Thread: The physiological experiences of near death

http://www.csicop.org/si/2004-05/nea...xperience.html
An interesting read, though maybe not of interest to those with a closed mind.

Darkness, Tunnels, and Light
Stories of darkness, tunnels, and bright light told by those who report near-death experiences actually have a basis in the structure and functioning of the eyes, the brain, and other sense organs that operate during these experiences.

Darkness, tunnels, and light are wondrous phenomena sometimes reported by the dying, as well as those recovering from near-death experiences (NDEs). These experiences have been reported since antiquity. Along with many others, I first learned of these experiences from a film I saw in 1990 called Flatliners. The film showed amazing and improbable medical apparati used in an equally improbable and dramatic location. Even so, I was stimulated to read more, and became fascinated by the possible physiology of all aspects of NDEs.

I am a physician specializing in anesthesiology, and have worked as a consultant anesthesiologist in Holland since 1980. An anesthesiologist is not someone who just knocks patients out, sits down, opens a newspaper, and waits for the surgeon to finish his work; instead, he or she keeps patients alive and insensible to pain during operations, and ensures that patients survive their operations in the best possible condition. This work requires me to view all bodily and mental phenomena from a very basic physiological perspective. So in my practice, I ask myself, “How can the functioning of the body generate this phenomenon? What is the mechanism? How does it work?” My approach to the study of NDE phenomena is very similar, which is why this article is about the ways the functioning of the body can generate darkness, tunnel, and light experiences.

Darkness, tunnel, and light experiences are part of the so-called “core-NDE” described by Kenneth Ring, an eminent NDE researcher in the 1970s and 1980s. He described the core-NDE as having the following components (Roberts and Owen 1988): feeling blissful sensations; leaving the body; entering a tunnel or darkness; perceiving a bright light; and entering the light.

Wonderful, fantastical experiences . . . but what is the mechanism? How can they be explained? Many people offer explanations ranging from the preposterous to those worthy of serious consideration. Among them:

Real experiences of a spiritual or immaterial realm. The immaterial is unseen, unheard, and unable to be sensed or measured empirically; it is unprovable.
A dream arising from the collective unconscious. The great psychoanalyst Carl Jung proposed this idea. Indeed, inculcation of cultural identities and myths certainly occurs during the upbringing of each person. But the invocation of these aspects of each person’s unconscious to explain tunnel, darkness, and light experiences is to use an explanation that is just as unprovable as is any immaterial or spiritual explanation.
Recollection of the birth experience; an explanation proposed by the late Carl Sagan. This is a curious and dubious explanation. After all, babies’ eyes are shut during birth, their brains and vision are undeveloped, and there is no way to know what a baby experiences. Furthermore, why should people undergo a repeated birth experience while dying?
The effects of drugs and medicines. Most people undergoing these experiences are not under the effects of any drugs or medicines.
Carbon dioxide intoxication or oxygen starvation. Many people undergoing these experiences are not suffering from carbon dioxide overload or oxygen starvation.
A flood of endorphins (morphine-like substances in the brain), released by the dying brain. This is a compelling idea, but an inadequate explanation, and at best very difficult to prove.
Susan Blackmore’s neural-noise theory. In 1989, Tom Troscianko and Susan Blackmore reasoned that there were more nerve cells within the visual cortex representing the central parts of the retina than there were representing its peripheral parts. A computer simulation of increasing neural noise in the visual cortex induced by drugs or disease revealed a blob of white light gradually increasing in size, which, when viewed on a screen, gave viewers the sensation of moving down a tunnel toward a bright light and finally being enveloped by the light (Blackmore 1991). An elegant idea, but it neglects basic facts, such as the relative oxygen consumptions of retina and brain, as well as the fact that people can “see the light” while at the same time seeing things around them. These facts render the neural-noise theory an inadequate explanation of tunnel and light experiences, except perhaps for situations where there is epileptic nerve activity within the visual cortex.
What is another explanation for these experiences? After all, they are real experiences. People who have undergone them are neither mad nor hysterical, and they really have undergone darkness, tunnel, and light experiences. But how? I began my study with the light experience, distilling its properties from the stories I heard and the many reports I read. These properties are: people see bright light; the light does not hurt the eyes; this light is seen not only during NDEs undergone by apparently unconscious people, but also reported by the conscious dying; and no one else can see this bright light.

‘The Lovely Brightness’
Any successful physiological theory about the light experience must be able to explain these properties. After my medical studies, I left Australia to specialize in anesthesiology in England. While there, I worked in the now defunct Hackney General Hospital, as well as the equally defunct Mothers’ Hospital. This latter was an obstetrics hospital in the impoverished district of Clapton in London’s East End. Because I’d practiced there, a report related by Sir William Barrett in the book Death-Bed Experiences attracted my attention (Barrett 1926). I knew the hospital, how the rooms appeared, and how they were lit, as well as the nature of the women who came there to have their babies delivered. In Barrett’s report, a dying woman first saw only darkness, and subsequently saw a “lovely brightness,” as well as “bright forms.” The obstetrician reported her observations. At one point she wrote: “But then she turned to her husband, who had come in, and said, ‘You won’t let the baby go to anyone who won’t love him, will you?’ Then she gently pushed him to one side, saying, ‘Let me see the lovely brightness.’”

