Distinct Montages of Slow Oscillatory Transcranial Direct Current Stimulation (so-tDCS) Constitute Different Mechanisms during Quiet Wakefulness

Abstract

Slow oscillatory- (so-) tDCS has been applied in many sleep studies aimed to modulate brain rhythms of slow wave sleep and memory consolidation. Yet, so-tDCS may also modify coupled oscillatory networks. Efficacy of weak electric brain stimulation is however variable and dependent upon the brain state at the time of stimulation (subject and/or task-related) as well as on stimulation parameters (e.g., electrode placement and applied current. Anodal so-tDCS was applied during wakefulness with eyes-closed to examine efficacy when deviating from the dominant brain rhythm. Additionally, montages of different electrodes size and applied current strength were used. During a period of quiet wakefulness bilateral frontolateral stimulation (F3, F4; return electrodes at ipsilateral mastoids) was applied to two groups: ‘Group small’ (n = 16, f:8; small electrodes: 0.50 cm2; maximal current per electrode pair: 0.26 mA) and ‘Group Large’ (n = 16, f:8; 35 cm2; 0.35 mA). Anodal so-tDCS (0.75 Hz) was applied in five blocks of 5 min epochs with 1 min stimulation-free epochs between the blocks. A finger sequence tapping task (FSTT) was used to induce comparable cortical activity across sessions and subject groups. So-tDCS resulted in a suppression of alpha power over the parietal cortex. Interestingly, in Group Small alpha suppression occurred over the standard band (8–12 Hz), whereas for Group Large power of individual alpha frequency was suppressed. Group Small also revealed a decrease in FSTT performance at retest after stimulation. It is essential to include concordant measures of behavioral and brain activity to help understand variability and poor reproducibility in oscillatory-tDCS studies.

Effect of the anodal transcranial direct current electrical stimulation on cognition of medical residents with acute sleep deprivation

“…current strength), size of electrodes, or montage used, and explain the variability of the clinical study results reported 49 . For example, a-tDCS applied bilaterally over the dorsolateral prefrontal cortex during wakefulness in healthy subjects with eyes-closed or eyes-open increase focal EEG theta power 49 , 50 . During sleep anodal slow-oscillatory transcranial direct current stimulation (so-tDCS) showed no effects in healthy young subjects or insomnia patients 51 , 52 , 53 .…”

“…Previous sleep a-tDCS clinical studies reported that the effects on brain rhythms are dependent on the brain state at the time of the therapy (wakefulness/sleep) as well as on stimulation parameters (e.g. current strength), size of electrodes, or montage used, and explain the variability of the clinical study results reported 49 . For example, a-tDCS applied bilaterally over the dorsolateral prefrontal cortex during wakefulness in healthy subjects with eyes-closed or eyes-open increase focal EEG theta power 49 , 50 .…”