Deltas' and spindles' cross-area synchronization and ripple subtypes

Hippocampal ripples, critical for sleep-related memory consolidation, are heterogeneous events with various sources and functions. However, their specific roles in supporting different forms of memory remain poorly understood. 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 identified: medium-sized, large-sized, and small-sized ripples. Small-sized ripples were associated with increased prefrontal cortex to hippocampus connectivity, followed hippocampal delta waves, and were related to simple learning. In contrast, large-sized ripples exhibited increased hippocampus to prefrontal cortex connectivity, occurred during hippocampal spindles together as a doublet with a small-sized ripple, and were related to 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. Together, these results underscore that distinct ripple subtypes and their specific patterns of hippocampal-cortical coordination during sleep underpin different memory consolidation processes.