The role of mediotemporal theta activity in memory processing has long been established by rodent studies showing that LTP in the hippocampus is optimally elicited with stimulation at theta frequency and that discrete lesions in the medial septum that abolish theta produce severe learning/ memory deficits (for an overview see e.g., Vertes and Kocsis, 1997 or Axmacher et al., 2006). A recent study using intracranial recordings from patients with temporal lobe epilepsy performing a word list-learning paradigm showed a significant increase in rhinal-hippocampal theta coherence for subsequently remembered versus forgotten words (Fell et al., 2003). Given our present findings, we can hypothesize that the independent theta rhythms found in the entorhinal cortex and hippocampus may need to be actively synchronized in order to facilitate the synaptic plasticity involved in memory encoding. This hypothesis is further backed by the fact that 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
