Forgetting does not necessarily reflect failure to encode information but can, to some extent, also be voluntarily controlled. Previous studies have suggested that voluntary forgetting relies on active inhibition of encoding processes in the hippocampus by the dorsolateral prefrontal cortex (DLPFC) [1-4]. During attentional and sensorimotor processing, enhanced DLPFC theta power alongside increased alpha/beta oscillations are a neural signature of an inhibitory top-down mechanism, with theta oscillations reflecting prefrontal control and alpha/beta oscillations occurring in areas targeted by inhibition [5-12]. Here, we used intracranial EEG recordings in presurgical epilepsy patients implanted in DLPFC (n = 13) and hippocampus (n = 15) during an item-method directed forgetting paradigm. We found that voluntary forgetting is associated with increased neural oscillations in the low theta band (3-5 Hz) in DLPFC and in a broad theta/alpha/beta (6-18 Hz) frequency range in hippocampus. Combining time-lagged correlation analysis, phase synchronization, and Granger causality in 6 patients with electrodes in both DLPFC and hippocampus, we obtained converging evidence for a top-down control of hippocampal activity by the DLPFC. Together, our results provide strong support for a model in which voluntary forgetting relies on enhanced inhibition of the hippocampus by the DLPFC.
Besides its relevance for declarative memory functions, hippocampal activation has been observed during disambiguation of uncertainty and conflict. Uncertainty and conflict may arise on various levels. On the perceptual level, the hippocampus has been associated with signaling of contextual deviance and disambiguation of similar items (i.e., pattern separation). Furthermore, conflicts can occur on the response level. Animal experiments showed a role of the hippocampus for inhibition of prevailing response tendencies and suppression of automatic stimulus-response mappings, potentially related to increased theta oscillations (3-8 Hz). In humans, a recent fMRI study demonstrated hippocampal involvement in approach-avoidance conflicts. However, the more general significance of hippocampal activity for dealing with response conflicts also on a cognitive level is still unknown. Here, we investigated the role of the hippocampus for response conflict in the Stroop task by combining intracranial electroencephalography (iEEG) recordings from the hippocampus of epilepsy patients with region of interest-based fMRI in healthy participants. Both methods revealed converging evidence that the hippocampus is recruited in a regionally specific manner during response conflict. Moreover, our iEEG data show that this activation depends on theta oscillations and is relevant for successful response conflict resolution.
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