A powerful method has allowed McGovern researchers to discover how the brain represents the complex world in simple shapes.
一种强大的方法让麦戈文的研究人员能够发现大脑如何以简单的形状表示复杂的世界。
作者：Sabbi Lall，MIT 麦戈文大脑研究所
The world is constantly bombarding our senses with information, but the ways in which our brain extracts meaning from this information remains elusive. How do neurons transform raw visual input into a mental representation of an object — like a chair or a dog?
这个世界不断地用信息轰炸我们的感官，但我们的大脑从这些信息中提取意义的方式仍然难以捉摸。神经元是如何将原始的视觉输入转换成对物体的心理表征的——比如椅子或狗？
In work published in Nature Neuroscience, MIT neuroscientists have identified a brain circuit in mice that distills “high-dimensional” complex information about the environment into a simple abstract object in the brain.
在发表于《自然神经科学》（Nature Neuroscience）杂志上的一项研究中，麻省理工学院（MIT）的神经科学家在老鼠身上发现了一个大脑回路，它能将有关环境的“高维”复杂信息提炼成大脑中的一个简单抽象物体。
“There are no degree markings in the external world; our current head direction has to be extracted, computed, and estimated by the brain,” explains Ila Fiete, an associate member of the McGovern Institute and senior author of the paper. “The approaches we used allowed us to demonstrate the emergence of a low-dimensional concept, essentially an abstract compass in the brain.”
“外部世界没有程度标记；我们当前的头朝向必须由大脑来提取、计算和估计。“我们使用的方法让我们展示了一个低维度概念的出现，本质上是大脑中的一个抽象指南针。”
This abstract compass, according to the researchers, is a one-dimensional ring that represents the current direction of the head relative to the external world.
根据研究人员的说法，这个抽象的指南针是一个一维的圆环，代表着头部相对于外部世界的当前方向。
A circuit of thousands of neurons in the mammalian brain traces out a spare one-dimensional ring during complex navigation behaviors in 3-D space (blue), representing an internal compass. At any time, the circuit state is one dot along the compass, representing the present estimated direction of heading in the external world. During deep, non-REM sleep (yellow), the circuit traces out the same ring, but scaled differently in size, to define a whole cone.
哺乳动物大脑中由数千个神经元组成的回路在三维空间的复杂导航行为（蓝色）中描绘出一个备用的一维环，代表一个内部罗盘。在任何时候，环路状态都是沿着罗盘的一个点，表示当前外界估计的航向。在深度非快速眼动睡眠（黄色）中，回路会追踪出相同的圆环，但大小不同，以定义一个完整的锥体。
Schooling fish
鱼群
Trying to show that a data cloud has a simple shape, like a ring, is a bit like watching a school of fish. By tracking one or two sardines, you might not see a pattern. But if you could map all of the sardines, and transform the noisy dataset into points representing the positions of the whole school of sardines over time, and where each fish is relative to its neighbors, a pattern would emerge. This model would reveal a ring shape, a simple shape formed by the activity of hundreds of individual fish.
试图证明一个数据云有一个简单的形状，比如一个环，有点像观察一群鱼。通过追踪一条或两条沙丁鱼，你可能看不到一个模式。但如果你能映射所有沙丁鱼，并将这些嘈杂的数据集转换成表示整个沙丁鱼群随时间变化的位置的点，以及每条鱼与其邻居的相对位置，那么就会出现一种模式。这个模型将揭示一个环形，一个由数百条鱼的活动形成的简单形状。
Fiete, who is also an associate professor in MIT’s Department of Brain and Cognitive Sciences, used a similar approach, called topological modeling, to transform the activity of large populations of noisy neurons into a data cloud in the shape of a ring.
菲特，麻省理工学院大脑和认知科学系的副教授，她使用了一种类似被称为拓扑建模的方法，将大量嘈杂神经元的活动转化为环状的数据云。
Simple and persistent ring
简单而持续的环
Previous work in fly brains revealed a physical ellipsoid ring of neurons representing changes in the direction of the fly’s head, and researchers suspected that such a system might also exist in mammals.
之前对果蝇大脑的研究揭示了一个表示果蝇头朝向变化的椭圆形神经元环，研究人员怀疑这种系统可能也存在于哺乳动物中。
In this new mouse study, Fiete and her colleagues measured hours of neural activity from scores of neurons in the anterodorsal thalamic nucleus (ADN) — a region believed to play a role in spatial navigation — as the animals moved freely around their environment. They mapped how the neurons in the ADN circuit fired as the animal’s head changed direction.
在这项新的小鼠研究中，菲特和她的同事测量了动物在环境中自由活动时，大脑丘脑前庭核（ADN）中数十个神经元的神经活动时间。他们绘制了当动物头部改变方向时，ADN回路中的神经元是如何激活的。
Together, these data points formed a cloud in the shape of a simple and persistent ring.
