Below, I share three passages from Pim van Lommel, Consciousness Beyond Life: The Science of the Near-Death Experience (New York: HarperCollins, 2010). Dr. van Lommel is a Dutch cardiologist.
Various naturalistic explanations have been proposed for the phenomena reported in accounts of near-death experiences. Here are Dr. van Lommel’s responses to two of them:
Because low doses of ketamine, a drug formerly used as an anesthetic, can cause hallucinations, it has been postulated that this kind of substance is released in the brain during a period of stress or oxygen deficiency. Ketamine produces hallucinations because it blocks certain receptors (NMDA) in the brain. A small quantity of ketamine gives some people a sense of detachment from the body or tunnel experiences. There are no known reports of an encounter with deceased persons or of a life review, nor have there been reports of positive changes [in life, which are common after NDEs]. Ketamine usually causes such frightful and bizarre images, which are recognized as hallucinations, that research subjects prefer not to have the substance administered a second time. Because naturally occurring ketaminelike substances have never been found in the brain, this potential explanation must be abandoned. However, we cannot rule out that in some cases the blockade or malfunction of NMDA receptors may play a role in the experience of an NDE. (118-119)
One of the first attempts at explaining an NDE was based on the fact that stress releases endorphins. These are morphines occurring naturally in the body in small quantities, which function as neurotransmitters. They are released in large quantities during stress. Endorphins can indeed get rid of pain and cause a sense of peace and well-being. However, the effects of endorphins usually last several hours whereas the absence of pain and the sense of peace during an NDE vanish immediately after regaining consciousness. Endorphins also fail to explain other elements of an NDE. (119)
Here’s another relevant consideration:
During a cardiac arrest the cerebral cortex, thalamus, hippocampus, and brain stem as well as all connections between them stop functioning . . . which prevents information from being integrated and differentiated — a prerequisite for communication and thus for the experience of consciousness. The experience of consciousness should be impossible during a cardiac arrest. All measurable electrical activity in the brain has been extinguished and all bodily and brain-stem reflexes are gone. And yet, during this period of total dysfunction, some people experience a heightened and enhanced consciousness, known as an NDE. (195)