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关于大脑如何形成世界的心理地图：这是一个必要的工具，以便能够在复杂的世界中正常活动。研究表明，这一心理图谱，即所谓的“认知图谱”，依赖于大脑深处被称为海马体结构的结构网络，而海马体本身就是其中的中心。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是至关重要的，这让神经学家认为大脑将其认知地图作为所有记忆的组织者。这些功能的重要性和相互关联性在对这个网络的破坏中显而易见，例如阿尔茨海默病，患者的第一个主诉经常是迷路，最终导致严重的健忘症。神经科学家用多种方法研究这个网络的功能，但最有用的技术之一是单神经元记录技术，即将细微电线无痛地引入动物(通常是大鼠，最近是小鼠)的大脑中，以记录动物探索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许多关于人类大脑的知识。对海马神经元活动的观察表明，当动物处于特定位置时，海马神经元尤其活跃，因此他们被命名为位置细胞。每个地方细胞都有自己喜欢的地方，而地方细胞如何“知道”动物在那个地方的问题一直是人们非常感兴趣的。最近，随着“网格细胞”的发现，它又向前迈进了一大步。“网格细胞”位于Place细胞上游的大脑区域。网格单元的作用就像微小的里程计，因为它们以非常规则的方式标记距离，产生类似于地图网格的活动模式(因此而得名)。网格细胞很重要，因为它们揭示了认知地图的基本结构。到目前为止，位置和网格细胞的研究重点是它们在二维、平面世界中的反应。然而，世界当然是三维的(3D)，事实证明，表示三维要比表示二维复杂得多，因为动物可以在3D空间内扭曲和旋转，这使得大脑很难跟踪方向。我们已经开始观察当动物攀登到第三个垂直维度时，位置和网格细胞如何反应，并发现令人惊讶的是，网格细胞的测距特性似乎并没有延伸到垂直维度。这就好像一个网格细胞不知道老鼠有多高--推而广之，认知地图作为一个整体可能也不知道这一点。因此，这意味着认知地图可能是“平面的”。这一结论表面上似乎令人惊讶，因为我们肯定有一种主观感觉，即我们拥有一张完整的3D空间地图。然而，这种感觉可能是虚幻的，本项目打算找出情况是否如此。我们将记录大鼠和小鼠探索各种环境时的位置和网格细胞，以找出这些细胞是否对高度敏感，或者地图是否真的是平的，以及动物是否能够以表明它们知道3D空间中位置的方式导航。事实上，认知地图可能真的是3D的，但我们之前的实验没有看到这一点，因为使用的设备种类有限。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是非常重要的。这不仅对试图了解大脑如何代表世界的科学家来说是正确的，对那些设计3D结构供人类探索的人也是如此，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.