环球科学：大脑暗能量
最新研究颠覆了我们对大脑的一贯看法：当我们休息或发呆时，大脑并未停止运行，一些神秘的“背景”神经活动始终存在。
大脑的默认模式
● 神经科学家长期认为，当我们休息时，大脑中的神经回路处于关闭状态。
●然而，大脑成像实验表明，大脑内存在一些持续的“背景活动”。
●这种默认模式能为人们对未来事件作出反应做好准备。
●默认模式相关大脑区域间的神经链接出错，可能导致从阿尔茨海默病到抑郁症的一系列疾病。
撰文 马库斯·E·雷切尔(Marcus E. Raichle)
翻译 冯泽君
你躺在屋外的摇椅上打盹儿，腿上放着一本杂志。突然，一只苍蝇停在胳膊上，你拿起杂志去拍它。在苍蝇停在你身上之后，你的大脑里发生了什么？在 停下来之前呢？长期以来，神经科学家一直认为，人在休息时，大脑中的神经回路基本处于关闭状态。从这个意义上说，此时的神经活动属于“随机噪声”，就像没 收到信号的电视机显示的雪花状图案。而当苍蝇停在你的胳膊上时，大脑恢复意识，准备执行“拍蝇任务”。但最近的神经成像研究揭示了一个完全不一样的事实： 当人们躺着休息时，大脑并未闲着，很多重要的神经活动仍在进行。
现已证实，当我们的大脑在休息时——比如坐在椅子上发呆、躺在床上睡觉，或接受了麻醉，各个脑区之间仍在不停地传递信息。这种不间断的信息传递 被称作大脑的默认模式，它所消耗的能量是我们拍打苍蝇，或有意识地对其他外在刺激作出反应时所耗能量的20倍。实际上，我们有意识去做的大多数事件，比如 吃饭和演讲等，都是对大脑默认模式下基准神经活动的背离。
理解大脑默认模式的关键，是要找到此前不为人知的大脑系统——默认模式神经网络(default mode network，DMN)。在组织神经活动的过程中，默认模式神经网络到底发挥了怎样的作用，现在仍在研究当中，但我们知道，大脑在形成记忆，组织其他各 种需要为未来事件做准备的神经系统时(比如感觉到苍蝇停在胳膊上就下意识地去拍打，这个动作就需要大脑的运动系统随时做好准备)，可能就是采用默认模式神 经网络预先设定好的方式。在使脑区行为同步方面，默认模式神经网络可能也发挥了重要作用——让各个脑区就像赛跑运动员一样，在发令枪打响的那一刹那，都处 于合理的“预备”状态。如果默认模式神经网络确实在为大脑的有意识活动做准备，那么研究这个网络的行为，也许能让科学家找到一些线索，揭示意识体验的本 质。另外，神经科学家还推测，默认模式神经网络遭到破坏，可能会引起精神错乱，以及从阿尔茨海默病到抑郁症的一系列复杂大脑疾病。
寻找暗能量
“大脑始终处于活跃状态”并非新观点。脑电图(electroencephalogram，记录大脑电活动的波形图)的发明者汉斯·伯格 (Hans Berger)就是这个观点的支持者。1929年，他在一系列开创性论文中，根据仪器检测到的不间断脑电波推测，“中枢神经系统始终处于相当活跃的状态， 而不仅仅是在人们清醒的时候”。
但伯格关于大脑如何工作的观点一直没有引起科学界的重视，甚至在非侵害性成像技术成为神经科学实验室的常规手段后仍是如此。上世纪70年代末， 正电子断层扫描技术(positron-emission tomography，PET)问世，它可以测量大脑中的葡萄糖代谢率、血流量和氧摄取量，在一定程度上，这些指标能反映大脑的神经活动水平；1992 年，功能性磁共振成像技术(magnetic resonance imaging，fMRI)诞生，通过测量大脑耗氧量，它也能帮助科学家实现同样的目的。尽管这些技术都不仅限于测量大脑活动，但大多数实验设计都在无意 间给人留下了这样一个印象：大部分脑区平常都很“安静”，直到需要执行某项具体任务时才会活跃起来。
一般来说，在做成像实验时，神经科学家都会想方设法地确定，产生特定知觉或与某种行为相关的是哪些脑区。而找到这些脑区的最好方式，就是在两种 相关状态下，直接比较大脑活动有何不同。如果研究人员想要知道，哪些脑区对“读单词”这一行为比较重要，他们就会比较在大声读单词 (试验组)和默默地看同一组单词(对照组)时，受试者的大脑成像图有何差异。为了准确找出这种差异，研究人员必须从读单词受试者的大脑成像图上，消除看单 词受试者大脑成像图上的内容。经过处理，成像图上仍显示为活跃状态的脑区的神经元活动，可能就是读单词这一行为所必需的。在这种情况下，大脑的基本活动， 也就是始终存在的“背景”神经活动都会被消除。以这种方式得到的实验结果很容易让人认为，只有在执行特定任务时，大脑的“开关”才被打开，而在其他时候， 大脑都处于非活跃状态。
人们在休息或发呆时，大脑里面到底发生了什么？过去几年，我们和其他一些研究小组对这个问题产生了极大的兴趣，因为多项研究都暗示，在这种状态下，大脑中存在一定程度的背景活动。
只须对大脑成像图进行肉眼观测，就能找到大脑背景活动存在的证据：无论来自对照组还是试验组，大脑成像图总是显示，多个脑区都处于相当忙碌的状 态。由于都存在背景“噪声”，通过肉眼观察原始图像，我们几乎不可能从两类大脑成像图上找出差别，而要完成这一任务，只有利用计算机进行精密的图片分析。
进一步分析发现，在执行特定任务时，大脑消耗能量的上升幅度不会超过基础神经活动的5%。在神经回路中，大部分神经活动都与外部事件无关，这些 活动消耗的能量占大脑总消耗能量的60%～80%。因此我们借鉴天文学家的说法，把这些固定存在的神经活动称为大脑的暗能量——看不见的暗能量占据了宇宙 中物质能量的绝大多数。
我们推测大脑暗能量可能存在的另一个理由是，研究发现只有极少的感官信息能够真正抵达大脑的中枢处理区域。视觉信息从眼睛传向视觉皮层的过程中，信号强度会大幅衰减。
我们周围存在无数信息，每秒约有上百亿比特的信息抵达视网膜，但与之相连的视觉输出神经连接只有100万个，每秒钟视网膜传向大脑的信息只有600万比特，最终能到达视觉皮层的信息只有1万比特。
经过进一步处理，视觉信息才能进入负责产生意识知觉的脑区。令人惊讶的是，最终形成意识知觉的信息每秒钟不足100比特。如果这些是大脑所能利用的全部信息，如此少的信息量显然不大可能形成知觉，因此固定存在的大脑神经活动必定在此过程中发挥了某种作用。
神经突触的数量也暗示大脑暗能量可能存在。突触是神经元间的连接点。在视觉皮层中，负责传递视觉信息的突触数量还不到全部突触的10%。因此，大部分突触肯定是用于建立视觉皮层内部神经元间的联系。
马库斯·E·雷切尔是美国华盛顿大学圣路易斯分校医学院的放射学和神经病学教授。多年来，他带领的研究小组一直通过正电子断层扫描和功能性磁共振成像技术研究人脑功能。 1992年，他入选美国医学院院士，4年后又成为美国科学院院士。
本文译者：
冯泽君，上海复旦大学神经生物学博士，主要从事大脑功能相关研究，长期翻译科技类文章，已在《环球科学》杂志上发表多篇译文。
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