疯狂体验：灵魂出窍，把身体扔一边

THE young man woke feeling dizzy. He got up and turned around, only to see himself still lying in bed. He shouted at his sleeping body, shook it, and jumped on it. The next thing he knew he was lying down again, but now seeing himself standing by the bed and shaking his sleeping body. Stricken with fear, he jumped out of the window. His room was on the third floor. He was found later, badly injured.　　In the 15 years since that dramatic incident, Brugger and others have come a long way towards understanding out-of-body experiences. They have narrowed down the cause to malfunctions in a specific brain area and are now working out how these lead to the almost supernatural experience of leaving your own body and observing it from afar. They are also using out-of-body experiences to tackle a long-standing problem: how we create and maintain a sense of self.
　　Dramatised to great effect by such authors as Dostoevsky, Wilde, de Maupassant and Poe - some of whom wrote from first-hand knowledge - out-of-body experiences are usually associated with epilepsy, migraines, strokes, brain tumours, drug use and even near-death experiences. It is clear, though, that people with no obvious neurological disorders can have an out-of-body experience. By some estimates, about 5 per cent of healthy people have one at some point in their lives.
　　People without any obvious neurological disorder can have an out-of-body experience
　　So what exactly is an out-of-body experience? A definition has recently emerged that involves a set of increasingly bizarre perceptions. The least severe of these is a doppelg?nger experience: you sense the presence of or see a person you know to be yourself, though you remain rooted in your own body. This often progresses to stage 2, where your sense of self moves back and forth between your real body and your doppelg?nger. This was what Brugger’s young patient experienced. Finally, your self leaves your body altogether and observes it from outside, often an elevated position such as the ceiling. "This split is the most striking feature of an out-of-body experience," says Olaf Blanke, a neurologist at the Swiss Federal Institute of Technology in Lausanne.
　　Surprisingly pleasant
　　Some out-of-body experiences involve just one of these stages; some all three, in progression. Bizarrely, many people who have one report it as a pleasant experience. So what could be going on in the brain to create such a seemingly impossible sensation?
　　The first substantial clues came in 2002, when Blanke’s team stumbled across a way to induce a full-blown out-of-body experience. They were performing exploratory brain surgery on a 43-year-old woman with severe epilepsy to determine which part of her brain to remove in order to cure her. When they stimulated a region near the back of the brain called the temporoparietal junction (TPJ), the woman reported that she was floating above her own body and looking down on herself.
　　This makes some kind of neurological sense. The TPJ processes visual and touch signals, balance and spatial information from the inner ear, and the proprioceptive sensations from joints, tendons and muscles that tell us where our body parts are in relation to one another. Its job is to meld these together to create a feeling of embodiment: a sense of where your body is, and where it ends and the rest of the world begins. Blanke and colleagues hypothesised that out-of-body experiences arise when, for whatever reason, the TPJ fails to do this properly (Nature, vol 419, p 269).
　　More evidence later emerged that a malfunctioning TPJ was at the heart of the out-of-body experience. In 2007, for example, Dirk De Ridder of University Hospital Antwerp in Belgium was trying to help a 63-year-old man with intractable tinnitus. In a last-ditch attempt to silence the ringing in his ears, Ridder’s team implanted electrodes near the patient’s TPJ. It did not cure his tinnitus, but it did lead to him experiencing something close to an out-of-body experience: he would feel his self shift about 50 centimetres behind and to the left of his body. The feeling would last more than 15 seconds, long enough to carry out PET scans of his brain. Sure enough, the team found that the TPJ was activated during the experiences.
　　Insights from neurological disorders or brain surgery can only take you so far, however, not least because cases are rare. Larger-scale studies are required, and to achieve this Blanke and others have used a technique called "own-body transformation tasks" to force the brain to do things that it seemingly does during an out-of-body experience. In these experiments, subjects are shown a sequence of brief glimpses of cartoon figures wearing a glove on one hand. Some of the figures face the subject, others have their back turned (see diagram). The task is to imagine yourself in the position of the cartoon figure in order to work out which hand the glove is on. To do this, you may have to mentally rotate you own body as one image succeeds another. As volunteers performed these tasks, the researchers mapped their brain activity with an EEG and found that the TPJ was activated when the volunteers imagined themselves in a position different from their actual orientation - an out-of-body position.
