Transcranial Electrical Currents to Probe EEG Brain Rhythms and Memory Consolidation during Sleep in Humans

Abstract

Previously the application of a weak electric anodal current oscillating with a frequency of the sleep slow oscillation (∼0.75 Hz) during non-rapid eye movement sleep (NonREM) sleep boosted endogenous slow oscillation activity and enhanced sleep-associated memory consolidation. The slow oscillations occurring during NonREM sleep and theta oscillations present during REM sleep have been considered of critical relevance for memory formation. Here transcranial direct current stimulation (tDCS) oscillating at 5 Hz, i.e., within the theta frequency range (theta-tDCS) is applied during NonREM and REM sleep. Theta-tDCS during NonREM sleep produced a global decrease in slow oscillatory activity conjoint with a local reduction of frontal slow EEG spindle power (8–12 Hz) and a decrement in consolidation of declarative memory, underlining the relevance of these cortical oscillations for sleep-dependent memory consolidation. In contrast, during REM sleep theta-tDCS appears to increase global gamma (25–45 Hz) activity, indicating a clear brain state-dependency of theta-tDCS. More generally, results demonstrate the suitability of oscillating-tDCS as a tool to analyze functions of endogenous EEG rhythms and underlying endogenous electric fields as well as the interactions between EEG rhythms of different frequencies.

Coordination of Slow Waves With Sleep Spindles Predicts Sleep-Dependent Memory Consolidation in Schizophrenia

“…In healthy individuals, increasing spindles with zolpidem 66,67 , increasing sigma activity with transcranial stimulation 68,69 and enhancing the synchronization of sigma activity with slow oscillations using auditory closed-loop stimulation 70 all improve memory, while transcranial stimulation that decreases sigma activity impairs memory 71 . Only a few studies have attempted to improve cognition in schizophrenia by manipulating sleep oscillations.…”

Simultaneous EEG Monitoring During Transcranial Direct Current Stimulation

“…There are several reasons to explain this variability. Studies on fMRI 4 and electroencephalography (EEG) 5,6 reveal that although tDCS has the most activating effect on the underlying cortex, the stimulation evokes widespread changes in other regions of the brain. In addition, it has been shown that tDCS effects depend on the state of baseline cortical activity 7 .…”

The Memory, Cognitive and PsychologicalFunctions of Sleep: Update from Electroencephalographic and Neuroimaging Studies

“…Also, it significantly decreased slow spindle activity (8-12 Hz) only frontally. These EEG changes were associated with a strong impairment of only declarative memory consolidation at recall after sleep (Marshall et al, 2011). Collectively, these findings strongly indicate that SO, SWA and frontal slow spindle activity reflect specific EEG mechanisms of hippocampus-dependent memory consolidation, which do not act beyond non-REM sleep/SWS.…”

“…The stimulation did enhanced gamma (25-45 Hz) EEG activity during REM sleep, but it did not improve either declarative or procedural memory consolidation at morning recalls. Instead, this increase in gamma EEG during REM sleep resulted in a worsened mood, as assessed by positive and negative affect scale, in the morning after sleep, accompanied by worsening on working memory, as assessed by a word fluency test (Marshall et al, 2011). Collectively, these studies demonstrate that specific REM sleep EEG rhythms are involved in affective behavior.…”