Oscillations in the “Other Cortex”: Navigation in Real and Memory Space
The hippocampus and associated structures are organized in multiple loops, with reciprocal connections to the neocortex. The most prominent collective pattern of hippocampal neurons is theta oscillation. In one-dimensional tasks, pyramidal cells fire maximally at particular positions, signifying the place field center. The assembly members that define the current location also contribute spikes to the representation of past and future positions in multiple theta cycles. Similarly, positional “distances” among items of an episodic list can be coded by the synaptic strengths between the cell assemblies. In two-dimensional environments, exploration leads to crossing the same positions from different directions. These junctions serve to establish a map and subsequent landmark (map-based) navigation. The hallmark of the cognitive map is the presence of omnidirectional place cells in the hippocampus and tessellating “grid cells” in the entorhinal cortex. Neuron members of an omnidirectional or explicit assembly collectively define or symbolize the semantic “meaning” of an item.
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