How do we know where we are without a phone map? In 1971, a scientist in London found brain cells in rats that became active only in one part of a room. He called them place cells.

In 2005, scientists in Norway found grid cells. They fire at many points that make a pattern of triangles on the floor. Some of these scientists won the Nobel Prize in 2014.

Even without GPS, we can sense where we are in a room. In 1971, O'Keefe and a colleague in London recorded cells in the hippocampus of rats moving freely. Some cells fired only when a rat was in a particular spot, and O'Keefe called them place cells. Over 30 years later, a Norwegian team led by the Mosers found grid cells in a nearby area.

When the spots where they fired were marked on the floor, they formed a grid of triangles, like graph paper. Grid cells seem to measure distance and direction, while place cells mark specific locations. Similar cells have been found in humans who played navigation games before brain surgery.

Even with our eyes closed, we can usually point toward the door of a familiar room, because the brain maintains its own positioning system. Its discovery began in 1971, when O'Keefe and Dostrovsky recorded hippocampal neurons in freely moving rats and found cells that fired only when the animal occupied a particular location; O'Keefe named them place cells. In 2005, a Norwegian team led by the Mosers, a husband and wife, identified grid cells in the neighboring entorhinal cortex. Each fires at multiple locations that form a lattice of equilateral triangles, as if the floor were covered with graph paper.

Grid cells are thought to calculate distance and direction from self-motion, while place cells represent specific positions; other cells encode head direction or nearby walls. O'Keefe and the Mosers shared the 2014 Nobel Prize. Evidence in humans has followed: epilepsy patients navigating a virtual town revealed place-responsive cells, fMRI detected grid-like signals, and direct recordings identified grid-like neurons. But the "GPS" label is only a metaphor, since the brain estimates position from movement and landmarks and can drift in darkness, and most human data come from virtual environments and surgical patients.