A role for oscillations in interregional communication has mostly been studied in terms of theta phase-phase relationships (i.e. phase locking or spectral coherence). Using scalp EEG, investigators have observed increased theta coherence between frontal and posterior electrodes during successful encoding of item-context pairs [63] and working memory [64] -- for a review, see [65]. Similarly, using iEEG, several studies have observed increased theta phase locking between a large set of brain regions during successful encoding [42,46,51,66] and retrieval [46,51] in free recall, as well as during encoding or retrieval of spatial or temporal relations between landmarks in a navigation task [55,67]. Furthermore, synchronized entrainment of theta oscillations in visual and auditory cortex using rhythmically modulated visual and auditory stimuli has been shown to improve performance in subsequent cued recall [68,69].
