Ripples during associative and non-associative memory retrieval in humans.

Poster Session D - Monday, April 15, 2024, 8:00 – 10:00 am EDT, Sheraton Hall ABC

Jude Thom¹ (jude.thom@linacre.ox.ac.uk), Bernhard Staresina¹; ¹Department of Experimental Psychology, University of Oxford

Ripples in the hippocampus are high frequency (80-120 Hz) bursts of synchronous activity which have been linked to memory retrieval in awake humans. However, it is unclear whether ripple events are regionally specific to the hippocampus and how they are related to the type of memory being retrieved. Further, it remains unknown how ripples impact the underlying neural firing in the hippocampus and surrounding cortex. Here, we investigate these questions using intracranial electroencephalography (iEEG) and micro-wire data from the hippocampus and entorhinal cortex (EC) of nine epilepsy patients who performed associative (scene-object cued recall) and non-associative (scene recognition) memory tasks. We first used an unsupervised method to explore the frequencies of gamma bursts (30-200 Hz) around memory retrieval in hippocampus and EC iEEG. In hippocampus (but not EC) the distribution of gamma-bursts had a peak at ~100 Hz, which aligns with the human ripple frequency range. Next, we algorithmically detected hippocampal ripples and found that the ripple rate increased after memory cue onset in both associative and non-associative tasks. Importantly though, this increase was more persistent in associative retrieval trials. Finally, multi-unit activity analysis indicated a peak in neuronal firing rates (FR) around ripples, but with an asymmetry between hippocampal and EC FR. Specifically, local hippocampal FR preceded hippocampal ripples whereas EC FR followed hippocampal ripples. Together, these results suggest ripples are a prominent oscillatory signal around retrieval particularly in hippocampus, and their dynamics are differentially modulated by memory type and underlying neuronal firing across hippocampus and EC.

Topic Area: LONG-TERM MEMORY: Episodic