Frequency-dependent effects of oscillatory-tDCS on EEG oscillations. A study with better oscillation detection method (BOSC)

Oscillating transcranial direct current stimulation (osc-tDCS) modulates the spontaneous brain activity in a frequency-specific manner. Most studies evaluated cortical effects of osc-tDCS through spectral analysis measures, without differentiating components associated with rhythmic and non-rhythmic activity. Since osc-tDCS mainly affects brain oscillatory activity, our aim was to investigate on the specific changes of EEG oscillations following a frontal osc-tDCS at 0.8 and at 5 Hz. 20 healthy subjects (26.8 ± 2.5 years) participated in one of two experiments (Exp.1= 0.8-Hz tDCS, n= 10; Exp.2= 5-Hz tDCS, n= 10), consisting of 3 within-subject sessions: two active conditions with different stimulation polarity (anodal osctDCS, cathodal osc-tDCS), and a control condition (sham). EEG oscillatory components (28 cortical derivations) at the stimulation frequency were measured by the Better OSCillation detection method (BOSC). Variations between before and after the osc-tDCS were compared between conditions as a function of polarity (anodal vs. cathodal vs. sham) and frequency (0.8 vs. 5 Hz) of stimulation. The main finding is a significant local increase of 0.81-Hz slow oscillations (F1,18=19.97; p=0.0004) and 5.3-Hz theta oscillations (F1,18=26.93; p= 0.0001) after 5 Hz compared to 0.8-Hz tDCS. Our study shows larger frequency-specific and cross-frequency effects of 5-Hz compared to 0.8-Hz stimulation, not revealed by conventional FFT analyses. This finding is consistent with a more effective induction of EEG synchronization during wakefulness by means of a stimulation in the theta range, and it suggests to combine measurement of EEG power and EEG oscillations in future studies involving transcranial stimulations.