Opposite behaviour of neocortex and hippocampus in the interhemispheric dialogue: a human stereo-EEG single case.

Previous studies showed a rhythmic oscillation in the low delta range during REM sleep in the human hippocampus. Compelling evidence point to the involvement of slow rhythms in the processes of memory consolidation and post-learning plasticity. The almost unique opportunity to record two homologue bilateral cortical and hippocampal derivations without EEG alterations in an epileptic patient undergoing presurgical evaluation allowed us to investigate the interemispheric communication within slow-frequency rhythms across the sleep wake cycle. We recorded intracerebral (bilateral hippocampal and frontal neocortical derivations) stereo-EEG of a night of sleep and of the preceding wake. In order to investigate the behaviour and phase relationship of slow rhythms across different derivations, power and coherence analyses, analysis of waveform triggered around ( ! 1 sec) the negative peak of slow waves (0.5– 2.0 Hz) and cross-correlation analysis were carried out. In neocortical leads, delta power was lower during wake and REM sleep than during NREM sleep. Interemispheric coherence of lowdelta showed a similar pattern, with high levels during slow-wave sleep concomitant with a phase synchronization of delta waves. Hippocampal derivations showed a very different pattern. Lowdelta power was high during all three states, indicating that lowdelta is a peculiar feature of human hippocampal EEG activity during all physiological states. Interemispheric coherence showed an opposite pattern to neocortical derivations, with slightly higher levels during wake and REM sleep and, unexpectedly, very low levels during NREM sleep. The latter result points to an interhippocampal functional disconnection during NREM sleep. Between neocortical and hippocampal leads no phase modulation and coherence emerged in the low delta range, indicating a functional differentiation between neocortical and hippocampal slow rhythms. Hippocampal bilateral delta synchronization during wake and REM sleep could have a role in the encoding memory process, and it could be the human equivalent of the animal hippocampal theta rhythm. The de-synchronization of Delta oscillations between the two hippocampi during NREM sleep is a novel and unexpected finding that deserves further investigation.