Hippocampal place cells are topographically organized, but physical space has nothing to do with it

Abstract

Topographical organization can be found in many areas of the cerebral cortex, although its presence in higher order cortices is debated. Some studies evaluated whether this pattern of organization is present in the hippocampus, trying to determine whether hippocampal place cells are organized around a topographical map of space. Those studies indicated that the topographical organization of hippocampal place cells is either very limited or simply nonexistent. In this paper, we argue for a different interpretation of available evidence and suggest that there is a topographical organization in hippocampal place cells, but the topographical map formed is not a map of the physical space. Although place cell firing is correlated with the animal’s position and is important to spatial navigation, place cells encode much more information than just location. Thus, we should not expect the topographical map to be organized around physical space, but around an abstract, multidimensional space containing the receptive fields of place cells. We show that this conclusion is supported by two of the main theories of hippocampal function–cognitive map theory and index theory–which, when carefully analyzed, make exactly the same predictions about hippocampal topography. Such abstract topographical map would be extremely hard to find using the methods commonly employed in the literature, but there are some approaches that may, in the future, make possible to characterize the topographical organization in the hippocampus and other high-order brain regions.