Hippocampal sharp-wave ripples linked to visual episodic recollection in humans

• Published in: Science (American Association for the Advancement of Science) Vol. 365; no. 6454; p. 657
• Main Authors: Norman, Yitzhak, Yeagle, Erin M., Khuvis, Simon, Harel, Michal, Mehta, Ashesh D., Malach, Rafael
• Format: Journal Article
• Language: English
• Published: United States American Association for the Advancement of Science 16.08.2019; The American Association for the Advancement of Science
• Subjects: Activation; Brain; Bursts; Cerebral cortex; Cognition; Cognitive ability; Consolidation; Engraving; Epilepsy; Hippocampus; Incidence; Memory; Multitasking; Navigation behavior; Patients; Questions; Recall; Recall (Psychology); Reinstatement; Representations; RESEARCH ARTICLE SUMMARY; Retrieval; Ripples; Temporal lobe; Viewing; Visual cortex; Visual discrimination learning; Visual Learning; Visual observation; Visual stimuli
• ISSN: 0036-8075; 1095-9203; 1095-9203
• DOI: 10.1126/science.aax1030

What are the brain mechanisms responsible for episodic memory retrieval? Norman et al. investigated epilepsy patients who had electrodes implanted in the hippocampus and a variety of cortical areas. Using a visual learning paradigm, they examined the temporal relationship between the incidence of hippocampal sharp-wave ripples and recall. Effective encoding of visual information was associated with higher incidence of ripples. Successful recall was preceded by an increased probability of ripples, which were also associated with transient reemergence of activation patterns in higher visual cortical areas. Hippocampal ripples may thus boost recollections during episodic memory retrieval. Science , this issue p. eaax1030 Ripples reinstate human memory during free recall. Hippocampal sharp-wave ripples (SWRs) constitute one of the most synchronized activation events in the brain and play a critical role in offline memory consolidation. Yet their cognitive content and function during awake, conscious behavior remains unclear. We directly examined this question using intracranial recordings in human patients engaged in episodic free recall of previously viewed photographs. Our results reveal a content-selective increase in hippocampal ripple rate emerging 1 to 2 seconds prior to recall events. During recollection, high-order visual areas showed pronounced SWR-coupled reemergence of activation patterns associated with recalled content. Finally, the SWR rate during encoding predicted subsequent free-recall performance. These results point to a role for hippocampal SWRs in triggering spontaneous recollections and orchestrating the reinstatement of cortical representations during free episodic memory retrieval.