Hippocampal ripples and their coordinated dialogue with the default mode network during recent and remote recollection

Hippocampal ripples, brief synchronized electrical events in the brain, are known to support navigation in rodents and short-term memory in humans. This study shows that ripples also increase in the human hippocampus just before people recall recent or remote autobiographical events and semantic facts. Ripple patterns across different hippocampal sites were specific to the type of memory being retrieved. Over time, ripple patterns for autobiographical memories became more similar to those for semantic memory, suggesting a process of semantization. During recollection, ripple-coupled activity was widespread across the neocortex, especially in the default mode network, indicating that hippocampal ripples help coordinate communication between the hippocampus and large-scale cortical networks involved in conscious recall.

Study at a glance

Abstract

SUMMARY Hippocampal ripples are prominent synchronization events generated by hippocampal neuronal assemblies. To date, ripples have been primarily associated with navigational memory in rodents and shortterm episodic recollections in humans. Here, we uncover different profiles of ripple activity in the human hippocampus during the retrieval of recent and remote autobiographical events and semantic facts. We found that the ripple rate increased significantly before reported recall compared to control conditions. Patterns of ripple activity across multiple hippocampal sites demonstrated remarkable specificity for memory type. Intriguingly, these ripple patterns revealed a semantization dimension, in which patterns associated with autobiographical contents become similar to those of semantic memory as a function of memory age. Finally, widely distributed sites across the neocortex exhibited ripple-coupled activations during recollection, with the strongest activation found within the default mode network. Our results thus reveal a key role for hippocampal ripples in orchestrating hippocampal-cortical communication across large-scale networks involved in conscious recollection.