Introduction to the special issue on place cells.

In 1971, John O’Keefe and Jonathan Dostrovsky discovered place cells, and the hippocampal world has not been the same since. In the nearly 30 years since that discovery, hundreds of studies have been published, and thousands of words have been written, yet full understanding of the function of these cells and the role they collectively play in the hippocampus, remains somewhat elusive. The collection of papers in this special issue of Hippocampus provides a sense of current thinking about place cells, and amply demonstrates the absence of complete consensus. This is not surprising, given there is no generally accepted view of the function of the hippocampus containing these cells. However, much progress has been made over the years, and there is broad agreement on a framework for understanding the hippocampus and its collaborating neighbors. Within this broadly agreed-upon framework, differences of opinion remain. As usual, the devil is in the details. Perhaps the most important detail concerns the essential ‘‘spatial’’ nature of these cells. Are they always and only about space? If so, why do they sometimes appear to fire in non-spatial situations? Are they about space in all species, or is there something specially spatial about rats that distinguishes them from primates, especially humans? What do collections of these cells do? Form maps, generate routes, integrate paths, establish relations that could be non-spatial as well as spatial, store episodes? In this Introduction, we seek, in separate comments, to address some of these concerns in the context of the articles comprising this issue. While we have rather different views, we are in complete agreement on the virtues of bringing a range of disparate approaches together in Hippocampus. We trust that our readership will find the ensuing ideas enlightening.