Place cells in the hippocampus fire when an animal is at a particular location — their "place field." The rate code (which cells fire) tells you where the animal is. But there's a second code layered on top: the hippocampus also oscillates at theta frequency (6–10 Hz), and when within the theta cycle a spike occurs carries additional information.
As an animal moves through a place field, the cell's spikes shift systematically to earlier phases of theta — entering the field at late phase, exiting at early phase. This is phase precession. Within a single theta cycle, a sequence of overlapping place cells fires in order from current position to future position: the cycle "sweeps" ahead. The temporal order of spikes within one cycle encodes trajectory, not just location.
rate code
Which cell fires? Each place cell has a receptive field — a region of space where it responds. The population of active cells tells you the animal's location with ~10–20cm resolution.
temporal code
When within theta? Late phase = just entered the field. Early phase = about to exit. The phase at which a cell fires encodes how far through the field the animal is — adding information the rate code alone doesn't carry.
theta sequences
Within one cycle, overlapping place cells fire in spatial order. Early-phase spikes are from cells whose fields lie
ahead of the current position; late-phase from fields
behind. One theta cycle (~125ms) encodes a ~1–2m trajectory compressed in time.
scatter (bottom panel)
The
phase-position plot shows this directly: each dot is one spike, its x-position = how far the animal was through that cell's field, its y-position = theta phase at that moment. The downward slope — backward precession — is the signature.