Stimulation Of Slow Oscillations Research Articles

Closed-loop transcranial alternating current stimulation of slow oscillations

Abstract Transcranial alternating current stimulation (tACS) is an emerging non-invasive tool for modulating brain oscillations. There is evidence that weak oscillatory electrical stimulation during sleep can entrain cortical slow oscillations to improve the memory consolidation in rodents and humans. Using a novel method and a custom built stimulation device, automatic stimulation of slow oscillations in-phase with the endogenous activity in a real-time closed-loop setup is possible. Preliminary data from neuroplasticity experiments show a high detection performance of the proposed method, electrical measurements demonstrate the outstanding quality of the presented stimulation device.

Transcranial slow oscillation stimulation during NREM sleep enhances acquisition of the radial maze task and modulates cortical network activity in rats

Slow wave sleep, hallmarked by the occurrence of slow oscillations (SO), plays an important role for the consolidation of hippocampus-dependent memories. Transcranial stimulation by weak electric currents oscillating at the endogenous SO frequency (SO-tDCS) during post-learning sleep was previously shown by us to boost SO activity and improve the consolidation of hippocampus-dependent memory in human subjects. Here, we aimed at replicating and extending these results to a rodent model. Rats were trained for 12 days at the beginning of their inactive phase in the reference memory version of the radial arm maze. In a between subjects design, animals received SO-tDCS over prefrontal cortex (PFC) or sham stimulation within a time frame of 1 h during subsequent non-rapid eye movement (NREM) sleep. Applied over multiple daily sessions SO-tDCS impacted cortical network activity as measured by EEG and behavior: at the EEG level, SO-tDCS enhanced post-stimulation upper delta (2–4 Hz) activity whereby the first stimulations of each day were preferentially affected. Furthermore, commencing on day 8, SO-tDCS acutely decreased theta activity indicating long-term effects on cortical networks. Behaviorally, working memory for baited maze arms was enhanced up to day 4, indicating enhanced consolidation of task-inherent rules, while reference memory errors did not differ between groups. Taken together, we could show here for the first time an effect of SO-tDCS during NREM sleep on cognitive functions and on cortical activity in a rodent model.

P 153. Influence of transcranial slow oscillation current stimulation (tSOS) on EEG, sleepiness and alertness

Transcranial slow oscillation stimulation (tSOS) has been shown to have an impact on EEG. It acts upon memory consolidation during sleep and on memory encoding during wakefulness [2] , [1] . The goal of our study was to reproduce the stimulation protocol of Kirov et al. during daytime to investigate whether we could find effects on EEG, sleepiness and alertness. We performed a randomized, sham-controlled, double-blind cross over trial with 20 healthy individuals. Every subject was stimulated during 2 visits with a washout phase of at least 10 days in between. Subjects were stimulated during daytime and received either real or sham stimulation. We stimulated with anodal tSOS ( f = 0.75 Hz, waveform: sinus, current: 250 μA, DC offset: 130 μA, current density: 13.8 μA/cm 2 , electrode positions: (F3)/(F4)-mastoids, stimulator: Neuroconn DC Stimulator Plus) and with sham mode provided by manufacturer. Group A ( n = 10) received only one stimulation per visit whereas group B ( n = 10) received 3. One stimulation session consisted of 5 blocks of stimulation, each with a length of 5 min and a stimulation free interval of 1 min after each block. Several cognitive tests were performed before and after stimulation (PVT, DSST, Digit Span, KSS). EEG was recorded before, during and after stimulation. The data were analyzed using Matlab, EEGLab and Fieldtrip. In both groups neither in the cognitive performance tests nor in EEG power relevant significant differences between real and sham stimulation were found. We observed a slight, however non-significant increase in theta power in the stimulation free intervals as well for active as for sham stimulation. This effect was only of a short duration. A possible reason for the lacking improvement of cognitive performance might be the stimulation during quiet wakefulness. It has been shown that the response to tSOS depends on the brain state. Increased memory encoding due to tSOS, for instance, can be reached by stimulation during learning. Effects of tSOS are known to be short-lasting, maybe too short-lasting to have an impact on the cognitive tests performed afterwards. The slight increase of theta activity matches with previous studies. The lacking significance may be due to the stimulation with a lower current density. Furthermore we used the sham mode provided by the manufacturer of our stimulator, which applies an impedance check with a frequency of 2 Hz. It might have an influence on EEG similar to our active stimulation. Further studies discarding these problems are necessary and might be able to reproduce results of previous studies. This study was funded by the European project HIVE within the 7th framework program.

