Optimized tACS parameters for modulation of alpha oscillation

Abstract

Transcranial alternating current stimulation (tACS) is a non-invasive brain stimulation technique allowing the administration of a sinusoidal current adjusted to one’s maximal endogenous frequency. Oscillations within the alpha band range (8-12 Hz) have various associations with brain functions and states. A number of studies have proven to be effective in increasing alpha power using tACS. However, the heterogeneity of stimulation parameters, notably the intensity, leads currently to no consensus on optimal stimulation parameters to modulate 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. 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, continuous tACS was 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) or F3-F4 , at either the Individual’s Alpha Frequency (IAF), the Individual’s Theta Frequency (ITF) or sham intensity, which were a priori determined via a 5-minute pre-stimulation EEG recording with eyes open at rest. Intensity was set according to a standardized unpleasantness scale (≤ 30 out of 100) and couldn’t exceed 6 mA. The second tACS condition was administered 180 minutes after. All conditions were counterbalanced. Results suggest that alpha tACS stimulation adjusted to IAF was effective in increasing alpha power. Of the two stimulating sites, posterior alpha tACS stimulation induced greatest increases in alpha power, maximal when set to IAF, whereas frontal alpha tACS stimulation showed minimal effect on alpha. This study provides preliminary evidence that posterior tACS stimulation adjusted to IAF at higher intensities is well-tolerated and of great potential to increase posterior alpha power.