Hippocampal Sleep Features: Relations to Human Memory Function

Abstract

The recent spread of intracranial 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 of its central role in the declarative memory formation. Recent data indicate that sleep contributes to memory formation. Therefore, it is relevant to understand whether specific patterns of activity taking place during sleep are related to memory consolidation processes. Fascinating similarities between different states of consciousness (wakefulness, REM sleep, non-REM sleep) in some electrophysiological mechanisms underlying cognitive processes have been reported. For instance, large-scale synchrony in gamma activity is important for waking memory and perception processes, and its changes during sleep may be the neurophysiological substrate of sleep-related deficits of declarative memory. Hippocampal activity seems to specifically support memory consolidation during sleep, through specific coordinated neurophysiological events (slow waves, spindles, ripples) that would facilitate the integration of new information into the pre-existing cortical networks. A few studies indeed provided direct evidence that rhinal ripples as well as slow hippocampal oscillations are correlated with memory consolidation in humans. More detailed electrophysiological investigations assessing the specific relations between different types of memory consolidation and hippocampal EEG features are in order. These studies will add an important piece of knowledge to the elucidation of the ultimate sleep function.

Increased mesiotemporal delta activity characterizes virtual navigation in humans

“…The presence of hippocampal delta activity during REM sleep as well as wakefulness was also demonstrated by other investigators using stereo-EEG recordings (Moroni et al, 2007Ferrara et al, 2012) while another study reported a phase-locking of single-neuron activity to hippocampal delta activity while patients performed a virtual navigation task (Jacobs et al, 2007). In a recent study we demonstrated a phase-coupling of high-frequency activity with the 1.5-3 Hz mesiotemporal delta oscillations during REM sleep (Clemens et al, 2009).…”

Sleep-spindle detection: crowdsourcing and evaluating performance of experts, non-experts and automated methods

“…These are significant changes, as spindles result from interactions of several regions of the brain, including the thalamic reticular nucleus, thalamocortical relay neurons, the hippocampus, and the cortex 25, 26 . During wake, these same circuits are linked to learning, behavioral arousal, and sensory gating 27 .…”

Slow-oscillatory Transcranial Direct Current Stimulation Modulates Memory in Temporal Lobe Epilepsy by Altering Sleep Spindle Generators: A Possible Rehabilitation Tool

“…The epileptogenic focus often originates from the hippocampus and anterior subregions (including the anterior parahippocampal cortex) [2e4], which are areas believed to be involved in both the encoding and consolidation of memory. Memory consolidation is thought to occur through ripples, which are fast frequency discharges (110e300 Hz) interpreted as electric discharges that promote synaptic rearrangement and replay of neural sequences learned during consolidation [5] on the basis of long-term potentiation (LTP) [6,7]. Scalp sleep spindles are believed to represent the cortical equivalent of hippocampal ripples [8,9] and to be a marker of memory consolidation [10e13].…”