瘋狂體驗：靈魂出竅，把身體扔一邊

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."

　　年輕人醒來時覺得頭暈。他站起來，轉身，發現自己依然躺在牀上。他朝自己熟睡的身體大叫，搖晃它，並跳上去。接下來他感到自己又躺下了，但看到自己站在牀邊猛搖晃自己睡着的身體。他驚恐地從三樓房間窗戶跳出去，後來人們發現了重傷的他。
　　這個21歲年輕人的經歷正是靈魂出竅，一種奇特的意識狀態。這很可能是由他的癲癇引起的（Journal of Neurology, Neurosurgery and Psychiatry, vol 57, p 838）。“他並不是想自殺，”年輕人的醫生，瑞士蘇黎世大學醫院的神經心理學專家Peter Brugger說，“他跳窗是爲了將自我和肉體聯繫起來。他一定是突然發病了。”
　　自那個戲劇性的事故後15年，Brugger和他的同伴做了許多努力，試圖理解靈魂出竅體驗。他們將起因縮小至大腦某特定區域的故障上，研究這種離開並從遠處觀察自己身體的近乎超自然的體驗是怎麼產生的。同時利用靈魂出竅體驗解決一個存在已久的問題：我們是怎樣產生並保持自我的。
　　陀斯妥也夫斯基、王爾德、莫泊桑和愛倫坡等作家都將在作品中誇張地描寫過這種體驗，他們中有人獲得相關的第一手資料。這種體驗多數與癲癇、偏頭痛、中風、腦瘤、藥品，甚至是瀕死體驗相關。但有一點很清楚，沒有明顯神經問題的人也可能出現靈魂出竅。約有5%的健康人有過這種體驗。
　　沒有明顯神經問題的人也會有靈魂出竅體驗
　　那麼靈魂出竅體驗究竟是什麼？最近有一個定義，包含了一系列愈來愈奇特的感受。最輕的是分身體驗：你感覺到一個你認識的人，或者看到他成爲你，儘管你仍然在自己身體裏。這個感受常會進展到第二階段，你的自我在本體和分身之間來回。這正是Brugger的年輕病人的體驗。最終，你的自我會完全離開你的身體，而從外面——通常是天花板一類的高處——看着它。位於洛桑的瑞士聯邦科技學院的神經學家Olaf Blanke說：“這種分離是靈魂出竅最明顯的特徵。”
　　令人驚訝的愉快
　　有些靈魂出竅體驗僅包含上述一個階段，有些則是逐步體驗所有三步。奇怪的是，許多體驗者說他們感到很愉快。那麼大腦是怎麼產生這種看似不可能的體驗的呢？
　　第一個重要線索發現於2002年，Blanke的團隊偶然發現了誘發完全靈魂出竅體驗的方法。當時他們正在給一位患有嚴重癲癇的43歲婦女進行腦手術，尋找需要切除的腦部分。當他們刺激靠近後腦顳部聯合區（TPJ）的一塊時，婦女說她感到自己飄出體外，俯視自己。
　　這很有意義。TPJ處理視覺和觸覺信號，從內耳發來的平衡和空間信息，以及從關節、肌腱發來的告知身體部位位置的感覺。它負責將這些信息結合，創造出總感：你的身體在哪裏，它與世界的邊界在哪裏。Blanke和他的同事假設，不論什麼理由，當TPJ出錯時，靈魂出竅體驗就產生了(Nature, vol 419, p 269)。
　　之後有更多證據證明，出錯的TPJ是靈魂出竅體驗的核心。比如，2007年比利時安特衛普大學醫院的Dirk De Ridder試圖治療63歲老人的嚴重耳鳴。經過一系列嘗試，最後他的團隊在病人的TPJ區域附近植入電極。這沒能治好耳鳴，但令他產生了類似靈魂出竅的感覺：他感到自己移動到了身體左後方50釐米處。這個感覺持續了15秒多，足夠對他的大腦進行PET掃描。Dirk的團隊確定TPJ在體驗產生時很活躍。
　　從神經錯誤方向開始的研究或大腦手術只能這樣了，因爲案例稀少。更大規模的研究是必要的，爲此Blanke和他的團隊使用了名爲“自體改造”的技術來強迫大腦作出類似靈魂出竅時的反應。實驗者向志願者快速展示一系列戴着一隻手套的卡通人物。其中一些面對志願者，其他則是背對着。志願者需要想像自己處在卡通人物的位置，以便弄明白手套戴在哪隻手上。爲此隨着卡通人物一個接一個晃過，你需要轉動自己的身體。當志願者這樣做時，研究員用EEG記錄他們的大腦活動，發現當志願者想像自己處在與實際位置不同的地方——即體外位置時，他們的TPJ很活躍。
