Are Near Death Experiences explainable in purely neurological terms?
I've never had a patient confess to
having had a Near Death Experience (NDE), but recently I came across a
fascinating book called The Spiritual Doorway in the Brain by
Kevin Nelson, M.D. that reports as many as 18 million Americans may have
had one. If true, the odds are not only that some of my patients have
been among them, but also some of my friends. Which got me wondering:
just what does science have to tell us about their cause?
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 unconscious (and even at this level, no permanent
injury will result). Nelson further observes that when our blood
pressure drops low and we faint, the vagus nerve (a large nerve that
connects to the heart) tilts consciousness toward REM 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 morphine), 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 G-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 out-of-body experiences. 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 neurosurgeon 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
sleeping in their beds was reconstructed from memory. 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.
Source :
Happiness in this World
Tidak ada komentar:
Posting Komentar