In 1971, John O'Keefe and Jonathon Dostrovsky reported the discovery of place cells in the rat hippocampus—cells that fire action potentials when an animal passes through a specific small region of space, which is called the place field of the cell. This discovery, although controversial at first, led to a series of investigations that culminated in the 1978 publication of a book by O'Keefe and his colleague Lynn Nadel called "The Hippocampus as a Cognitive Map" (a phrase that also appeared in the title of the 1971 paper)—the book argued that the hippocampal neural network instantiates cognitive maps as hypothesized by the psychologist Edward C. Tolman. This theory aroused a great deal of interest, and motivated hundreds of experimental studies aimed at clarifying the role of the hippocampus in spatial memory and spatial navigation.