　　研究員同樣用經顱磁刺激來檢驗TPJ，這是一種無損害的暫時使大腦部分失效的技術。TPJ混亂後的志願者在做換位想像時明顯花費了更多時間（The Journal of Neuroscience, vol 25, p 550）。
　　大腦的其他部分也被牽連，包括離TPJ很近的一個區域。研究員一致認爲，當這些區域良好運作時，我們感到在自己體內。但如果幹擾它們，就會產生靈魂脫體感。
　　但這不能解釋靈魂出竅體驗最顯著的特徵。”有靈魂出竅體驗的人不僅能看到自己的身體，還能看到周圍的事物及人，這令人很困惑。“Brugger說，”這些信息是從哪裏來的？“
　　另一個證據來自睡眠麻痹，睡後，有時健康的人們會發覺身體無法移動，儘管意識是清醒的 (詳見 "The twilight zone")。調查了接近12000體驗過睡眠麻痹的人之後，加拿大安大略省滑鐵盧大學的Allan Cheyne發現許多人的體驗接近靈魂出竅。包括意識脫離身體，並轉身回望。
　　Cheyne認爲這可能是大腦中信息的矛盾造成的。在睡眠麻痹中，你很可能進入了夢到移動或飛翔時的REM狀態。在這種情況下，你會有移動感，但你的大腦很清楚你的身體沒有動。爲了解決這種矛盾，大腦會切斷失控感（Cortex, vol 45, p 201）。“將自我從身體上切離，問題就解決了。”Cheyne說，“自我似乎在運動，而身體留在原處。”可能類似的感覺矛盾導致了典型的靈魂出竅體驗。
　　大腦將自我切離身體來解決感覺矛盾。身體留在原地。
　　同時Brugger提出了假設，解釋爲什麼有些人眼睛閉着卻能看到東西，依據是一名有靈魂出竅體驗的病人的報告。坐在牀邊的病人父親說他的眼睛是閉着的。而他之後說他從牀上方看到父親走到洗手間，拿着溼毛巾回來並擦拭了他的額頭。
　　可能這名病人聽到父親走進洗手間的聲音和水聲，然後感到額頭上的潮溼感。Brugger猜測他的大腦可以將這些感覺轉換爲圖像，正如牽連感覺。但這依然不能解釋病人的外人視角。“我們依然不清楚大腦是怎麼做到的。”德國美茵茨約翰內斯古滕貝格大學的認知哲學家Thomas Metzinger說。
　　Metzinger 有個假設。想像最近一個假期的片段，你是從第一人視角，還是包括你自己在畫面中的第三人視角？令人驚訝的是，我們大部分人選擇後者。“在編碼圖像記憶時，大腦已經使用了外人視角。”Metzinger說，“我們不知道爲什麼和怎樣做到的，但如果大腦是從這樣的數據庫提取信息，（在靈魂出竅體驗中）可能就有自己從外面看自己的素材了。”
　　不管是怎樣的機制，靈魂出竅體驗的研究將有助於回答一個神經學和哲學的深層問題：自我意識是怎麼產生的？很清楚的是，自我意識大部分時間在我們身體裏。但是在靈魂出竅體驗中，自我意識會脫離身體。那麼自我和身體是怎麼關聯的？
　　爲了解答這個問題，Metzinger與Blanke及B的同事共同進行了一個實驗，誘發健康的志願者產生靈魂出竅感。他們從背後拍攝志願者，將圖像發送到志願者戴在頭上的顯示屏上，這樣志願者就能看到自己站在前方2米左右。然後實驗者拍志願者的背——志願者可以在顯示屏上看到這個。這會造成感覺矛盾，許多志願者報告說感到自己脫離了身體，向視線中的自己飄過去(Science, vol 317, p 1096)。
　　對Metzinger來說，這些實驗證明了自我意識是由擁有一個身體而開始的，但是自我意識比單純的感覺統一更復雜。“自我意識有許多構成，”Metzinger說，“我們正試圖弄明白它們，一點點地構建整體。現在還只是開端。”