By Jane Dreaper
Health correspondent, BBC News
Many people report seeing a bright light
A large study is to examine near-death experiences in cardiac arrest patients.
Doctors at 25 UK and US hospitals will study 1,500 survivors to see if people with no heartbeat or brain activity can have "out of body" experiences.
Some people report seeing a tunnel or bright light, others recall looking down from the ceiling at medical staff.
The study, due to take three years and co-ordinated by Southampton University, will include placing on shelves images that could only be seen from above.
To test this, the researchers have set up special shelving in resuscitation areas. The shelves hold pictures - but they're visible only from the ceiling.
Dr Sam Parnia, who is heading the study, said: "If you can demonstrate that consciousness continues after the brain switches off, it allows for the possibility that the consciousness is a separate entity.
"It is unlikely that we will find many cases where this happens, but we have to be open-minded.
"And if no one sees the pictures, it shows these experiences are illusions or false memories.
"This is a mystery that we can now subject to scientific study."
Dr Parnia works as an intensive care doctor, and felt from his daily duties that science had not properly explored the issue of near-death experiences.
Process of death
He said: "Contrary to popular perception, death is not a specific moment.
"It is a process that begins when the heart stops beating, the lungs stop working and the brain ceases functioning - a medical condition termed cardiac arrest.
"During a cardiac arrest, all three criteria of death are present. There then follows a period of time, which may last from a few seconds to an hour or more, in which emergency medical efforts may succeed in restarting the heart and reversing the dying process.
"What people experience during this period of cardiac arrest provides a unique window of understanding into what we are all likely to experience during the dying process."
Dr Parnia and medical colleagues will analyse the brain activity of 1,500 cardiac arrest survivors, and see whether they can recall the images in the pictures.
Hospitals involved include Addenbrookes in Cambridge, University Hospital in Birmingham and the Morriston in Swansea, as well as nine hospitals in the US.