麻醉藥幫助我們理解什麼是意識大綱

More than a decade ago, a 43-year-old woman went to a surgeon for a hysterectomy. She was put under, and everything seemed to be going according to plan, until, for a horrible interval, her anesthesia stopped working. She couldn't open her eyes or move her fingers. She tried to breathe, but even that most basic reflex didn't seem to work; a tube was lodged in her throat. She was awake and aware on the operating table, but frozen and unable to tell anyone what was happening.

十多年前的一天，外科醫生們正在對一名43歲的女性進行子宮切除術。患者在麻醉下失去了意識，所有的一切似乎都按部就班有條不紊——直到她身上的麻醉突然失效了一段時間，她無法睜開眼睛，無法動一動手指。她試圖呼吸，可即使是這樣最基本的反射也不能正常進行；一根管子卡在她的喉嚨裏。她的神智清醒着，知道自己正在手術檯上，只是一動也不能動，不能向任何人訴說發生了多麼可怕的事情。

Studies of anesthesia awareness are full of such horror stories, because administering anesthesia is a tightrope walk. Too much can kill. But too little can leave a patient aware of the procedure and unable to communicate that awareness. For every 1,000 people who undergo general anesthesia, there will be one or two who are not as unconscious as they seem — people who remember their doctors talking, and who are aware of the surgeon's knife, even while their bodies remain catatonic and passive. For the unlucky 0.13 percent for whom anesthesia goes awry, there's not really a good preventive. That's because successful anesthetization requires complete unconsciousness, and consciousness isn't something we can measure.

在關於麻醉覺醒(anesthesia awareness)的研究裏，充滿了此類恐怖故事，這是由於給予麻醉的確是一項令麻醉醫師如履薄冰的棘手任務。用藥過量可以致人於死地。但藥量不足卻可能使患者在手術中醒來，而無法向其他人訴說自己的困境。每1000名接受全身麻醉的患者中，平均會有一或兩人(0.13%)不幸並非如表面所見的那樣不省人事——他們記得醫生們的交談，感覺得到外科醫生的刀劃過血肉，儘管此時他們的身體仍然毫無知覺，被動地任人擺弄。目前還沒有什麼真正有效的方法可以防止麻醉過程中出現這種岔子。因爲，成功的麻醉需要實現完全無意識，而我們現有的技術還無法對意識加以衡量。

There are tools that anesthesiologists use to get a pretty good idea of how well their drugs are working, but these systems are imperfect. For most patients receiving inhaled anesthesia, they're no better at spotting awareness than dosing metrics developed half a century ago, says George Mashour, a professor of anesthesiology at the University of Michigan Medical School. There are two intertwined mysteries at work, Mashour told me: First, we don't totally understand how anesthetics work, at least not on a neurological basis. Second, we really don't understand consciousness — how the brain creates it, or even what, exactly, it is.

現在已經有一些工具可以幫助麻醉醫師較好地把握自己所用藥物的效力，但這些系統還不夠完善。密歇根大學醫學院(University of Michigan Medical School)的麻醉學教授喬治·馬舒爾(George Mashour)表示，對於大多數接受吸入麻醉的患者而言，尚無比半個世紀前研發的用藥劑量指標效果更好的知覺觀察方法。馬舒爾還告訴我，在麻醉工作中存在着兩個相互交織的未解謎團：首先，我們並不完全瞭解麻醉的工作機制，至少在神經學基礎的層面上是如此。其次，我們對意識也沒有實現真正的理解——大腦如何創造了意識，以及，“意識”的確切定義究竟是什麼。

Lacking a way to measure consciousness directly, anesthesiologists monitor for proxies of it — the presence of certain types of brain waves, physical responses and sensitivity to pain — and adjust the dosage if they arise. To improve on this method, neuroscientists are searching for what they call neural correlates of consciousness — changes in brain function as a person transitions from being apparently conscious to apparently unconscious. The more they know about these, the better they hope to understand what consciousness is.

