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

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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.