　　The team also zapped the TPJ with transcranial magnetic stimulation, a non-invasive technique that can temporarily disable parts of the brain. With a disrupted TPJ, volunteers took significantly longer to do the own-body transformation task (The Journal of Neuroscience, vol 25, p 550).
　　Other brain regions have been implicated too, including ones close to the TPJ. The emerging consensus is that when these regions are working well, we feel at one with our body. But disrupt them, and our sense of embodiment can float away.
　　This does not, however, explain the most striking feature of out-of-body experiences. "It’s a great puzzle why people, from their out-of-body locations, visualise not only their bodies but things around them, such as other people," says Brugger. "Where does this information come from?"
　　One line of evidence comes from the condition known as sleep paralysis, in which healthy people find their body immobilised as in sleep despite being conscious (see "The twilight zone"). In a survey of nearly 12,000 people who had experienced sleep paralysis, Allan Cheyne of the University of Waterloo in Ontario, Canada, found that many reported sensations similar to out-of-body experiences. These included floating out of their body and turning back to look at it.
　　Cheyne suggests that this might be the result of conflicts of information in the brain. During sleep paralysis, it is possible to enter a REM-like state in which you dream of moving or flying. Under these circumstances you are conscious of a sensation of movement, yet your brain is aware that your body cannot move. In an attempt to resolve this sensory conflict, the brain cuts the sense of self loose (Cortex, vol 45, p 201). "It resolves by splitting the self from its body," says Cheyne. "The self seems to go with the movement and the body gets left behind." Perhaps similar sensory conflicts cause classic out-of-body experiences.
　　The brain resolves sensory conflict by splitting the self from the body. The body gets left behind
　　Brugger, meanwhile, has a suggestion for how someone might see things even though their eyes are shut, based on one of his patients who reported an out-of-body experience. According to this patient’s father, who was sitting by the bedside, he had his eyes closed. Yet he later reported seeing, from a perspective above his bed, his father going to the bathroom, returning with a wet towel and towelling his forehead.
　　The patient presumably heard his father walk to the bathroom and run some water, and must have felt the wet towel on his head. Brugger speculates that his brain converted those stimuli into a visual image, not unlike what happens in synaesthesia. This still does not, however, explain the external vantage point. "It’s not clear how the brain constructs that," says cognitive philosopher Thomas Metzinger of the Johannes Gutenberg University in Mainz, Germany.
　　Metzinger does have a suggestion. Imagine an episode from a recent holiday. Do you visualise it from a first-person perspective, or from a third-person perspective with yourself in the scene? Surprisingly, most of us do the latter. "In encoding visual memories, the brain already uses an external perspective," says Metzinger. "We don’t know much about why and how, but if something is extracted from such a database [during an out-of-body experience], there may be material for seeing oneself from the outside."
　　Whatever the mechanism, the study of out-of-body experiences promises to help answer a profound question in neuroscience and philosophy: how does self-consciousness emerge? It’s abundantly clear to us that we have a sense of self that resides, most of the time, in our bodies. Yet it is also clear from out-of-body experiences that the sense of self can seemingly detach from your physical body. So how are the self and the body related?
　　To address that question, Metzinger has teamed up with Blanke and his colleagues in an experiment that induces an out-of-body experience in healthy volunteers. They film each volunteer from behind and project the image into a head-mounted display worn by the volunteer so that they see an image of themselves standing about 2 metres in front. The experimenters then stroke the volunteer’s back - which the volunteers see being done to their virtual self. This creates sensory conflict, and many reported feeling their sense of self migrating out of their physical bodies and towards the virtual one (Science, vol 317, p 1096).
　　To Metzinger, these experiments demonstrate that self-consciousness begins with the feeling of owning a body, but there is more to self-consciousness than the mere feelings of embodiment. "Selfhood has many components," says Metzinger. "We are trying to fill them in, building block by building block. This is just the beginning."

