Thalamic spindles and upstates, but not ripples, coordinate cortico-cortical and hippocampo-cortical co-ripples in humans

Abstract

The co-occurrence of brief [~]90Hz oscillations (co-ripples) may be important for integrating information across the neocortex and hippocampus and, therefore, 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, and/or if they are coordinated by lower frequency thalamic waves. Using human intracranial recordings, we found that ripples are generated in the anterior and posterior thalamus during local spindles on the down-to-upstate transition in non-rapid eye movement sleep, with similar characteristics as cortical and hippocampal ripples. However, thalamic ripples only infrequently co-occur or phase-lock, with cortical and hippocampal ripples. In contrast, thalamo-cortical spindles and upstates were strongly coordinated with cortico-cortical and hippocampo-cortical co-rippling. Thus, while thalamic ripples may not directly drive multiple cortical or hippocampal sites at ripple frequency, thalamo-cortical spindles and upstates may provide the input necessary for spatially distributed co-rippling to integrate information in the cortex. Significance StatementWidespread networks of [~]90 Hz oscillations, called "ripples," have recently been identified in humans and may help to bind information in the cortex and hippocampus for memory. However, it is not known whether the thalamus generates ripples, and if so whether they, or other thalamic waves, coordinate networks of co-occurring cortical and hippocampal ripples. Here, we show that the human thalamus generates [~]90 Hz ripples during NREM sleep. While thalamic ripples do not appear to directly synchronize ripple co-occurrence in the cortex and hippocampus, our data provide evidence that propagating thalamo-cortical spindles and upstates organize these networks. Thus, the thalamus projects slower frequency waves that modulate higher frequency hippocampo-cortical oscillatory networks for memory in humans.