Hippocampal ripples signal contextually-mediated episodic recall

High-frequency oscillatory events, termed ripples, represent synchrony of neural activity in the brain. Recent evidence suggests medial temporal lobe (MTL) ripples support memory retrieval. However, it is unclear if ripples signal the reinstatement of episodic memories. Analyzing electrophysiological MTL recordings from 219 neurosurgical participants performing episodic recall tasks, we find that the rate of hippocampal ripples rises just prior to the free recall of recently-formed memories. This pre-recall ripple effect (PRE) is stronger in the CA1 and dentate gyrus (DG) subfields of hippocampus than neighboring MTL regions entorhinal and parahippocampal cortex. The PRE is also stronger prior to the retrieval of temporally and semantically clustered as compared with unclustered recalls, indicating the involvement of ripples in contextual reinstatement, which is a hallmark of episodic memory. One sentence summary High frequency human hippocampal ripples occur prior to those memories most likely retrieved via episodic mechanisms.