The basic phase precession phenomenon is apparent when you look at the timing of place cell spikes relative to the hippocampal theta rhythm:
(Figure from Malhotra et al. 2012) Each spike is assigned a color based on the /phase/ of the theta rhythm (a component of the LFP or “local field potential”) at which it occurs. Note the smooth color change as the animal runs through the place field.
Thus, when looking at a typical place cell like the above, phase precession manifests as (1) a relationship between *location* on the maze and the theta phase of a place cell's spikes, as shown when plotting spikes “in space” and (2) a relationship between *time* and the theta phase of a place cell's spikes, as shown when plotting spikes “in time” aligned to the LFP.
When a rat runs through a place field, space and time will be correlated, so it is not a priori clear if either one is somehow “primary”. There are numerous papers that explore this issue.
An important conceptual step is to extend this idea to what happens in a population of place cells which are all phase precessing:
(Figure from Skaggs & McNaughton 1996, which is well worth reading)
Note how in each theta cycle, there is now a *sequence* of place cells occurring!