Caractérisation de la technique de stimulation transcrânienne par courant alternatif pour optimiser l’augmentation de la puissance alpha

Transcranial alternating current stimulation (tACS) is a non-invasive brain stimulation technique increasingly used for its modulating effect on intrinsic brain oscillations. In comparison to transcranial direct current stimulation (tDCS), tACS allows the administration of a sinusoidal current adjusted to one’s endogenous measured frequency. Oscillations within the alpha band range (8-12 Hz) are among the most studied, given their various associations with brain functions and states. A number of studies have proven to be effective in increasing alpha power using tACS through diverse methods. However, the heterogeneity of stimulation parameters, notably the intensity, makes it difficult to implement new tACS protocol. Indeed, there is currently no consensus on optimal stimulation parameters to modulate the alpha rhythm. The current project aimed to document the differential impact of controlling for key tACS stimulation characteristics, namely the stimulation intensity, the stimulation frequency and the stimulation site (anterior or posterior). To this end, we conducted a study, in which 20 healthy participants underwent four different tACS conditions conducted over two non-consecutive days (2 blocks per day). In each experimental condition, tACS stimulation was continuously delivered via two electrodes for a total duration of 20 minutes. Two active tACS conditions were administered at electrode sites PO7-PO8 (10-10 International System) at either the Individual’s Alpha Frequency (IAF) or at the Individual’s Theta Frequency (ITF), which were a priori determined via a 5-minute pre-stimulation EEG recording with eyes open at rest. Two stimulation conditions were performed with stimulating electrodes positioned over F3-F4 electrode sites, at IAF or sham intensity (ramp-up of 15 seconds). The stimulation intensity was set according to the participant’s own rating of unpleasantness on a standardized unpleasantness scale (≤ 40 out of 100) and could not exceed 6 mA. The second tACS condition was administered 180 minutes after the first tACS condition. To assess for fatigue levels, participants were asked to perform a psychomotor vigilance task (PVT) during tACS. All conditions were counterbalanced. Results suggest that alpha tACS stimulation adjusted to IAF was effective in increasing alpha power. Of the two stimulating sites, anterior alpha tACS stimulation induced greatest increases in alpha power, maximal when set to IAF, although specific to alpha generators’ site. Posterior alpha tACS stimulation showed overall increase both over frontal and posterior brain areas. These effects persisted at the 60-minute recording for the anterior tACS only. The current pilot study provides preliminary evidence that posterior tACS stimulation adjusted to IAF at higher intensities is well tolerated and shows potential as an effective brain stimulation technique to increase posterior alpha power.

Keyphrases: Oscillation alpha, tACS, EEG, Stimulation non-invasive cérébrale, Alpha power modulation, Alpha wave, Brain oscillations, Non-invasive brain stimulation