Hippocampal cells called "complex spike cells" have distinctive firing patterns that respond to where an animal is in its environment. These "place cells," first described in the rat hippocampus by O'Keefe and Dostrovsky (1971), produce action potentials when the animal is in a specific place, for example, in one corner of an enclosure. A place cell is active only in a restricted region, becoming electrically quiet when the animal moves out of that region. When the animal returns to the same region, the place cell resumes firing. Place cells thus have specific physical locations as their receptive fields, and different hippocampal place cells respond to different locations. O'Keefe and Burgess (1996) have shown how the geometric properties ofthe environment influence the firing patterns of place cells. Place cells respond to the distance of the animal from an edge and produce a graded firing pattern that varies with distance from the edge. By moving the walls ofan arena while recording
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