Hippocampal ripples and memory consolidation

During slow wave sleep and quiet wakefulness, the hippocampus generates high frequency field oscillations (ripples) during which pyramidal neurons replay previous waking activity in a temporally compressed manner. As a result, reactivated firing patterns occur within shorter time windows propitious for synaptic plasticity within the hippocampal network and in downstream neocortical structures. This is consistent with the long-held view that ripples participate in strengthening and reorganizing memory traces, possibly by mediating information transfer to neocortical areas. Recent studies have confirmed that ripples and associated neuronal reactivations play a causal role in memory consolidation during sleep and rest. However, further research will be necessary to better understand the neurophysiological mechanisms of memory consolidation, in particular the selection of reactivated assemblies, and the functional specificity of awake ripples.

Research highlights
► Consolidation refers to the long-term stabilization of initially labile memory traces.
► Consolidation is believed to occur during sleep and rest (‘off-line’ states).
► During off-line states, the hippocampus generates fast oscillations (ripples).
► Awake neuronal activity is reactivated in the hippocampus and cortex during ripples.
► Ripples and reactivations play a prominent role in off-line memory consolidation.