Cortical and Hippocampal Low Delta Activity: a human stereo-EEG study.

Introduction: Slow wave activity, as assessed by scalp electroencephalogram (EEG), represents the fundamental phenomenon of NREM sleep. Recently, it was observed that hippocampal intracranial Stereo-EEG (SEEG) activity in humans is characterized by an evident low-delta (0.5 2.0 Hz) rhythm during both slow wave sleep and REM sleep. Objectives: Since slow hippocampal activity was only recently described, here we asked if it has independent and peculiar features respect to cortical delta or is related to it. Methods: Thus, we investigated the time course and phase synchronization of hippocampal and neocortical slow rhythms (<2 Hz) in 4 epileptic patients undergoing SEEG recordings, during presurgical electro-clinical evaluation, from two homologous bilateral neocortical and hippocampal derivations. Time course and intra- and inter-hemispheric synchronization across the wake sleep cycle were evaluated by means of power and coherence analysis and analysis of waveform temporally referenced to the negative peak of the slow waves. Results: We found that, unlike cortical SEEG activity, hippocampal activity is characterized by the presence of a high amplitude slow activity during both wake and REM sleep. Moreover, slow hippocampal activity appears to be synchronized across hemispheres during the active states (wake and REM sleep). During NREM sleep, when both neocortical and hippocampal derivations show a high amplitude slow activity, no synchronization of phase emerged between neocortex and hippocampus. Finally, the time-course of cortical and hippocampal inter-hemispheric coherence of slow activity across wake, NREM and REM sleep states showed a clear opposition of phase. Conclusions: Altogether, these findings suggest that slow activity is independently regulated in the hippocampus and neocortex, and raise new questions about the functional role of hippocampal slow activity during active states (wake and REM sleep).