First of all, I had better tell you what this is not about: near death experiences (NDEs). Despite their variability, NDEs have an overall similarity. They consist of several stages in a specific order, the number of stages depending on the length of the experience: (1) an out-of-body experience, (2) a rapid movement down a dark tunnel, (3) a meeting with a Being of Light, whom most people identify with God in whatever way they conceive of Him, (4) a life review, which appears to be instantaneous, in which the person's life is rated against the criterion of love, and (5) finally, the coming to a barrier, before being sent back to the world. Generally, you have to be fairly near to death to have a near death experience. Dr Melvin Morse (Closer to the Light, 1991) compared 12 children who had required resuscitation with 121 children who had been critically (and I mean critically!) ill, but were never in any danger of dying, and 37 who had taken mind-altering medication. Most of the 12 test children had experienced at least one aspect of an NDE; none of the control groups had.
However, there appears to be another, much more variable and much rarer phenomenon: having complex visions during periods of unconsciousness which, for want of a better word, I shall call comas, though that might not be the correct medical term. The ones you hear about are those which impact on the person's attitude to life, although they tend to be based on beliefs already held. Here are the few I have come across.