由於無法直接對意識加以測量，麻醉師們只好退而求其次，監測意識的替代物——特定類型的腦電波、生理反應和疼痛敏感度的存在情況，並在上述指標升高時增加麻醉劑量。此外，爲了改善這一方法，神經科學家正試圖尋找“意識的神經機制”(neural correlates of consciousness)，即，人在從明顯的意識知覺狀態轉變爲明顯無意識狀態的過程中腦功能的改變。他們認爲，對此瞭解得越深入，就越有希望解開意識之謎。

Michael Alkire, associate professor of anesthesiology at the University of California, Irvine, was one of the first people involved in the search for neural correlates of consciousness, back in the 1990s. He's particularly excited now about a study published in August by an international team of researchers based at the University of S?o Paulo and the University of Wisconsin, Madison. They compared the brain activity of patients from the full spectrum of consciousness — awake, asleep, drugged with anesthetics, in comas or suffering from "locked-in syndrome," in which the body appears trapped in a comalike state but the brain is active and aware. The researchers stimulated these subjects' brains with a magnetic field and used EEG to trace the pulse's path. The brains we might think of as conscious and those we think of as unconscious reacted to the stimulus in distinct ways. "If the patient is awake, the electrical ‘ping' can travel all around the brain," Alkire said. "But if they're unconscious, the ‘ping'tends to stay localized and just fades away like a sonar blip."

關於意識的神經機制的研究興起於20世紀90年代。美國加州大學歐文分校(University of California, Irvine)的麻醉學副教授邁克爾·阿爾基爾(Michael Alkire)是最早參與此類研究的科學家之一。八月份由聖保羅大學(University of S?o Paulo)和威斯康星大學麥迪遜分校(University of Wisconsin, Madison)的研究人員組成的國際研究團隊發表了一項研究，令他格外興奮。該研究比較了患者在整個意識譜系——醒覺、睡眠、接受麻醉劑後、昏迷或罹患“閉鎖綜合徵”（此時，患者的軀體陷入類似昏迷的狀態，但大腦仍在活動且醒覺）——的多種狀態下大腦活動的異同。研究人員採用磁場來刺激受試者們的大腦，並使用腦電圖(EEG)跟蹤了腦電脈衝的路徑。結果顯示，在我們通常認定爲有意識或無意識的大腦中，這些刺激引起的反應方式截然不同。“如果患者處於醒覺狀態，電脈衝信號可以傳遍整個大腦，”阿爾基爾說。“但是，如果他們失去了意識，脈衝信號往往只在局部短暫停留，然後便如曇花一現般消失無蹤。”

This finding excites Alkire because it bolsters an existing theory of how consciousness works. Mashour, who also studies neural correlates of consciousness, has repeatedly found evidence that — contrary to conventional- wisdom — sensory networks in the brains of unconscious people remain locally functional, but intrabrain communication has broken down. The neighborhood's lights are on, in other words, but the Internet and phone lines have all been cut.

這些研究結果令阿爾基爾十分振奮，因爲它很好地支持了一個現有的意識運作理論。在馬舒爾對意識的神經機制進行研究的過程中，與傳統觀點相背離的證據一再出現，這些證據表明，在無意識者的大腦中，感覺網絡仍在局部地發揮作用，只是大腦內部的通信系統完全崩壞了。換句話說，鄰近街區的燈照常亮着，可互聯網和電話線都被切斷了。

The S?o Paulo-Madison study could be showing that unconsciousness is what happens when different parts of the brain can't connect: The signal simply dies. This also suggests that anesthetics work best when they cut those lines of communication. What's more, it provides insight into a vexing question: How can the entirety of human experience arise from tiny electrical impulses?

這項聖保羅-麥迪遜研究顯示，無意識是大腦的不同部位間失去聯繫的外在表現：只是信號無法傳遞而已。它還表明，當麻醉劑恰好切斷那些通信線路時，麻醉效果最好。更重要的是，該研究爲我們探討一個深奧的問題提供了啓示：人類的體驗作爲一個整體，是如何從微小的電脈衝中誕生的？

Neuroscientists do know that consciousness does not reside in any one part of the brain — there is no region where all information is aggregated together — but they don't know much more than that. Consciousness is difficult to study by its very nature, so it has been left mostly to philosophers for the last hundred years — and they don't agree on much. For instance, the philosopher John Searle describes consciousness as a purely subjective experience — what you have when you wake up in the morning, and what disappears when you fall asleep at night (or die, depending on how bad your day was). The philosopher Daniel Dennett wrote a book called "Consciousness Explained," which plays down the subjective experience of consciousness so much that critics dubbed it "consciousness explained away."