A matron was also present, and reported: “Her husband was leaning over her and speaking to her, when pushing him aside she said, ‘Oh, don’t hide it; it’s so beautiful.’”

These two sentences reveal that this woman saw the “lovely brightness” because her medical condition caused her pupils to widen. The woman was dying of heart failure, and lethal heart failure causes oxygen starvation; severe oxygen starvation causes the pupils to widen. Furthermore, sympathetic nervous system activity is maximal during lethal heart failure, and this also causes the pupils to widen.

No one else in the room saw the bright and wonderful light, and nowhere does this report mention that the lighting in the room was increased. The size of the pupils of the other people in the room did not change because the level of illumination in the room did not change, so they did not see the bright light. This woman pushed her husband aside because he did indeed block the light. So she saw bright light because her pupils widened, admitting more light into her eyes. Light enters the eyes through the pupils, and the diameter of the human pupil varies from 1 millimeter to as much as 10 mill limeters. A small calculation reveals the magnitude of the effect of pupil widening: the area of the pupil through which light is admitted into the eye is a circle, and pupil area = pi (pupil radius)2. This means that the amount of light entering each eye can increase by as much as 100 times.

Pupil widening is indeed a likely reason she saw a "lovely brightness." Another sentence in this same report also caught my eye: "She lived for another hour, and appeared to have retained to the last the double consciousness of the bright forms she saw, and also of those tending her at the bedside. . . .”

Fascinating—and also very revealing. As an amateur photographer, I realized this was also an effect of pupil widening. Pupil widening reduces the depth of field. A person whose pupils are widely dilated not only sees bright light, but only clearly sees people upon whom the eyes are focused, while all other people are seen as bright and blurry forms. So this unfortunate woman interpreted the bright and blurry images of out-of-focus people elsewhere in the room as “bright forms.”

The optical effects of pupil widening were very likely the cause of the “bright light” and “bright forms” seen by this woman. However, to her and her family, as well as to all observers, the experiences and observations she reported while dying were not just mental and optical manifestations of a mundane biological event. Instead, they were an intense and wondrous confirmation of deeply held socio-cultural beliefs in a life after death. This story beautifully illustrates how pupil widening due to a multitude of causes can arouse visions of “bright light” and “figures of light.”

Oxygen Deprivation
At the same time as I read this account, I was also studying the effects of oxygen poisoning and oxygen starvation, and learned that both tunnel and darkness experiences could be caused by oxygen starvation. Oxygen is an essential ingredient in complex chemical reactions within all cells of the body, generating vital, energy-rich chemicals. Oxygen starvation can occur when there is: too little oxygen in the air; abnormal lung function; abnormal heart function; too little blood; anemia; abnormal red blood cell function; obstructed or severed blood vessels; abnormal cell function; or any combination of one or more of these factors.

Oxygen starvation causes failure of all the organs and tissues of the body, and the eyes and brain are most sensitive to its effects, failing before any other organs. Furthermore, the retina has a higher oxygen consumption than the brain, which is why oxygen starvation causes the functioning of the retina to fail before significantly affecting the functioning of the brain.

Oxygen starvation does not cause all parts of the brain to fail at the same time. The brain stem, which generates consciousness, is the part of the brain most resistant to oxygen starvation. Therefore, oxygen starvation will cause vision to fail before causing a loss of consciousness. And indeed, the experience of fainting proves this. Fainting is due to a sudden, fortunately temporary, failure of blood flow to the head, resulting in oxygen starvation of the brain and loss of consciousness. Just before losing consciousness, many people notice that everything suddenly “went gray,” “went dark,” or “went black.” Perception of grayness or darkness is a conscious experience, so these people are actually saying that their vision failed before they lost consciousness. Some people also report tunnel vision just before losing consciousness.

Oxygen starvation can cause both tunnel and darkness experiences. The reason for this lies in the structure and functioning of the blood supply of the retina. The macula is the optical center of the retina; it has the greatest blood supply, while the flow of blood to the retina decreases with distance from the macula according to the inverse square law. Yet the oxygen consumption of each part of the retina is much the same, so oxygen starvation will cause failure of peripheral vision before causing total visual failure. Indeed, experiments with oxygen starvation in human volunteers prove this fact. This is why tunnel experiences occur only in NDEs caused by oxygen starvation, while toxins and poisons cause a “pit experience” before causing failure of vision. So oxygen starvation explains why not everyone has a tunnel experience during an NDE. Oxygen starvation also explains why the tunnel experience is not a true component of the NDE, but is instead a manifestation of the cause of the NDE (Greyson 1983).

