Pharmological explanations suggest that drugs administered during medical care, especially anesthetics, are responsible for a NDE. This explanation is problematic primarily because many NDE occur without medical care and drugs. Furthermore, many researchers have pointed out that those patients under pain killers or anesthetics have fewer and less detailed NDE. Lastly, accounts of hallucinations under the influence of drugs are different in key ways than a NDE (Life After Life, pg. 145).
Neurological explanations posit that NDEs are the outcome of brain activity during the death process. The Dying Brain Hypothesis is most famously attributed to Susan Blackmore. The feelings of bliss are triggered by the release of endorphines under stress, anoxia (lack of oxygen in the brain) and hypercarbia (high carbon dioxide) may be responsible for the tunnel vision and light, and temporal lobe stimulation may cause the life review and out-of-body distortions. However, Blackmore admits that these explanations do not account for the entire experience, and . A critique of her book can be found here. Among the problems with these type of explanations is that many cases of NDE occured when the patient was clinically dead under medical supervision. Some suggest that the brain wasn't totally dead and that there must be lingering "fragments of conscioussness" or "neural activity that is so minimal [that] it goes undetected." Yet if that were the case we would except a fragmentary or minimal experience instead of life-transforming, inspiring and lucid visions.
Psychological explanations are too many to count and generally not convincing in the slightest; NDEs are caused by expectations about death, or they are really a memory of one's birth, or an evolutionary advantage by feigning death, or memes etc. A more reasonable explanation is that a life-threatening emergency evokes a crises response wherein their REM state hijacks perception and the person will disassociate (The Spiritual Anatomy of Emotion, pg. 424). This explanation is lacking because not all NDEs occur under stress, or it happens so sudden that there is not enough time to have a crises response (for example, being blindsided by an oncoming car).
Now let us turn to some positive evidence for a spiritual explanation: NDEs demonstrate that consciousness can exist independent of the brain. Notice I do not suggest that NDEs conclusively demonstrate that there is life after death, because we do not know what lies beyond "the border" from which all returnees inevitably return. To begin I will quote some NDEs themselves.
"Pam Reynolds...had a giant basilar artery aneurysm...Neurosurgeon Robert Spetzler...was a specialist and pioneer in a rare, dangerous, but sometimes necessary technique called hypothermic cardiac arrest, or 'Operation Standstill.' He would take her body down to a temperature so low that she was essential dead, but then bring her back to a normal temperature before irreversible damage set in...As the surgery began, her heart was stopped, and her EEG brain waves flattened into total silence....and her temperature fell to 60 degrees Fahrenheit.
"When all of Reynolds' vital signs were stopped, the surgeon began to cut through her skull with a surgical saw. At that point, she reported that she felt herself "pop" outside her body and hover above the operating table. From her out-of-body position, she could see the doctos working on her lifeless body. She observed, 'I thought the way they had my head shaved was very peculiar. I expected them to take all of the hair, but they did not.' She described, with considerable accuracy for a person who knew nothing of surgical practice, the Midas Rex bone saw used to open skulls. Reynolds also heard and reported later what was happening during the operation and what the nurses in the operating room had said.
"At a certain point, she became conscious of floating out of the operating room and traveling down a tunnel with a light. Deceased relatives and friends were waiting at the end of this tunnel, including her long-dead grandmother. She entered the presence of a brilliant, wonderfully warm and loving Light and sensed that her soul was part of God and that everything in existence was created from the Light (the breathing of God). This extraordinary experience ended when Reynolds' deceased uncle led her back to her body (The Spiritual Brain, pg. 154; See here for more discussion of this case).
This is one such where there is "veridical perception" of verifiable objects. Kenneth Ring describes five such cases, including a case where a nurse at Hartford Hospital states that she worked with a patient who described a NDE in which she saw a red shoe on the roof of the hospital during her OBE, which a janitor then retrieved. Pim van Lommel reports a case in which a coronary-car nurse removed dentures froma cyanotic and comatose heart-attack victim and placed them in a drawer. The patient was revived by CPR, and a week later the nurse saw him again in the cardiac ward. "The moment he sees me he says: 'O, that nurse knows where my dentures are" (The Spiritual Brain, 155, 322). Skeptics of course discount such evidence as anecdotal and sometimes even poke holes in the stories. But I wonder at which point it becomes highly unlikely that all such stories are hoaxes, misinterpertations, false memories etc.
In the post of OBEs I already pointed out similar cases not involving NDEs which demonstrate veridical perception. There too I mentioned Ring's collected stories of blind people seeing during OBEs and NDEs.
"Third," I quote from The Spiritual Anatomy of Emotion, pg. 425, "is the frequency with which people who are near death report meeting deceased friends and relatives...Surely some of us would wish to converse with Elvis, or John F. Kennedy, or Marilyn Monroe, or Kurt Cobain." It seems that there is something more here than just a hallucination.
In conclusion, while I do not claim that all NDEs, or all aspects of NDEs, must occur in reality, I do believe that the evidence is on the side of the survival theory. It is possible that there is a continuum of experiences - some are fully internal and hallucinatory, while other are real external experiences that one's consciousness perceives when one makes the journey to the Light.