In subsequent decades, a vast theoretical and empirical enterprise has grown around the theta rhythm. It appears in several mammalian species, including humans, and is most commonly observed during active exploration. Theta phase has been linked to the firing of MTL neurons that represent specific locations in space. The finding that hippocampal place cells fire at progressively earlier phases of the theta rhythm as an animal traverses a cell’s firing field [4] inspired a powerful conceptualization of MTL function [5]. In this framework, the ongoing theta rhythm forms a consistent reference for distributed cellular activity, allowing for coding of information not only in firing rates but also in spike-phase relations. Furthermore, the systematic co-activation of sequentially visited places allows for spike-timing dependent plasticity (STDP) to strengthen associations between sequentially activated cells with a bias for forward-associations (see Box 2 and Figure 1). This mechanism does not have to be specific to spatial memory but may be important for establishing temporal associations between arbitrary stimuli. And indeed, it can be related to two hallmark findings of episodic free recall: temporal contiguity,
