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The medial entorhinal cortex, populated by grid cells, projects both directly and indirectly to the CA1 and CA3 cortices, the sites of hippocampal place cells. The study by Ormond and McNaughton (1) investigates a potential mechanism by which grid cells of the medial entorhinal cortex (MEC) exert their influence on the formation of firing fields of place cells. Specifically, the authors propose a “Fourier hypothesis” by which the spatial frequencies from several grid cell modules converge on the place-cell layer, molding the contours of firing fields to a composite of those several frequencies; Ormond and McNaughton tested the theory by inactivating specific input components and obtained fascinating results, largely supporting the model. The work (1) is best understood in the context of the broad “hippocampal cognitive map” project. In 1971, O’Keefe and Dostrovsky, while recording single neurons in the hippocampus of freely moving rats, discovered cells that fired when the animal crossed a restricted region of space; these neurons were called “place cells” (2). A typical place cell will fire in one location, termed the “place field” (Fig. 1A). Soon after, John O’Keefe and Lynn Nadel set the framework of the project with the publication of The Hippocampus as a Cognitive Map (3). The book set out a bold theory that the hippocampus was a map, performing cognitive functions critical for navigation and other cognitive operations including episodic memory. The book also presented an explicit neural model for the creation of place cells. As noted in the book, virtually all of the processing layers were unexplored, and the model highly speculative. The theory set out a goal of understanding how neural dynamics produces cognition (Fig. 1B). (A) Firing-rate maps for a place cell recorded over 15 min from a rat in a cylinder (Left) and a grid cell recorded …

Do the spatial frequencies of grid cells mold the firing fields of place cells?