Only one study [56] observed exclusive increases in hippocampal and rhinal theta power using an SME contrast. These increases occurred in the pre-stimulus interval; theta power appears to fall below baseline in the period after stimulus onset but statistics are not reported. Other studies found a mix of increases and decreases in memory success contrasts. Notably, several of these report positive effects in a “low theta” range, either using statistics at the electrode level [37,41] or specifically localized to the posterior hippocampus [40]. However, in almost all studies, theta decreases were far more widespread and stronger than reported increases. For example, [43] found significant theta power increases prior to item retrieval, but these effects were isolated to the right temporal pole and evolved to widespread, bilateral decreases shortly before and during item retrieval. A similar study [42] found patches of theta power increases in the SME, localized to prefrontal and lateral temporal cortices, which vanished amid strong theta decreases (including MTL) after 500 ms post-stimulus onset. Similarly, [32] found small but significant increases in frontal theta only within 500 ms
