These are five potential explanations in Dr. Alexander book. Some of them may not make sense to laymen like us, so here are the most common explanations he refutes.
– A primitive brainstem program to ease the terminal pain and suffering. This didn’t explain the rich nature of his recollections.
– The distorted recall of memories from the deeper parts of the system like the amygdala that has enough overlying brain to be protected from the bacterial inflammation which occurs at the brain surface. This did not explain the robust, richly interactive nature of the recollections.
– DMT dump. DMT is a natural serotonin that causes visions. He is personally familiar to the serotonin related drug experiences (LSD) from his teen years. He has not had a DMT experience, but he has seen patients under the influence. The rich ultra-reality still needs a functioning audio and visual neocortex as target regions in which to generate a rich experience like he had. The prolonged coma has destroyed his neocortex, where all the serotonin from the raphe nuclei in his brainstem would have effects on the audiovisual experiences. However, his cortex was off so the DMT would have no place in the brain to act.
– A reboot phenomenon – a random dump of fragmented memories due to old memories in the affected neocortex, which might occur on restarting the cortex into consciousness after a prolonged failure like a meningitis. This seems most unlikely when you take into account the details of his vivid recollections.
– An unusual memory generation through an archaic visual pathway is used in birds, but not in humans. Humans can experience it only in cases when they’re cortically blind. It provided no clues to the ultra-reality he witnessed and failed to explain the auditory-visual incorporation.
Dr. Alexander’s near death experience account is the most credible because he has a scientific background.