Place cells: How your brain creates maps of abstract spaces

Hippocampal “place cells” don’t just light up when an animal is somewhere—they can be manipulated to change behavior, providing direct evidence that these neurons help build the brain’s internal maps of space. In classic recordings from rats, individual hippocampal pyramidal neurons fire at specific locations, creating a tiled representation of where the animal is within an environment. The key distinction is that a “place cell” is the neuron, while its “place field” is the region of the outside world that reliably drives that neuron’s spikes.

Place fields are not limited to flat ground. Experiments with freely flying bats show that place fields can form roughly spherical patterns in three-dimensional space, and rats navigating cubic mazes represent the full volume, including vertical movement. This matters because it suggests the hippocampal circuitry supports navigation in the real geometry animals experience, not just in simplified 2D layouts.

The strongest causal evidence comes from virtual reality experiments. Mice run on a spherical treadmill while a computer updates a surrounding visual scene, producing a convincing illusion of movement through a virtual track. Even though the animal’s body stays fixed, hippocampal activity still forms place fields that tile the virtual space. Researchers then used targeted stimulation to force certain neurons to spike at the wrong times. Stimulating neurons that normally fire near the end of the track—while the mouse is still far away—can trigger the mouse to start licking for reward as if it had already reached the end. Conversely, stimulating neurons that normally fire near the start can make the mouse “believe” it is earlier in the track, sometimes causing it to run past the virtual boundary, like crashing through a wall. The behavioral shift links hippocampal place-cell activity to the construction of the cognitive map rather than mere correlation.

Once place fields exist, the next question is how they change. Hippocampal representations “remap” across environments: a neuron that fires in one location in one setting may fire elsewhere—or not at all—in a different setting. The extent of remapping depends on how similar two environments are. Large changes like repainting walls and moving furniture can reorganize many place fields, while subtle changes such as adding a small picture may leave most fields intact. In some cases, remapping follows environmental transformations: rotating a cue card in a cylindrical chamber can rotate place fields while preserving their relative structure, and altering wall aspect ratios can stretch place fields with the box.

Remapping also responds to non-spatial information. Changing odor or inducing fear through mild electric shocks can cause place cells to shift even when the chamber itself stays the same—turning a familiar “restaurant” map into something drastically altered after a traumatic event. This points to a broader role: place-cell activity carries information not only about position, but also about context and events tied to that position.

That broader role challenges the term “place cells.” In a sound-frequency joystick task, hippocampal neurons show firing peaks for preferred frequencies and near-silent activity for distant ranges, behaving like place cells but in an abstract acoustic space. The hippocampus appears to track continuous variables relevant to the task—spatial coordinates in navigation, sound frequency in the joystick experiment—suggesting a flexible mapping system that can support memory and planning beyond physical location.