Bilateral 5 Hz transcranial alternating current stimulation on fronto-temporal areas modulates resting-state EEG

Abstract

Rhythmic non-invasive brain stimulations are promising tools to modulate brain activity by entraining neural oscillations in specific cortical networks. The aim of the study was to assess the possibility to influence the neural circuits of the wake-sleep transition in awake subjects via a bilateral transcranial alternating current stimulation at 5 Hz (θ-tACS) on fronto-temporal areas. 25 healthy volunteers participated in two within-subject sessions (θ-tACS and sham), one week apart and in counterbalanced order. We assessed the stimulation effects on cortical EEG activity (28 derivations) and self-reported sleepiness (Karolinska Sleepiness Scale). θ-tACS induced significant increases of the theta activity in temporo-parieto-occipital areas and centro-frontal increases in the alpha activity compared to sham but failed to induce any online effect on sleepiness. Since the total energy delivered in the sham condition was much less than in the active θ-tACS, the current data are unable to isolate the specific effect of entrained theta oscillatory activity per se on sleepiness scores. On this basis, we concluded that θ-tACS modulated theta and alpha EEG activity with a topography consistent with high sleep pressure conditions. However, no causal relation can be traced on the basis of the current results between these rhythms and changes on sleepiness.

Figure 1

Experimental design. (a) EEG recording montage and stimulation electrodes montage (grey circles). (b) 3D finite element modeling of current density distribution with the bilateral fronto-temporal stimulation montage, using the COMETS toolbox for MATLAB. (c) Experimental design (KSS: Karolinska Sleepiness Scale; θ-tACS: 5 Hz transcranial Alternating Current Stimulation; AC: Alternating Current).

Figure 2

Topographic distribution of baseline and post-stimulation EEG spectral powers for θ-tACS and Sham conditions. Topographic maps of mean EEG spectral powers (log-trasformed) at the baseline and after stimulation associated with θ-tACS (first column) and sham (second column) protocols. Maps are plotted for the following frequency bands: delta (1–4 Hz), theta (5–7 Hz), alpha (8–12 Hz), beta (13–24 Hz). Values are colour coded and plotted at the corresponding position on the planar projection of the scalp surface and are interpolated (biharmonic spline) between electrodes. The maps are scaled between minimal and maximal values considering the two experimental conditions within each frequency band.

Figure 3

Cortical effects of θ-tACS: EEG frequency bands resolution. Topographic maps of mean EEG variations in spectral powers associated with θ-tACS (Δ θ-tACS, first column) and sham (Δ sham, second column) conditions and statistical maps of comparisons assessed by paired t-tests (Δ θ-tACS vs. Δ sham, third column). Maps are plotted for the following frequency bands: delta (1–4 Hz), theta (5–7 Hz), alpha (8–12 Hz), beta (13–24 Hz). Values are colour coded and plotted at the corresponding position on the planar projection of the scalp surface and are interpolated (biharmonic spline) between electrodes. The topographic maps (first and second columns) are scaled between minimal and maximal values considering the two experimental conditions within each frequency band. The statistical maps are scaled symmetrically according to the absolute maximal t-value across the statistical comparisons in all frequency bands. White dots represent significant statistical differences between θ-tACS and Sham conditions surviving the TFCE correction for multiple comparisons (p_{TFCE-corrected} < 0.05).

Figure 4

Cortical effects of θ-tACS: single-Hz frequency bin resolution. Topographic maps of mean EEG variations in spectral powers with 1-Hz resolution associated with θ-tACS (Δ θ-tACS, first column) and sham (Δ sham, second column) conditions and statistical maps of comparisons assessed by paired t-tests (Δ θ-tACS vs. Δ sham, third column). Maps are plotted in the theta and alpha frequency range (5–12 Hz) with a single-Hz resolution. Values are colour coded and plotted at the corresponding position on the planar projection of the scalp surface and are interpolated (biharmonic spline) between electrodes. The topographic maps (first and second columns) are scaled between minimal and maximal values across the two experimental conditions and all frequency bins. The statistical maps are scaled symmetrically according to the absolute maximal t-value across the statistical comparisons in all frequency bins. White dots represent significant statistical differences between θ-tACS and Sham conditions surviving the TFCE correction for multiple comparisons (p_{TFCE-corrected} < 0.05).

Figure 5

Correlations between changes in EEG power and subjective sleepiness. Topographic distribution of the correlation coefficients (Pearson’s r) between changes in subjective sleepiness and EEG power changes expressed as (Δθ-tACS – ΔSham) differences.