Cortico-cortical and hippocampo-cortical co-rippling are facilitated by thalamo-cortical spindles and upstates, but not by thalamic ripples

The co-occurrence of brief ~90 Hz oscillations (co-ripples) may be important in integrating information across the cortex and hippocampus, essential for sleep consolidation, and cognition in general. However, how such co-ripples are synchronized is unknown. We tested if cortico-cortical and hippocampal-cortical ripple co-occurrences are due to the simultaneous direct propagation of thalamic ripples, or if they are instead facilitated by lower frequency waves. Using human intracranial recordings, we found that ripples are generated in the thalamus during nonrapid eye movement sleep with similar characteristics as cortical and hippocampal ripples. However, thalamic ripples only infrequently and weakly co-occur, and never phase-lock, with cortical and hippocampal ripples. In contrast, thalamo-cortical spindles and upstates strongly facilitated cortico-cortical and hippocampo-cortical co-rippling. Thus, while thalamic ripples may not directly drive multiple cortical or hippocampal sites at ripple frequency, these sites may ripple synchronously in response to widespread activation from thalamo-cortical spindles and upstates.