　　年轻人醒来时觉得头晕。他站起来，转身，发现自己依然躺在床上。他朝自己熟睡的身体大叫，摇晃它，并跳上去。接下来他感到自己又躺下了，但看到自己站在床边猛摇晃自己睡着的身体。他惊恐地从三楼房间窗户跳出去，后来人们发现了重伤的他。
　　这个21岁年轻人的经历正是灵魂出窍，一种奇特的意识状态。这很可能是由他的癫痫引起的（Journal of Neurology, Neurosurgery and Psychiatry, vol 57, p 838）。“他并不是想自杀，”年轻人的医生，瑞士苏黎世大学医院的神经心理学专家Peter Brugger说，“他跳窗是为了将自我和肉体联系起来。他一定是突然发病了。”
　　自那个戏剧性的事故后15年，Brugger和他的同伴做了许多努力，试图理解灵魂出窍体验。他们将起因缩小至大脑某特定区域的故障上，研究这种离开并从远处观察自己身体的近乎超自然的体验是怎么产生的。同时利用灵魂出窍体验解决一个存在已久的问题：我们是怎样产生并保持自我的。
　　陀斯妥也夫斯基、王尔德、莫泊桑和爱伦坡等作家都将在作品中夸张地描写过这种体验，他们中有人获得相关的第一手资料。这种体验多数与癫痫、偏头痛、中风、脑瘤、药品，甚至是濒死体验相关。但有一点很清楚，没有明显神经问题的人也可能出现灵魂出窍。约有5%的健康人有过这种体验。
　　没有明显神经问题的人也会有灵魂出窍体验
　　那么灵魂出窍体验究竟是什么？最近有一个定义，包含了一系列愈来愈奇特的感受。最轻的是分身体验：你感觉到一个你认识的人，或者看到他成为你，尽管你仍然在自己身体里。这个感受常会进展到第二阶段，你的自我在本体和分身之间来回。这正是Brugger的年轻病人的体验。最终，你的自我会完全离开你的身体，而从外面——通常是天花板一类的高处——看着它。位于洛桑的瑞士联邦科技学院的神经学家Olaf Blanke说：“这种分离是灵魂出窍最明显的特征。”
　　令人惊讶的愉快
　　有些灵魂出窍体验仅包含上述一个阶段，有些则是逐步体验所有三步。奇怪的是，许多体验者说他们感到很愉快。那么大脑是怎么产生这种看似不可能的体验的呢？
　　第一个重要线索发现于2002年，Blanke的团队偶然发现了诱发完全灵魂出窍体验的方法。当时他们正在给一位患有严重癫痫的43岁妇女进行脑手术，寻找需要切除的脑部分。当他们刺激靠近后脑颞部联合区（TPJ）的一块时，妇女说她感到自己飘出体外，俯视自己。
　　这很有意义。TPJ处理视觉和触觉信号，从内耳发来的平衡和空间信息，以及从关节、肌腱发来的告知身体部位位置的感觉。它负责将这些信息结合，创造出总感：你的身体在哪里，它与世界的边界在哪里。Blanke和他的同事假设，不论什么理由，当TPJ出错时，灵魂出窍体验就产生了(Nature, vol 419, p 269)。
　　之后有更多证据证明，出错的TPJ是灵魂出窍体验的核心。比如，2007年比利时安特卫普大学医院的Dirk De Ridder试图治疗63岁老人的严重耳鸣。经过一系列尝试，最后他的团队在病人的TPJ区域附近植入电极。这没能治好耳鸣，但令他产生了类似灵魂出窍的感觉：他感到自己移动到了身体左后方50厘米处。这个感觉持续了15秒多，足够对他的大脑进行PET扫描。Dirk的团队确定TPJ在体验产生时很活跃。
　　从神经错误方向开始的研究或大脑手术只能这样了，因为案例稀少。更大规模的研究是必要的，为此Blanke和他的团队使用了名为“自体改造”的技术来强迫大脑作出类似灵魂出窍时的反应。实验者向志愿者快速展示一系列戴着一只手套的卡通人物。其中一些面对志愿者，其他则是背对着。志愿者需要想像自己处在卡通人物的位置，以便弄明白手套戴在哪只手上。为此随着卡通人物一个接一个晃过，你需要转动自己的身体。当志愿者这样做时，研究员用EEG记录他们的大脑活动，发现当志愿者想像自己处在与实际位置不同的地方——即体外位置时，他们的TPJ很活跃。
