Interactions between hippocampus, prefrontal cortex and visual association cortex during short-term memory maintenance

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Visual short-term memory (STM) refers to the maintenance of visual information for a short time interval. Key brain regions of visual STM include the prefrontal cortex and parietooccipital regions. Furthermore, increasing evidence suggests that the hippocampus is recruited when multiple items need to be maintained; however, it is still unclear if and how STM-related mechanisms in hippocampus, prefrontal cortex and parietooccipital regions are interrelated. Maintenance of specific items can be investigated by multivariate pattern classification analyses (MVPA) which can be applied to both EEG and fMRI data. However, no previous study has investigated stimulus-specific reactivation of multiple items during STM. Furthermore, it is unknown if and how hippocampus and prefrontal cortex interact with regions supporting stimulus-specific representations, and if they control these representations. Here, we will address these open questions. According to an influential computer model, STM depends on a multiplexing buffer in which items are consecutively reactivated during subsequent phase ranges of neural oscillations in the theta (3-8 Hz) frequency range. Specifically, we will thus test the hypothesis that hippocampus and/or prefrontal cortex control representations of specific items via oscillatory interactions with distributed activity patterns in visual association areas. We will investigate two groups of participants: Presurgical epilepsy patients who are implanted with hippocampal and/or neocortical intracranial EEG (iEEG) electrodes for planning of resective surgery, and healthy participants undergoing simultaneous EEG/fMRI. Stimulus-specific representations will be assessed using MVPA on iEEG data (in the iEEG experiment) and on scalp EEG data (in the simultaneous EEG/fMRI experiment). Simultaneously, hippocampal activation will be analyzed using either iEEG from the hippocampus or fMRI recordings in the healthy participants. This project will help clarifying the neural mechanisms underlying visual STM and the role of the hippocampus for multi-item STM.