The observed gap in inter-structural synchrony implies the existence of independent delta/theta rhythms in the human MTL, produced by autonomous generators. Regarding the potential generating mechanisms of these rhythms, findings from rodent electrophysiology have provided multiple indications of local theta generators in different subregions of the hippocampus as well as in the entorhinal cortex (Acker et al., 2003; Alonso and Llinas, 1989; Giocomo et al., 2007). The interplay between these subregions, however, is complex and not entirely understood. Perhaps the most important findings in this context were derived from recordings in awake rats, where partial coherence analyses of hippocampal recordings provided evidence for at least two hippocampal theta generators in the hippocampus of which only one is mediated by inputs from the entorhinal cortex (Kocsis et al., 1999). In the light of this study, the two independent theta rhythms found in humans could thus be interpreted such that the entorhinal rhythm may represent intrinsic entorhinal rhythmicity mediated by sensory and other cortical inputs, while the hippocampal rhythm could reflect theta activity generated autonomously within the hippocampus itself, mediated by inputs
