That NDEs happen isn't in dispute. The sequence and type of events of which they're composed are similar enough among people who report them that NDEs could be considered a syndrome of sorts akin to a disease lacking a known cause. But just because millions of people have experienced NDEs doesn't mean the most commonly believed explanation for them—that souls leave bodies and encounter God or some other evidence for the afterlife—is correct. After all, people misinterpret their experience all the time (an optical illusion representing the most basic example). Without a doubt, many people who report NDEs are profoundly affected by them, but usually more as a result of their interpretation of the experience (i.e., the afterlife is real) than as a result of the experience itself. It turns out, however, that a number of reproducible observations combined with a bit of conjecture yield an entirely plausible neurological explanation for all the reported experiences that comprise NDEs.

In his book Nelson, notes that normally 20% of the heart's blood flow is directed to the brain, but that it can drop as low as 6% or so before we fall (and even at this level, no permanent injury will result). Nelson further observes that when our blood pressure drops low and we faint, the (a large nerve that connects to the heart) tilts consciousness toward sleep—but interestingly in some people not all the way. A number of subjects seem to be susceptible to what he calls "REM intrusion." REM intrusion most typically occurs, when it does, in the transition from wakefulness to sleep. Nelson found in his research that the functioning of the mechanism that flip-flops people between REM sleep and wakefulness tended to be different in people who reported NDEs. In those people, he found the switch was more likely to "fragment and blend" those two states of consciousness (control of our state of consciousness is found in the brainstem and is tightly regulated), causing such people to simultaneously exhibit features of both. During REM intrusion people have found themselves paralyzed ("sleep paralysis"), fully awake but experiencing light, out-of-body sensations, and stunningly vivid narratives. During REM sleep, many of the brain's pleasure centers are stimulated as well (animals that have had their REM regions injured lose all interest in food and even ), which may explain the feelings of peace and unity also reported during NDEs.

Neurophysiology can also explain the feeling of moving through a tunnel so commonly mentioned in NDEs. People are well known to experience "tunnel vision" immediately before fainting. Experiments with pilots spun around in giant centrifuges have reproduced the tunnel vision phenomena by increasing -forces and decreasing blood flow to their retinas (the periphery of the retina is more susceptible to drops in blood pressure than its center, so that the visual field appears compressed, making scenes appear as if viewed through a tunnel). When special goggles that generate suction were applied to the pilots' eyes to counteract the blood pressure lowering effect of the centrifuge, the pilots lost consciousness without developing the tunnel vision effect-proving the experience of tunnel vision to be caused by decreased blood flow to the eye.

Perhaps the most intriguing aspect of NDEs is how often they're associated with . This, too, however, turns out to be an illusion. Evidence that out-of-body experiences have nothing to do with souls leaving bodies can be found in the observation that they've also been reported by people just awakening from sleep, recovering from anesthesia, while fainting, during seizures, during migraines, and while at high altitudes (there's no reason to think the souls of people are leaving their bodies during any of those non-life-threatening situations). But the most fascinating evidence that out-of-body experiences are neurological phenomena comes from studies initially performed in the 1950s by a named Penfield. He was interested in figuring out how to distinguish between normal brain tissue and brain tumors or scars that were responsible for causing seizures. So he stimulated the brains of hundreds of awake patients in an effort to map the cerebral cortex and figure out where in our brains our physical body is represented.

One patient suffered from temporal lobe seizures and when Penfield stimulated the temporoparietal region of his brain, he reported leaving his body. When the stimulation stopped, he "returned," and when Penfield stimulated the temporoparietal region again, he left his body once more. Penfield also found when he varied the current and stimulus location that he could make his patient's limbs seem to shorten or create a body double that existed next to him! In The Tell-Tale Brain, V.S. Ramanchandran describes a patient who had a tumor removed from his right frontoparietal region and developed a "phantom twin" attached to the left side of his body. When Ramanchandran applied cold water to his ear (a procedure known as cold-water caloric testing, which stimulates the brain's balance system, known to have connections to the frontoparietal region), the patient's twin shrunk, moved, and changed positions!

Neurologists have since recognized that the temporoparietal region of the brain is responsible for maintaining our body schema representation. When external current is applied to this region, it ceases to function normally and our body schema "floats." Further evidence that this phenomenon is an illusion comes from experiments in which people who've had out-of-body experiences when transitioning from sleep to wakefulness were unable to identify objects placed in the room after they'd fallen asleep, strongly suggesting the picture they viewed of themselves in their beds was reconstructed from . Though no evidence yet exists that low blood oxygen levels cause dysfunction of the temperopareital region in the same way as does applied current, this remains a testable hypothesis and the most likely explanation.

In sum then, though far from proven, as an explanation for what actually explains near-death experiences, the REM intrusion hypothesis has far more evidence to support it than does the idea that we actually do leave our bodies when death looms near.

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