ABSTRACT
Background The maintenance of items in working memory (WM) relies on a widespread network of cortical areas and hippocampus where synchronization between electrophysiological recordings reflects functional coupling. We investigated the direction of information between sensory areas and hippocampus during encoding and maintenance of WM items.
Methods Participants (N=15) performed a WM task where a string of letters was presented all at once, thus separating the encoding period from the maintenance period. Participants mentally replayed the letters during maintenance. We recorded sEEG from the hippocampus, scalp EEG and, additionally in 3 participants, temporo-parietal ECoG.
Results When analyzing the information flow to and from auditory cortex by Granger causality, the flow was from ECoG over auditory cortex to hippocampus with a peak in the 12-24 Hz beta range while letters were presented, and this flow was subsequently reversed during maintenance, while letters were maintained in memory. The same pattern appeared to and from hippocampus with ECoG over temporo-parietal cortex. For scalp EEG, the pattern appeared on temporal sites, albeit in the 4-12 Hz theta-alpha range. While the pattern was significantly structured for correct trials, it was unstructured for incorrect trials.
Conclusions The functional interaction between hippocampus and cortex and the reversal of information flow provide a physiological basis for the encoding of memory items and their active replay during maintenance.
SIGNIFICANCE STATEMENT How do we encode and recall memories to guide action within seconds? Here, we investigated electrical activity in hippocampus and cortex while the participants mentally replayed a set of letters to maintain them working memory with the aim to respond correctly. We found clear evidence that during the encoding of the letters, the information flow was from sensory cortex to hippocampus (bottom-up). The flow was subsequently reversed during maintenance (top-down), thus providing a physiological basis for the recall of the memory items. This functional interaction provides the first evidence of bidirectional communication during encoding and the active replay of memory items.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
vasileios.dimakopoulos{at}usz.ch, lennart.stieglitz{at}usz.ch, pierre.megevand{at}unige.ch, johannes.sarnthein{at}usz.ch