The 1990s research on place cells and the later discovery of grid cells showed that the mammalian brain builds internal maps by combining two kinds of signals: “where am I now” and “how far and in what direction have I moved.” This matters because navigation is not only about finding roads; it is a basic brain function that supports memory, planning, and decision-making.
Place cells were first reported in 1971 by John O’Keefe at University College London, when he recorded neurons in the hippocampus of rats. In the 1990s, improved recording methods and careful experiments strengthened the conclusion that these neurons code for location. A place cell increases its firing rate when an animal is in one specific region of an environment, called a place field. The same cell can stay quiet elsewhere, even when the animal is moving, sniffing, or turning. When the rat enters a different room or the cues change strongly, the hippocampus can “remap,” meaning many place cells change where they fire. This supported the idea of a cognitive map: the hippocampus does not just store a list of landmarks, but forms a flexible representation of space.
A key step in the 1990s was showing how place-cell activity depends on both external cues and self-motion. Experiments by O’Keefe and Lynn Nadel’s followers, and by other labs, separated visual landmarks from movement information. When landmarks were rotated, place fields often rotated with them, showing control by external cues. But when rats moved in the dark or when cues were reduced, place cells could still keep a stable pattern for a while, showing an internal updating process. This internal updating is called path integration: the brain estimates current position by adding up small movements over time, using signals related to speed and direction.
The later discovery of grid cells explained how path integration could be computed with high precision. In 2005, Edvard and May-Britt Moser and colleagues at the Norwegian University of Science and Technology reported grid cells in the medial entorhinal cortex, a major input to the hippocampus. A grid cell fires at many locations, but those locations form a regular pattern that looks like a hexagonal lattice across the floor. The spacing between firing fields can be tens of centimeters in a rat, and different grid cells have different spacings and orientations. This repeating structure suggested a built-in coordinate system that can measure distance and direction in a way that is not tied to one particular landmark.
Place cells and grid cells together clarified that “mapping” is distributed across a hippocampal–entorhinal network. The entorhinal cortex provides structured signals such as grid patterns, and the hippocampus uses them to create distinct place representations that can separate similar locations. This division of labor helps explain both stability and flexibility: grid-like codes support continuous tracking during movement, while hippocampal codes can rapidly change when context changes, like entering a new room. Other related cell types strengthened this picture. Head-direction cells, first described in the 1980s by James Ranck and later studied by Jeffrey Taube and others, fire when the animal’s head points in a particular direction, acting like a neural compass. Border cells, reported in 2008, fire near walls or edges, anchoring the map to boundaries. Speed-related signals also influence these circuits, helping update position during movement.
This research also connected navigation to memory in a concrete, testable way. The hippocampus is essential for episodic memory in humans, as shown by the well-known patient H.M., whose hippocampal damage caused severe inability to form new long-term memories. Place-cell activity provided a mechanism for how experiences could be organized by context: events happen somewhere, and “where” can serve as an index that links people, objects, and actions into a retrievable episode. In humans, brain imaging and intracranial recordings have found place-like and grid-like signals in the hippocampus and entorhinal cortex, supporting the idea that these systems are conserved across mammals.
The broader importance today is medical and technological. Early Alzheimer’s disease often affects the entorhinal cortex and hippocampus, and spatial disorientation is a common early symptom; understanding these circuits helps explain why navigation fails. In artificial intelligence and robotics, researchers use grid-like representations and path-integration ideas to build systems that navigate efficiently and generalize to new environments. The main lesson from the 1990s onward is that internal maps are not pictures in the brain; they are coordinated patterns of neural activity that can combine landmarks with self-motion to support both navigation and memory.