这些数据点一起形成了一个简单而持久的环形云。
“This tells us a lot about how neural networks are organized in the brain,” explains Edvard Moser, director of the Kavli Institute of Systems Neuroscience in Norway, who was not involved in the study. “Past data have indirectly pointed towards such a ring-like organization, but only now has it been possible, with the right cell numbers and methods, to demonstrate it convincingly,” says Moser.
“这告诉我们很多关于神经网络在大脑中是如何组织的信息”，没有参与这项研究的挪威卡弗里系统神经科学研究所（Kavli Institute of Systems Neuroscience）所长爱德华·莫泽（Edvard Moser）解释说。莫泽说：“过去的数据间接指向了这样一个环状组织，但直到现在，通过正确的细胞数量和方法，才有可能令人信服地证明这一点。”
Their method for characterizing the shape of the data cloud allowed Fiete and colleagues to determine which variable the circuit was devoted to representing, and to decode this variable over time, using only the neural responses.
他们描述数据云形状的方法使菲特和同事们能够确定环路用来表示哪个变量，并且仅使用神经反应来解码这个变量随时间的变化。
“The animal’s doing really complicated stuff,” explains Fiete, “but this circuit is devoted to integrating the animal’s speed along a one-dimensional compass that encodes head direction. Without a manifold approach, which captures the whole state space, you wouldn’t know that this circuit of thousands of neurons is encoding only this one aspect of the complex behavior, and not encoding any other variables at the same time.”
“动物做的事情真的很复杂，”菲特解释说，“但是这个环路是用来整合动物沿着一个一维指南针走动时的速度，这个指南针可以编码头部方向。如果没有一种能够捕捉整个状态空间流形的方法，你就不会知道这个由数千个神经元组成的环路仅仅编码了复杂行为的这一方面，而没有同时编码任何其他变量。”
Even during sleep, when the circuit is not being bombarded with external information, this circuit robustly traces out the same one-dimensional ring, as if dreaming of past head-direction trajectories.
即使在睡眠中，当环路没有受到外部信息的轰击时，这个环路也能有力地追踪出相同的一维环，就好像梦到了过去的头朝向轨迹。
Further analysis revealed that the ring acts an attractor. If neurons stray off trajectory, they are drawn back to it, quickly correcting the system. This attractor property of the ring means that the representation of head direction in abstract space is reliably stable over time, a key requirement if we are to understand and maintain a stable sense of where our head is relative to the world around us.
进一步的分析表明，这个环起着吸引子的作用。如果神经元偏离了轨道，它们就会被拉回到轨道上，从而迅速修正系统。圆环的这种吸引子特性意味着，在抽象空间中头部方向的表示是可靠稳定的，如果我们想要理解和保持头部相对于周围世界的稳定感，这是一个关键的要求。
“In the absence of this ring,” Fiete explains, “we would be lost in the world.”
“如果没有这个环，”费特解释说，“我们就会迷失在这个世界上。”
Shaping the future
塑造未来
Fiete’s work provides a first glimpse into how complex sensory information is distilled into a simple concept in the mind, and how that representation autonomously corrects errors, making it exquisitely stable.
菲特的工作让我们第一次看到了复杂的感官信息是如何在头脑中被提炼成一个简单的概念，以及这种表达是如何自动纠正错误，使其极其稳定的。
But the implications of this study go beyond coding of head direction.
但这项研究的意义不仅限于头部方向的编码。
“Similar organization is probably present for other cognitive functions, so the paper is likely to inspire numerous new studies,” says Moser.
莫泽说：“类似的组织结构可能存在于其他认知功能中，所以这篇论文很可能会激发许多新的研究。”
Fiete sees these analyses and related studies carried out by colleagues at the Norwegian University of Science and Technology, Princeton University, the Weitzman Institute, and elsewhere as fundamental to the future of neural decoding studies.
菲特认为，挪威科技大学、普林斯顿大学、威茨曼研究所以及其他地方的同事所做的这些分析和相关研究，对于神经解码研究的未来是至关重要的。
With this approach, she explains, it is possible to extract abstract representations of the mind from the brain, potentially even thoughts and dreams.
她解释说，有了这种方法，就有可能从大脑中提取思维的抽象表征，甚至可能是思想和梦。
“We’ve found that the brain deconstructs and represents complex things in the world with simple shapes,” explains Fiete. “Manifold-level analysis can help us to find those shapes, and they almost certainly exist beyond head-direction circuits.”
“我们发现大脑以简单的形状解构并代表世界上复杂的事物，”菲特解释说。“流形分析可以帮助我们找到这些形状，它们几乎肯定存在于头朝向环路之外。”
新闻原文链接：http://news.mit.edu/2019/finding-the-brain-compass-0812