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.
Highlights
An increasing number of studies are investigating the impact of applied oscillatory electric currents or fields, inducing or estimated to induce subthreshold membrane potential oscillations, on the activity of cortical networks and/or individual neurons as well as their functional implications [1,2,3,4,5,6,7,8]
Weak electric currents oscillating within the frequency range of the sleep slow oscillation in humans (,0.75 Hz) and an anodal DC bias applied at the transition into slow-wave sleep (SWS) enhanced EEG power of the slow oscillation and sleep spindle activity as well as declarative memory consolidation [2]
We find that effects of theta-transcranial direct current stimulation (tDCS) on brain electric activity and memory consolidation are strongly dependent upon ongoing brain state
Summary
An increasing number of studies are investigating the impact of applied oscillatory electric currents or fields, inducing or estimated to induce subthreshold membrane potential oscillations, on the activity of cortical networks and/or individual neurons as well as their functional implications [1,2,3,4,5,6,7,8]. We have focused on the functional implications of applied currents during sleep on sleepdependent memory consolidation [2,9] as well as on the state dependence of these effects [5]. Slow-wave sleep (SWS), which is hallmarked by slow oscillatory activity (,1 Hz) in the human electro-encephalogram (EEG) and is most pronounced during the first part of nocturnal sleep, has been implicated in the consolidation of declarative memories. Weak electric currents oscillating within the frequency range of the sleep slow oscillation in humans (,0.75 Hz) and an anodal DC bias applied at the transition into SWS enhanced EEG power of the slow oscillation and sleep spindle activity as well as declarative memory consolidation [2]. Theta oscillations are a hallmark of the EEG during REM sleep in rodents [15,16] and are characteristic for REM sleep in humans, though in a more transient form [17,18,19]
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