Is the cognitive map flat? A neurobiological study of spatial encoding in three dimensions.

目前大脑研究中最紧迫的问题之一是大脑如何形成世界的心理地图：这是在复杂的世界中活动时能够正常运作的必要工具。研究表明，这种心理地图，即所谓的“认知地图”，依赖于大脑深处被称为海马体结构的结构网络，海马体本身是其中的中心。有趣的是，这个海马体网络对于形成和存储生活事件的记忆也至关重要，这使得神经科学家认为大脑使用其认知地图作为其所有记忆的组织者。这些功能的重要性和相互关联性在神经网络的损伤中表现得很明显，比如阿尔茨海默病，患者的第一个症状通常是迷路，最终导致严重的健忘症。神经科学家用许多方法研究这个网络的功能，但最有用的方法之一是单神经元记录技术，在动物（通常是大鼠，最近是小鼠）的大脑中无痛地引入精细的微电线，以便记录动物探索周围世界时这些区域的脑细胞（神经元）的活动。由于所有哺乳动物的大脑都有相同的基本计划，我们也从这些研究中了解了很多关于人类大脑的知识。对海马体神经元活动的观察表明，当动物在一个特定的地方时，它们特别活跃，因此它们被称为“位置细胞”。每个位置细胞都有自己喜欢的位置，而位置细胞是如何“知道”动物在那个位置的，这个问题一直很有趣。最近，随着在位置细胞上游的大脑区域发现了“网格细胞”，它又向前迈进了一大步。网格细胞的作用就像微型里程表，它们以一种非常规则的方式标出距离，产生类似地图网格的活动模式（因此得名）。网格细胞很重要，因为它们揭示了认知地图的基本结构。迄今为止，对位置和网格细胞的研究主要集中在它们在二维平面世界中的反应。然而，世界当然是三维的，表现三维比表现二维要复杂得多，因为动物可以在三维空间中扭曲和转动，这使得大脑很难跟踪方向。我们已经开始观察当动物爬到第三个垂直维度时，位置细胞和网格细胞是如何反应的，并且发现，令人惊讶的是，网格细胞的距离测量特性似乎并没有延伸到垂直维度。这就好像一个网格细胞并不“知道”老鼠有多高——推而广之，整个认知地图可能也不知道这一点。因此，这意味着认知地图可能是“平坦的”。从表面上看，这个结论似乎令人惊讶，因为我们肯定有一种主观感觉，即我们拥有一张完整的三维空间地图。然而，这种感觉可能是虚幻的，而本项目旨在找出是否是这种情况。我们将记录大鼠和小鼠探索不同环境时的位置和网格细胞，以便发现细胞是否对高度敏感，或者地图是否真的是平坦的，以及动物是否能够以表明它们知道3D空间位置的方式进行导航。事实上，有可能认知地图确实是3D的，但我们之前的实验并没有看到这一点，因为我们使用的仪器种类有限。回答认知地图是二维还是三维的问题对于理解我们的空间感非常重要。这不仅适用于那些试图理解大脑如何代表世界的科学家，也适用于那些设计供人类探索的3d结构的人，包括建筑师、空间站和3d虚拟现实的设计师。

One of the current most pressing questions in brain research concerns how the brain forms a mental map of the world: a necessary tool in order to be able to function normally while moving around in a complex world. Research has shown that this mental map, the so-called "cognitive map", depends on a network of structures deep inside the brain known as the hippocampal formation, of which the hippocampus itself is central. Interestingly, this hippocampal network is also critical for forming and storing memory for life events, leading neuroscientists to think that the brain uses its cognitive map as an organiser for all its memories. The importance and interrelatedness of these functions is evident in damage to this network, such as occurs in Alzheimer's disease, in which the first complaint of patients is often getting lost, and the culmination of which is profound amnesia.Neuroscientists study the functioning of this network in a number of ways, but one of the most useful has been the technique of single neuron recording, where fine microwires are painlessly introduced into the brains of animals (usually rats, and more recently mice), in order to record the activity of brain cells (neurons) in these areas as the animal explores the world around. Since all mammal brains have the same basic plan, we also learn much about the human brain from these studies. Observation of the activity of hippocampal neurons has revealed that they are particularly active when the animal is at a particular place, hence their name "place cells". Each place cell has its own preferred place, and the question of how a place cell "knows" the animal is at that place has been of great interest. It recently took a great leap forward with the discovery of "grid cells", in a brain area immediately upstream of the place cells. Grid cells act like tiny odometers, in that they mark out distances in a very regular way, producing activity patterns that resemble the grid of a map (hence their name). Grid cells are important because they reveal the basic structure of the cognitive map.Work on place and grid cells to date has focused on how they respond in a two-dimensional, flat world. However, the world is of course three-dimensional (3D), and it transpires that representing three dimensions is far more complicated than representing two, because animals can twist and turn within 3D space, making it very hard for the brain to keep track of orientation. We have begun to look at how place and grid cells respond when animals climb into the third, vertical dimension and have found that, amazingly, the distance-measuring properties of grid cells do not seem to extend into the vertical dimension. It is as if a grid cell does not "know" how high the rat is - and by extension, the cognitive map as a whole may not know this either. The implication is therefore that the cognitive map may be "flat".This conclusion seems superficially surprising because we certainly have the subjective feeling that we possess an integrated 3D map of space. However, this feeling may be illusory, and the present project intends to find out if this is the case. We will record place and grid cells as rats and mice explore various environments, in order to find out whether the cells are sensitive to height or whether the map really is flat, and whether animals can navigate in ways that suggest they know about locations in 3D space. It may be, in fact, that the cognitive map really is 3D but that our previous experiments did not see this because of the restrictive kinds of apparatus that were used. Answering the question of whether or not the cognitive map is 2- or 3D is of great importance in understanding our sense of space. This is true not only for scientists who seek to understand how the brain represents the world, but also for those who design 3d structures for humans to explore, including architects, and designers of space stations and of 3D virtual realities.