Over 35 years ago John O’Keefe discovered place cells in the hippocampus and, along with Lynn Nadel, proposed the idea that the hippocampal formation was the location of the brain’s “cognitive map”. We will start with an exploration of the allocortical structures that compose the hippocampal formation and contrast the organization of allocortex with neocortex. Next we will explore the three cell types that support the navigational hypothesis. First, the place cells discovered by O’Keefe are pyramidal cells in CA1 and CA3 that exhibit “location-specific firing”. A typical place cell will fire in one or two regions of an environment and be silent in other locations. Place cells are driven by a combination of sensory cues and path integration. Next, Jim Ranck discovered head-direction cells in presubiculum. Later these cells were found in widespread regions including the anterior nucleus of the thalamus, retrosplenial cortex, lateral mammillary nucleus and entorhinal cortex —  a set of structures resembling the original “Papez circuit”. A typical head-direction cell will fire when the rat’s nose is pointed in a compass direction, for example North West, with an angular range of about 90°.  The head-direction system is considered to be a circular (1D) attractor, driven by the vestibular system and prevented from drift by visual cues. The grid cells of entorhinal cortex are a third cell type. Grid cells, discovered by the Moser group, act like place cells in exhibiting location-specific firing, but rather than having a single firing field, firing-bumps are distributed in a regular hexagonal pattern. The spatial regularity of bumps suggest that firing is driven by path integration. There is rough agreement on the general function of the three cell types: Place cells segregate the world into environments and identify locations within an environment, head-direction cells orient the spatial framework and serve as navigational guides, and grid cells provide a metric for translations that updates the animal’s location on its “map” and calculates optimal paths between map points. We will explore possibilities of how the hippocampal network performs these navigational computations.

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