Hippocampal 4–12 Hz rhythm, phase precession, and temporal sequence coding
The hippocampal theta rhythm (4–12 Hz in rodents, ~4–8 Hz in humans) is a prominent LFP oscillation generated by the medial septum. The key discovery of O'Keefe & Recce (1993) is phase precession: as an animal traverses a place field, the cell fires progressively earlier relative to the ongoing theta cycle. This means place cells fire at a slightly higher frequency (~8.5 Hz) than the LFP theta (~8 Hz), causing the firing phase to precess from late to early theta over ~360° as the animal crosses the field. This sweeping phase pattern creates a temporal sequence code where each theta cycle replays the animal's trajectory — a "theta sequence."