The hippocampus as a spatial map. Preliminary evidence from unit activity in the freely-moving rat.

See also: O'Keefe, "Place units in the hippocampus of the freely moving rat," Experimental Neurology, 51 (1976) 78–109.

O’Keefe and his student Jonathan Dostrovsky discovered place cells in the hippocampus, and that they show a specific kind of temporal coding in the form of theta phase precession. 

In 2014 O'Keefe shared the Nobel Prize in Physiology or Medicine with May-Britt Moser and Edvard I. Moser “for their discoveries of cells that constitute a positioning system in the brain.”