神經科學家已然確知，意識並不存在於大腦的任何一個特定部位——並沒有一個腦區專司彙總所有的信息——但他們所知的也僅限於此。意識的本質使其難以進行直接研究，因此，在過去的一百年中這一領域的絕大部分都是哲學家們的舞臺，可惜他們各執己見，沒有達成多少一致意見。例如，哲學家約翰·塞爾(John Searle)將意識描述成了一種純粹的主觀體驗——當你在早晨醒來時，你就獲得了意識；等你在夜間入睡（或者死亡，這取決於你這一天過得怎麼樣）時它便消失了。而在哲學家丹尼爾·丹尼特(Daniel Dennett)撰寫的名爲《意識的解釋》(Consciousness Explained)論著中，則大大貶低了意識的主觀體驗，以至有評論家戲稱這本書“把意識給解釋沒了”。

Scientists largely ignored these sorts of debates for most of the 20th century. But in 1994, an interdisciplinary conference at the University of Arizona brought them together for the first time. That conference led to ongoing research studying the links between anesthesia and consciousness. Stuart Hameroff, an anesthesiologist and the director of the school's Center for Consciousness Studies, was an organizer of the conference. Anesthesia, he told me, is a great example of why scientists have to think about consciousness. It's not enough, he said, just to assume your patient is unconscious because she doesn't respond to pain.

在20世紀的絕大部分時間裏，科學家們在很大程度上一直對這些爭論視若無睹。但是，1994年美國亞利桑那大學(University of Arizona)舉行的一場跨學科會議第一次將科學家和哲學家們聚在了一起。這次會議引發人們就麻醉和意識之間的聯繫展開了持續的研究。該校意識研究中心(Center for Consciousness Studies)的主任斯圖爾特·哈姆魯夫(Stuart Hameroff)是這次會議的組織者。他告訴我，麻醉是可解釋科學家們爲何需要思考意識問題的絕佳例子。他說，單憑患者無法對疼痛作出反應並不足以判定他已經失去了意識。

That's because, while you need a brain to have consciousness, you might not need a brain to experience pain. In the 1990s, scientists ran tests on rats and goats, studying how the effects of anesthetics changed as different parts of the brain were intentionally damaged or removed. The amount of the drugs necessary to prevent the animals from moving in response to pain didn't change as the cortex, the thalamus and the brain stem were destroyed. "Turns out, they were measuring a spinal-cord reflex, which is a much more primitive thing and has nothing to do with consciousness," Hameroff said. Outward signs of consciousness may or not may not have anything to do with actual awareness.

這是因爲，雖然保持意識需要大腦，但感受痛苦卻未必需要大腦的參與。20世紀90年代，科學家們在大鼠和山羊身上進行試驗，他們故意損傷或去除了不同的大腦部位，然後研究麻醉劑的作用發生了怎樣的改變。當皮層、丘腦和腦幹受損後，要防止動物因疼痛而掙扎所需的藥物劑量並沒有發生改變。“事實證明，他們之前檢測的只是脊髓反射，這是一種非常原始的反應，跟意識沒有任何關係，”哈姆魯夫說。意識的外在表現與真正的覺醒狀態之間可能存在關聯，也可能完全是兩碼事。

In everyday life, it's nearly impossible to ever know whether someone is conscious or not, Hameroff said, even if it seems glaringly obvious. Philosophers are fond of pointing out that, for all you know, you're surrounded by people who appear to be fully conscious but who experience none of it subjectively. (They frequently trot out these beings for thought experiments and call them "philosophical zombies.") But for those under anesthesia and the knife, such sophistry offers little solace.

在日常生活中，想要確知某個人的意識狀態幾乎是不可能的，哈姆魯夫說，儘管它似乎是如此顯而易見。哲學家們很喜歡這樣講：你只知道你周圍的人看起來都具有完整的意識，但沒有一個人對此有任何主觀體驗。（他們經常在這些人面前炫耀所謂的“思想實驗”，並稱他們爲“哲學殭屍”。）只是對於那些身在麻醉和手術刀下的患者而言，這樣的詭辯起不到什麼寬慰效果。

Increasingly, research on what happens to the brain under anesthesia suggests that the synthesis and integration of information among many different parts of the brain is the best measure of consciousness. Some people, Mashour said, go so far as to say that this communication among regions is consciousness itself. Our subjective experience of the world might arise as a byproduct of the brain's piecing together different sensory inputs.

