Uncovering Sleep's Hippocampal-Cortical Dialogue: The Role of Deltas' and Spindles' Cross-Area Synchronization and Ripple Subtypes

ippocampal ripples, critical for sleep-related memory consolidation, are heterogeneous events with various sources and functions. Here we applied principal component analysis to identify ripple sub-types and relate them to hippocampal-cortical interactions as well as their role in consolidating simple and complex semantic-like memories in rats. Three main ripple types were discovered: baseline, large-input, and small-input ripples. Small-input ripples, were associated with increased prefrontal cortex to hippocampus connectivity, followed hippocampal delta waves, and were sufficient for simple learning. In contrast, large-input ripples exhibited increased hippocampus to prefrontal cortex connectivity, occurred during hippocampal spindles together as a doublet with a small-input ripple, and were critical for complex memory consolidation. Finally, learning induced heightened coupling between hippocampal delta and spindle oscillations and their cortical counterparts, consequently leading to an increased synchronization of ripples with cortical oscillations.