Napping to renew learning capacity: enhanced encoding after stimulation of sleep slow oscillations

As well as consolidating memory, sleep has been proposed to serve a second important function for memory, i.e. to free capacities for the learning of new information during succeeding wakefulness. The slow wave activity (SWA) that is a hallmark of slow wave sleep could be involved in both functions. Here, we aimed to demonstrate a causative role for SWA in enhancing the capacity for encoding of information during subsequent wakefulness, using transcranial slow oscillation stimulation (tSOS) oscillating at 0.75Hz to induce SWA in healthy humans during an afternoon nap. Encoding following the nap was tested for hippocampus-dependent declarative materials (pictures, word pairs, and word lists) and procedural skills (finger sequence tapping). As compared with a sham stimulation control condition, tSOS during the nap enhanced SWA and significantly improved subsequent encoding on all three declarative tasks (picture recognition, cued recall of word pairs, and free recall of word lists), whereas procedural finger sequence tapping skill was not affected. Our results indicate that sleep SWA enhances the capacity for encoding of declarative materials, possibly by down-scaling hippocampal synaptic networks that were potentiated towards saturation during the preceding period of wakefulness.

Slow-oscillatory transcranial direct current stimulation can induce bidirectional shifts in motor cortical excitability in awake humans

Constant transcranial direct stimulation (c-tDCS) of the primary motor hand area (M1 HAND) can induce bidirectional shifts in motor cortical excitability depending on the polarity of tDCS. Recently, anodal slow oscillation stimulation at a frequency of 0.75 Hz has been shown to augment intrinsic slow oscillations during sleep and theta oscillations during wakefulness. To embed this new type of stimulation into the existing tDCS literature, we aimed to characterize the after effects of slowly oscillating stimulation (so-tDCS) on M1 HAND excitability and to compare them to those of c-tDCS. Here we show that so-tDCS at 0.8 Hz can also induce lasting changes in corticospinal excitability during wakefulness. Experiment 1. In 10 healthy awake individuals, we applied c-tDCS or so-tDCS to left M1 HAND on separate days. Each tDCS protocol lasted for 10 min. Measurements of motor evoked potentials (MEPs) confirmed previous work showing that anodal c-tDCS at an intensity of 0.75 mA (maximal current density 0.0625 mA/cm2) enhanced corticospinal excitability, while cathodal c-tDCS at 0.75 mA reduced it. The polarity-specific shifts in excitability persisted for at least 20 min after c-tDCS. Using a peak current intensity of 0.75 mA, neither anodal nor cathodal so-tDCS had consistent effects on corticospinal excitability. Experiment 2. In a separate group of ten individuals, peak current intensity of so-tDCS was raised to 1.5 mA (maximal current density 0.125 mA/cm2) to match the total amount of current applied with so-tDCS to the amount of current that had been applied with c-tDCS at 0.75 mA in Experiment 1. At peak intensity of 1.5 mA, anodal and cathodal so-tDCS produced bidirectional changes in corticospinal excitability comparable to the after effects that had been observed after c-tDCS at 0.75 mA in Experiment 1. The results show that so-tDCS can induce bidirectional shifts in corticospinal excitability in a similar fashion as c-tDCS if the total amount of applied current during the tDCS session is matched.

Slow oscillation electrical brain stimulation during waking promotes EEG theta activity and memory encoding

The application of transcranial slow oscillation stimulation (tSOS; 0.75 Hz) was previously shown to enhance widespread endogenous EEG slow oscillatory activity when applied during a sleep period characterized by emerging endogenous slow oscillatory activity. Processes of memory consolidation typically occurring during this state of sleep were also enhanced. Here, we show that the same tSOS applied in the waking brain also induced an increase in endogenous EEG slow oscillations (0.4-1.2 Hz), although in a topographically restricted fashion. Applied during wakefulness tSOS, additionally, resulted in a marked and widespread increase in EEG theta (4-8 Hz) activity. During wake, tSOS did not enhance consolidation of memories when applied after learning, but improved encoding of hippocampus-dependent memories when applied during learning. We conclude that the EEG frequency and related memory processes induced by tSOS critically depend on brain state. In response to tSOS during wakefulness the brain transposes stimulation by responding preferentially with theta oscillations and facilitated encoding.

Oscillating current stimulation – slow oscillation stimulation during sleep

Oscillating current stimulation – slow oscillation stimulation during sleep