　　研究员同样用经颅磁刺激来检验TPJ，这是一种无损害的暂时使大脑部分失效的技术。TPJ混乱后的志愿者在做换位想像时明显花费了更多时间（The Journal of Neuroscience, vol 25, p 550）。
　　大脑的其他部分也被牵连，包括离TPJ很近的一个区域。研究员一致认为，当这些区域良好运作时，我们感到在自己体内。但如果干扰它们，就会产生灵魂脱体感。
　　但这不能解释灵魂出窍体验最显著的特征。”有灵魂出窍体验的人不仅能看到自己的身体，还能看到周围的事物及人，这令人很困惑。“Brugger说，”这些信息是从哪里来的？“
　　另一个证据来自睡眠麻痹，睡后，有时健康的人们会发觉身体无法移动，尽管意识是清醒的 (详见 "The twilight zone")。调查了接近12000体验过睡眠麻痹的人之后，加拿大安大略省滑铁卢大学的Allan Cheyne发现许多人的体验接近灵魂出窍。包括意识脱离身体，并转身回望。
　　Cheyne认为这可能是大脑中信息的矛盾造成的。在睡眠麻痹中，你很可能进入了梦到移动或飞翔时的REM状态。在这种情况下，你会有移动感，但你的大脑很清楚你的身体没有动。为了解决这种矛盾，大脑会切断失控感（Cortex, vol 45, p 201）。“将自我从身体上切离，问题就解决了。”Cheyne说，“自我似乎在运动，而身体留在原处。”可能类似的感觉矛盾导致了典型的灵魂出窍体验。
　　大脑将自我切离身体来解决感觉矛盾。身体留在原地。
　　同时Brugger提出了假设，解释为什么有些人眼睛闭着却能看到东西，依据是一名有灵魂出窍体验的病人的报告。坐在床边的病人父亲说他的眼睛是闭着的。而他之后说他从床上方看到父亲走到洗手间，拿着湿毛巾回来并擦拭了他的额头。
　　可能这名病人听到父亲走进洗手间的声音和水声，然后感到额头上的潮湿感。Brugger猜测他的大脑可以将这些感觉转换为图像，正如牵连感觉。但这依然不能解释病人的外人视角。“我们依然不清楚大脑是怎么做到的。”德国美茵茨约翰内斯古滕贝格大学的认知哲学家Thomas Metzinger说。
　　Metzinger 有个假设。想像最近一个假期的片段，你是从第一人视角，还是包括你自己在画面中的第三人视角？令人惊讶的是，我们大部分人选择后者。“在编码图像记忆时，大脑已经使用了外人视角。”Metzinger说，“我们不知道为什么和怎样做到的，但如果大脑是从这样的数据库提取信息，（在灵魂出窍体验中）可能就有自己从外面看自己的素材了。”
　　不管是怎样的机制，灵魂出窍体验的研究将有助于回答一个神经学和哲学的深层问题：自我意识是怎么产生的？很清楚的是，自我意识大部分时间在我们身体里。但是在灵魂出窍体验中，自我意识会脱离身体。那么自我和身体是怎么关联的？
　　为了解答这个问题，Metzinger与Blanke及B的同事共同进行了一个实验，诱发健康的志愿者产生灵魂出窍感。他们从背后拍摄志愿者，将图像发送到志愿者戴在头上的显示屏上，这样志愿者就能看到自己站在前方2米左右。然后实验者拍志愿者的背——志愿者可以在显示屏上看到这个。这会造成感觉矛盾，许多志愿者报告说感到自己脱离了身体，向视线中的自己飘过去(Science, vol 317, p 1096)。
　　对Metzinger来说，这些实验证明了自我意识是由拥有一个身体而开始的，但是自我意识比单纯的感觉统一更复杂。“自我意识有许多构成，”Metzinger说，“我们正试图弄明白它们，一点点地构建整体。现在还只是开端。”