Stability of ripple events during task engagement in human hippocampus

Abstract

Periods of cognitive disengagement, such as rest or sleep, are thought to support the progressive consolidation of episodic memories. During these states, the hippocampus displays transient high-frequency oscillatory bursts, known as ripples, which are thought to promote interactions with the neocortex, consolidating memory traces. More recent findings have suggested ripples in the human hippocampus may also occur during task engagement, particularly for tasks requiring episodic memory processes. However, it is unclear if hippocampal ripples occur during other cognitive states or whether ripple properties are modulated by specific types of task demands. In addition, identifying genuine hippocampal ripple events in the human brain can be methodological challenging. To address these questions, we used intracranial recordings from the human hippocampus to quantify ripple events across perceptual, memory and resting task states. Using spectro-temporal identification of hippocampal ripples, we observed highly similar ripple event properties across tasks, with a modest yet significant increase in ripple properties (rate, duration & amplitude) during resting task states. These ripple event attributes did not differ between hemisphere, nor across or within the time of day examined. Supporting data further highlighted that while hippocampal ripples occurred during all task states, these rates were typically lower than that observed during sleep. Together, these findings highlight that hippocampal ripples occur consistently, but sparsely, during a broad range of cognitive task states. Such findings may be incorporated into existing models of systems consolidation, whereby hippocampal ripples help to initially establish latent memory traces.