Beyond Hypnograms: Assessing Sleep Stability Using Acoustic and Electrical Stimulation

Personalized Theta Transcranial Alternating Current Stimulation and Gamma Transcranial Alternating Current Stimulation Bring Differential Neuromodulatory Effects on the Resting Electroencephalogram: Characterizing the Temporal, Spatial, and Spectral Dimensions of Transcranial Alternating Current Stimulation

OBJECTIVES

The neuromodulatory effects of transcranial alternating current stimulation (tACS) on electroencephalogram (EEG) dynamics are quite heterogenous. The primary objective of the study is to comprehensively characterize the effects of two tACS protocols on resting-state EEG.

MATERIALS AND METHODS

A total of 36 healthy participants were recruited and were randomized into three groups. Two groups received either personalized theta (4-8 Hz) or gamma (40 Hz) stimulation bilaterally in the frontal regions for 20 minutes (4 minutes ON, 1 minute OFF, four cycles). The third group performed relaxed breath watching for 20 minutes. Artifact-free, 1-minute EEG segments from the baseline, during tACS, and after stimulation resting EEG were characterized to see the effects of tACS. Threshold-free cluster enhanced permutation tests (for spectral measures) and two-way mixed analysis of variance (for aperiodic slope) were used for statistical inferences.

RESULTS

Current modeling simulation using ROAST with preset parameters (800 μA, AF3 AF4 locations) showed that induced electric fields can activate frontal cortical regions. During the stimulation period, personalized theta tACS entrained theta band power in the centro-parietal areas. There was a compensatory power decrease in the beta and gamma bands after theta tACS. No entrainment effects were observed for gamma tACS during stimulation, but a significant entrainment was observed in the theta and beta bands in the parieto-occipital regions after stimulation. The delta band power decreased in the central regions. No spectral modulations were seen after breath watching. The spectral slope, which measures aperiodic activity, was not affected by either breath watching or tACS.

CONCLUSIONS

Characterizing the effects of multiple tACS protocols is critical to effectively target specific neural oscillatory patterns and to personalize the protocols. The study can be extended to target specific oscillatory patterns associated with cognitive deficits in neuro-psychiatric conditions.

The influence of transcranial alternating current stimulation on EEG spectral power during subsequent sleep: A randomized crossover study

OBJECTIVE

To evaluate the instant impact of transcranial alternating current stimulation (tACS) on sleep brain oscillations.

METHODS

Thirty-six healthy subjects were randomly assigned to receive tACS and sham stimulation in a crossover design separated by a one-week washout period. After stimulation, a 2-h nap polysomnography (PSG) was performed to obtain Electroencephalogram (EEG) data and objective sleep variables, and self-reported subjective sleep parameters were collected at the end of the nap. EEG spectral analyses were conducted on the EEG data to obtain the absolute and relative power for each sleep stage during the nap. The associations between power values and objective and subjective measurements were analyzed using Spearman or Pearson correlation coefficients.

RESULTS

The tACS group presented higher power in slow wave activity (SWA) and delta frequency bands and lower alpha, sigma and beta power values compared to the sham group during the N2 and N3 sleep stages. SWA and delta power were positively associated with sleep duration and sleep efficiency relevant parameters; while alpha, sigma and beta power were positively associated with prolonged sleep latency and wakefulness related variables. PSG, self-reported and sleep diary measured objective and subjective sleep parameters were comparable between the tACS and the sham groups.

CONCLUSION

Our results support that tACS could promote sleep depth in microstructure of sleep EEG, manifesting as an increase in EEG spectral power in low frequency bands and a decrease in high frequency bands. The registration number of this study is ChiCTR2200063729.

The Effects of Transcranial Electrical Stimulation of the Brain on Sleep: A Systematic Review

Transcranial Electrical Stimulation (tES) is a promising non-invasive brain modulation tool. Over the past years, there have been several attempts to modulate sleep with tES-based approaches in both the healthy and pathological brains. However, data about the impact on measurable aspects of sleep remain scattered between studies, which prevent us from drawing firm conclusions. We conducted a systematic review of studies that explored the impact of tES on neurophysiological sleep oscillations, sleep patterns measured objectively with polysomnography, and subjective psychometric assessments of sleep in both healthy and clinical samples. We searched four main electronic databases to identify studies until February 2020. Forty studies were selected including 511 healthy participants and 452 patients. tES can modify endogenous brain oscillations during sleep. Results concerning changes in sleep patterns are conflicting, whereas subjective assessments show clear improvements after tES. Possible stimulation-induced mechanisms within specific cortico-subcortical sleep structures and networks are discussed. Although these findings cannot be directly transferred to the clinical practice and sleep-enhancing devices development for healthy populations, they might help to pave the way for future researches in these areas. PROSPERO registration number 178910.