Causal Relationship Between SPWRs and Spatial Learning and Memory

Abstract

Consolidation is the process by which recently acquired memories are gradually strengthened and reorganized for long-term storage, potentially involving a transfer from the hippocampus to cortical areas. Hippocampal sharp-wave ripples (SPWRs) are transient fast oscillations (200 Hz) occurring during slow-wave sleep, rest, and short immobility periods. Because place cells are sequentially reactivated during SPWRs, and because their frequency would be propitious to induce long-term potentiation, SPWRs have been considered a likely candidate mechanism for spatial learning and memory consolidation. Establishing a causal link between the two requires closed-loop interventional experiments where SPWRs are detected online and suppressed in a spatiotemporally selective manner. This was achieved in three studies where SPWRs were suppressed either during sleep and rest following training on a spatial task or during training on the spatial task. The resulting impairments in performance in all three studies established for the first time a causal link between sleep and rest SPWRs and memory consolidation on the one hand, and awake SPWRs and working spatial memory on the other hand. These studies also raised the question of a differential role for awake and sleep/rest ripples that will require further investigation. Similarly, conclusive evidence for a role of SPWRs in hippocampo-cortical information transfer remains elusive.KeywordsHippocampusSharp wave-ripplesMemory consolidationSpatial memoryCausal neuroscienceNeuronal replayClosed-loop stimulation