We found only seven EEG/MEG studies that reported any decrease in theta power (including those that also show increases) for subsequently remembered items or during successful retrieval [21,31–35]. While most scalp EEG studies use either one or two common reference electrodes or average reference, [32] used a bipolar reference scheme. Since increases in theta power are often observed with a broad topography across the scalp, these effects might have been attenuated with a bipolar reference that acts as a spatial high-pass filter. Furthermore, task strategy and its interaction with effort or attention may also play a role; one study [31], for instance, explicitly instructed subjects to engage in certain mnemonic strategies (Loci or pegword method). We will return later to a more in-depth discussion of the potential relevance of these variables (see “Why the scalp/invasive discrepancy?” and “Low-frequency power decreases may be a general marker of activation”). In summary, evidence from non-invasive scalp EEG and MEG recordings seem to be broadly consistent with the hypothesis that theta oscillations facilitate successful memory operations (see Figure 2).
