Hippocampal sleep spindles preceding neocortical sleep onset in humans

- Sleep spindles occur in the hippocampus several minutes before sleep onset.
- Hippocampal sleep spindles consistently preceded neocortical events.
- Sleep onset is characterized by cortical and subcortical dissociated EEG activities.

The coexistence of regionally dissociated brain activity patterns -with some brain areas being active while other already showing sleep signs- may occur throughout all vigilance states including the transition from wakefulness to sleep and may account for both physiological as well as pathological events. These dissociated electrophysiological states are often characterized by multi-domain cognitive and behavioral impairment such as amnesia for events immediately preceding sleep. By performing simultaneous intracerebral electroencephalographic recordings from hippocampal as well as from distributed neocortical sites in neurosurgical patients, we observed that sleep spindles consistently occurred in the hippocampus several minutes before sleep onset. In addition, hippocampal spindle detections consistently preceded neocortical events, with increasing delays along the cortical antero-posterior axis. Our results support the notion that wakefulness and sleep are not mutually exclusive states, but rather part of a continuum resulting from the complex interaction between diffuse neuromodulatory systems and intrinsic properties of the different thalamocortical modules. This interaction may account for the occurrence of dissociated activity across different brain structures characterizing both physiological and pathological conditions.