intervals, enabling the researchers to time how long the experience lasted.
"This is significant, since it has often been assumed that experiences in relation to death are likely hallucinations or illusions, occurring either before the heart stops or after the heart has been successfully restarted, but not an experience corresponding with 'real' events when the heart isn't beating," said Parnia, now at the State University of New York at Stony Brook.
"In this case, consciousness and awareness appeared to occur during a three-minute period when there was no heartbeat.
"This is paradoxical, since the brain typically ceases functioning within 20-30 seconds of the heart stopping and doesn't resume again until the heart has been restarted. Furthermore, the detailed recollections of visual awareness in this case were consistent with verified events."
Parnia said more work was needed to see whether awareness persisted into clinical death.