The hippocampus as the switchboard between perception and memory

Adaptive memory recall requires a rapid and flexible switch from external perceptual reminders to internal mnemonic representations. However, owing to the limited temporal or spatial resolution of brain imaging modalities used in isolation, the hippocampal-cortical dynamics supporting this process remain unknown. We thus employed an object/scene cued recall paradigm across three studies, spanning single neuron recordings, intracranial as well as high-density EEG and fMRI. First, a hippocampal gamma power/spike rate increase distinguishing successful vs. unsuccessful recall emerged 500 ms after cue onset. Intriguingly, hippocampal gamma peaks marked the moments at which extrahippocampal activation patterns shifted from perceptual cue towards mnemonic target representations. In parallel, source-localised EEG (corroborated by fMRI) revealed a recall signal progressing from hippocampus to posterior parietal cortex and then to medial prefrontal cortex. These results identify the hippocampus as the switchboard between perception and memory and elucidate the ensuing hippocampal-cortical dynamics supporting the recall process.