有越來越多關於麻醉狀態下大腦功能的研究表明，對來自大腦多個不同部位的信息加以綜合的能力是衡量意識的最佳指標。馬舒爾指出，有些人甚至認爲這種不同腦區之間的通訊交流就是意識本身。我們對世界的主觀體驗很可能是大腦試圖拼湊不同的感覺輸入信息時產生的副產物。

It's easy to see the connections between this idea and the "ping" study. When the brain falls asleep, drifts into a coma or comes under the influence of anesthetic drugs, the ability to synthesize information disappears, though the brain doesn't cease to function. Figuring out a method for measuring intrabrain communication will be crucial for preventing operating-table awareness.

很容易看出，上述觀點與腦電脈衝研究之間存在一定關聯。當大腦陷入熟睡、昏迷或麻醉藥物的影響之下時，雖然它並沒有停止運作，但其綜合信息的能力卻消失了。找出一種可測定腦內通訊的方法對於防止患者在手術檯上醒覺至關重要。

In the June 2013 issue of the journal Anesthesiology, Mashour proposed just that: a monitor that focuses on the brain's ability to communicate within itself. It's similar to the "ping" study but tracks a different signal. Activity in conscious brains has been shown to loop between sensory areas (the visual cortex in the rear of the brain, for example) and the higher-level parts of the brain associated with processing information (like the temporal lobe, just behind your ears). Mashour and others call this "recurrent processing": Signals travel from the sensory areas to the processing areas and back again. When somebody is unconscious, the recurrent proc-essing disappears. Mashour's study showed that this pattern — or lack thereof — is present in the brains of people anesthetized with three different classes of drugs. It's not just a side effect of one kind of medication. His work suggests that anesthesia monitors might be more effective if, rather than measuring the presence of electrical waves produced bythe brain, they monitor how electrical signals move around the brain.

在2013年6月的《麻醉學》(Anesthesiology)雜誌上，馬舒爾建議：使用一臺監測儀來專門監測大腦內部的通訊能力。這與腦電脈衝研究有點類似，但它們跟蹤的是不同的信號。研究顯示，在意識清醒的大腦中，感覺區（例如，大腦後部的視覺皮層）與信息加工相關的高級區域（比如位於耳後側的顳葉）之間存在腦部活動環路。馬舒爾等稱其爲“回返加工”：信號從感覺區進入加工區，然後再返回感覺區。在人失去意識後，這種“回返加工”也隨之煙消雲散。馬舒爾的研究表明，在三種不同類別藥物的麻醉下，人的大腦中都呈現了這種模式的存在和缺失。這並不是某一種藥物的副作用。這項工作表明，如果麻醉監測器能夠監測電信號如何在大腦之中傳送，可能會比單純測定大腦是否產生電波更爲有效。

Should such a device be developed, it would be good news not only for those 0.13 percent of patients but also for fans of Enlightenment philosophy. Mashour told me that the synthetic model of consciousness dates back to Immanuel Kant — his "Critique of Pure Reason" might be vindicated by neuroscience. "Kant said there has to be some process that takes individual processing and connects it together into a unified experience," Mashour said. "Over the years, we've teased out the parts of the brain necessary for appreciating vision, color, motion. They're all mediated by different brain areas. But how does the brain put all that together into single perception?" For Kant, this was clear long before EEG monitors and anesthetics. "Without this synthesis," he wrote in 1781, "the manifold would not be united in one consciousness."

如果能夠開發這樣的設備，對於那0.13%的不幸患者以及啓蒙哲學的粉絲們都是大好消息。馬舒爾告訴我，意識的綜合模式最早可以追溯到伊曼紐爾·康德(Immanuel Kant)——他的《純粹理性批判》(Critique of Pure Reason)或可因神經科學的發展而獲得平反。“康德認爲，必有一定的過程將單獨的信息整理加工，使其彼此聯繫並結合爲一體，即成爲經驗，”馬舒爾說。“多年來，我們已經逐步確定景象、色彩和運動的評鑑由大腦的不同區域所介導。然而，大腦是如何將它們整合爲統一的感知印象的？”康德所處的時代遠在腦電監測儀及麻醉劑的發明和使用之前，1781年，他寫道，很明顯，“如果沒有這種綜合作用，多姿多彩而散亂的感覺信息將不可能在一個意識中實現統一。”