Intracranial studies of theta activity during episodic memory processing paint a complicated picture. Contrary to findings from scalp EEG/MEG, almost all intracranial studies of human memory report at least some decreases in theta power [32,37–44] associated with memory success, and many report exclusively decreases in theta power [35,45–51] (see Figure 2). These decreases are often accompanied by increases in high-frequency power (30+ Hz) so that the effect can be visualized as a “tilt” of the power spectrum during successful compared to unsuccessful memory operations (see Box 4 and Figure 3).The tilt is typically observed across widespread brain regions, including frontal, temporal, and medial temporal lobe. Furthermore, the tilt has been found during encoding and retrieval [48,51]. While this effect goes in the opposite direction of scalp studies, and undercuts the hypothesized role of theta in episodic memory formation, it is undeniable -- more than a dozen studies since 2007 have reported the spectral tilt during memory tasks, including high-powered studies with over 150 subjects [42,51]. We will later return to a discussion of differences between scalp and intracranial EEG that might explain these discordant findings (See “Why the scalp/invasive discrepancy?”).