Somatic Sensations
Aha, say the critics, but during a “tunnel experience” people feel themselves moving, flying, or being drawn through a tunnel toward a light or entering the light. So they say oxygen starvation cannot be the cause of tunnel and light experiences. Yet oxygen starvation explains these sensations very well. Furthermore, a close study of the way oxygen starvation affects conscious perception of sensations explains all these things without the necessity of invoking a human soul, paranormal sensations, or immaterial spirit worlds.

The human brain is about one and a half kilos of jelly-like tissue contained within the protective confines of the hard bones of the skull. Mind is a product of brain function, and all sensations enter the brain as signals conducted into the brain along sensory nerves. The mind only knows what is happening within the body, to the body, and in the world around the body by interpreting the sensory nerve signals conveying sense data into the brain. So if these sensory nerves transmit signals into the brain indicating that the body is moving, falling, or flying, the conscious mind perceives the body to be moving, falling, or flying. Furthermore, if the brain malfunctions, even normal sensory signals transmitted into the brain may be interpreted incorrectly. And the malfunctioning brain may even misinterpret normal sensory signals as sensations of movement, falling, or flying. Oxygen starvation is a common cause of brain malfunction, as well as the cause of the terminal loss of consciousness of more than nine in ten dying persons (Murray 1997). And oxygen starvation causes malfunction of muscle spindles, the sense organs that provide the brain with most of its information about body position and movement. Muscle spindles are special muscle structures sandwiched between the fibers of every muscle. There is about one muscle spindle per 1,000 ordinary muscle fibers. Muscle spindles are both sense organs and muscle fibers, sensing and transmitting to the brain sensations of weight, of movement, of falling, of floating, and of flying. Moreover, the tensing and relaxing of muscle spindles relative to the surrounding muscle fibers also generates similar sensations.

Severe oxygen starvation causes convulsions. Muscle spindles sense these movements and transmit the sensations to the brain. Victims may also sense others attempting to aid them. The brain malfunctions during oxygen starvation, causing muscle spindle tension to differ from the tension of the surrounding muscle fibers. Body parts where muscle spindles are relaxed relative to surrounding muscle fibers feel heavier than normal, while body parts where muscle spindles are tenser than the surrounding muscle fibers feel lighter than normal. Again, all these phenomena have been well established experimentally.

During NDEs caused by oxygen starvation, a combination of brain malfunction, abnormal muscle spindle function, random movements due to convulsions caused by oxygen starvation, and movements of the oxygen-starved person’s body made by people treating and helping the person all combine to generate sensations of movement. When this is combined with a total loss of vision, tunnel vision, or the effects of pupil widening, sensations of moving through darkness or a tunnel toward light can occur. Some people also say they felt themselves being “drawn to the light.” This is quite possibly a result of the initial restoration of central vision, followed by an increasing restoration of peripheral vision as oxygen supplies to the eyes increase. A person undergoing such an experience would first see a small spot of light at the end of a tunnel which would gradually increase in size to envelop the whole visual field. This would give the illusion of moving toward a light at the end of a tunnel, and even of entering the light as retinal function was restored.

All these things make it possible to explain and understand the sequence of events during NDEs caused by oxygen starvation. Consider a report in Return From Death (1986), written by Margot Grey. Grey reported the story of a woman who nearly died in childbirth. This woman reported that “I was moving very rapidly down a long, dark tunnel. I seemed to be floating. I saw faces which came and went and who looked at me kindly, but did not communicate. I did not recognize them. As I got nearer to the end of the tunnel I seemed to be surrounded by a wonderful warm glowing light.”

The terminal loss of consciousness while dying during childbirth is always due to oxygen starvation caused by convulsions that can occur in late pregnancy (due to a toxic condition called “pre-eclampsia”), massive blood loss, heart failure, amniotic fluid embolus, hyperventilation, etc. An explanation of the sequence of events related by this woman is that she lost consciousness and all memory of events. Her resuscitation restored consciousness and her memory, but the oxygen supply to her retina was still insufficient to restore vision and normal sensory function. All this caused sensations of moving in a dark tunnel. Further restoration of the oxygen supply to her body restored central vision, which together with widened pupils caused her to sense movement toward a bright light at the end of a tunnel. Her recovery progressed, and retinal function was fully restored, but not normal vision or brain function—so she felt herself moving out of the tunnel to be enveloped by the light. Finally, delivery rooms are kept warm so the newly born babies do not cool down after birth. This was the warmth she felt.

Darkness, tunnel, and light experiences are wondrous, seemingly paranormal experiences. Nonetheless, it is evident that they can be explained by the body’s responses to oxygen starvation. The combination of tunnel and light experiences can only be explained by oxygen starvation, and nothing else. Other associated experiences, such as darkness and out-of-body experiences, can also be generated by other changes in body function induced by a wide range of different conditions.

My favourite bit, which pretty much sums up my feelings on the matter.

This explanation of tunnel-and-light experiences does not constitute conclusive proof that this is the only mechanism by which these experiences can arise. After all, this explanation does not preclude paranormal or immaterial explanations. But it is an alternative, provable physical explanation that accounts for all aspects of these experiences, as well as making it possible to predict when these experiences are likely to occur.