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, the tendency to successively remember items experienced in temporal proximity; and forward-asymmetry, a bias for forward-transitions [6]. In essence, the stream of sensory inputs to the brain is compressed by the theta rhythm, allowing MTL circuitry to form associations between sequential inputs. Over time and repeated encounters of the same stimuli in different sequences, this mechanism may lead to the formation of “cognitive maps” that reflect long-standing associations between all kinds of stimuli (such as places or concepts; see Box 2) [5,7].
