Electrical stimulation of the frontal cortex enhances slow-frequency EEG activity and sleepiness

Abstract

Our aim was to enhance the spontaneous slow-frequency EEG activity during the resting state using oscillating transcranial direct currents (tDCS) with a stimulation frequency that resembles the spontaneous oscillations of sleep onset. Accordingly, in this preliminary study, we assessed EEG after-effects of a frontal oscillatory tDCS with different frequency (0.8 vs. 5 Hz) and polarity (anodal, cathodal, and sham). Two single-blind experiments compared the after effects on the resting EEG of oscillatory tDCS [Exp. 1=0.8 Hz, 10 subjects (26.2 ± 2.5 years); Exp. 2=5 Hz, 10 subjects (27.4 ± 2.4 years)] by manipulating its polarity. EEG signals recorded (28 scalp derivations) before and after stimulation [slow oscillations (0.5-1 Hz), delta (1-4 Hz), theta (5-7 Hz), alpha (8-12 Hz), beta 1 (13-15 Hz) and beta 2 (16-24 Hz)] were compared between conditions as a function of polarity (anodal vs. cathodal vs. sham) and frequency of stimulation (0.8 vs. 5 Hz). We found a significant relative enhancement of the delta activity after the anodal tDCS at 5 Hz compared to that at 0.8 Hz. This increase, even though not reaching the statistical significance compared to sham, is concomitant to a significant increase of subjective sleepiness, as assessed by a visual analog scale. These two phenomena are linearly related with a regional specificity, correlations being restricted to cortical areas perifocal to the stimulation site. We have shown that a frontal oscillating anodal tDCS at 5 Hz results in an effective change of both subjective sleepiness and spontaneous slow-frequency EEG activity. These changes are critically associated to both stimulation polarity (anodal) and frequency (5 Hz). However, evidence of frequency-dependence seems more unequivocal than evidence of polarity-dependence.

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

“…Nevertheless, our results are difficult to reconcile with those of D’Atri et al who also applied a protocol of the anodal oscillating-tDCS at 0.8 Hz during eyes-closed resting state and found a qualitative increase in alpha power [58]. However, several parameters differed between the studies.…”

The Efficacy of Transcranial Current Stimulation Techniques to Modulate Resting-State EEG, to Affect Vigilance and to Promote Sleepiness

“…Since resting-state EEG is characterized by alpha and theta activity (the latter rhythm considered as a marker of sleepiness) it is reasonable to wonder if theta-tDCS (5 Hz) could induce a larger resonance effect than 0.75 Hz tDCS, also affecting the sleepiness level. In line with this, D’Atri and coworkers [ 46 ] compared these two frequencies of stimulation and found a post-stimulation significant enhancement of delta and theta activity after anodal stimulation at 5 Hz, compared to 0.75 Hz stimulation, combined to an increase of subjective sleepiness.…”

“…Regarding the role of the stimulation polarity, empirical findings seem to support a more robust efficacy of the anodal oscillatory stimulation compared to the cathodal one [ 39 , 40 , 45 ]. This is directly shown in a study that compared the two oscillating stimulations (cathodal vs. anodal) showing a larger effectiveness of the anodal one [ 46 ].…”

<p>Bilateral Theta Transcranial Alternating Current Stimulation (tACS) Modulates EEG Activity: When tACS Works Awake It Also Works Asleep</p>

“…In all these studies, however, self-reported sleepiness was not affected,6–9 suggesting that neurophysiological changes of sleep-like slow-frequency EEG activity do not necessarily affect subjective sleepiness ratings. In contrast, we found that a frontal anodal o-tDCS at 5 Hz significantly enhanced sleepiness and correlated with stimulation-induced variations of slow-wave activity (SWA) in a narrow scalp/brain region encircling the stimulated site 10…”