in the rodent hippocampus the theta phase at which a stimulus arrives determines the direction of plasticity, i.e., whether it results in long term potentiation or depression. This effect as been shown both for artificially induced theta activity in vivo (Pavlides et al., 1988) and in vitro (Huerta and Lisman, 1995) as well as in behaving animals (Holscher et al., 1997; Hyman et al., 2003). In this context it is conceivable that synchronization of theta activity between the hippocampus and entorhinal cortex creates a slowly modulated facilitating state that provides the temporal basis for encoding or retrieval of separate items (Hasselmo et al., 2002; Jensen and Lisman, 1998; Rizzuto et al., 2006). Such a synchronization process could, e.g., be instantiated by a simultaneous phase reset of ongoing oscillatory activity in both regions, driven by sensory inputs (Givens, 1996; McCartney et al., 2004). In a recent study we demonstrated such a stimulus-induced theta phase reset in humans during a continuous word recognition paradigm. This reset occurred approximately 190 ms after stimulus onset and was observed simultaneously in the hippocampus and rhinal cortex (Mormann et al., 2005).
