CENTER FOR RESEARCH IN
COGNITION & NEUROSCIENCES/ConnexConnex

The recent spread of intracranial/intracerebral electroencephalographic (EEG) recording techniques for presurgical evaluation of drug-resistant epileptic patients is providing new information on the activity of different brain structures during both wakefulness and sleep. The interest has been mainly focused on the medial temporal lobe, and in particular the hippocampal formation, whose peculiar local sleep features have been recently described, providing support to the idea that sleep is not a spatially global phenomenon. The study of the hippocampal sleep electrophysiology is particularly interesting because declarative memory and, in particular, spatial learning, seem to specifically benefit from NREM sleep. A peculiar bimodal distribution of hippocampal activity in the delta range, made up by a low frequency non-oscillatory activity (up to 2 Hz) and a faster oscillatory rhythm (2–4 Hz) has been recently described. Interestingly, the faster oscillatory activity at around 3 Hz is present in the spontaneous human hippocampal EEG activity during both resting wakefulness and sleep and also during the execution of a spatial navigation task. Aimed to evaluate the effects of spatial learning on human hippocampal sleep EEG activity, we recently recorded hippocampal Stereo-EEG (SEEG) activity in a group of refractory epilepsy patients undergoing presurgical clinical evaluation, after a training on a spatial navigation task. We observed that hippocampal high-delta (2-4 Hz range) activity increases during the first NREM episode after learning compared to the baseline night. Moreover, the amount of hippocampal NREM high-delta power was correlated with task performance at retest. The effect involved only the hippocampal EEG frequencies inasmuch no differences were observed at the neocortical electrodes and in the traditional polysomnographic measures. These findings support the hypothesis that, in humans, hippocampal EEG activity in the delta band is involved in spatial memory formation processes during sleep. The presence of hippocampal delta rhythms around 3 Hz during a virtual maze exploration, observed also in other studies, and its increase during the following sleep, supports the idea that this EEG activity, strictly related to spatial navigation, could be involved in replay mechanisms.