Transcranial alternating current stimulation affects several alpha components depending on their frequencies relative to the stimulation frequency

Background The aftereffects of transcranial alternating current stimulation (tACS), especially targeting occipital alpha oscillations, have been reported to show large individual differences in behavioural effects and neural responsiveness. We predicted that this variance at least partly originates from the fact that multiple alpha components are affected differently by alpha-tACS. Objective To test the above prediction, we decomposed several alpha components from the data and evaluated the aftereffects separately for each component and participant. More specifically, we tested how the difference between the stimulation frequency and peak frequency influences the aftereffects of tACS on each alpha component. Methods Eighteen participants received 20-min tACS or sham stimulation on separate days. Ten minutes of magnetoencephalography data were collected before and after stimulation, and spectral analysis was performed with a high-frequency resolution (0.1 Hz) to disentangle different alpha components based on the difference in peak frequencies and spatial patterns. Results: The results revealed three alpha components with slightly different frequencies. tACS increased or decreased the power of these alpha components depending on the relative frequency difference from electrical stimulation. Furthermore, observable components differed among participants, possibly because of anatomical differences in each alpha source. Conclusion When each alpha component is not analysed separately, the change in overall alpha power will be a combination of decreased or increased components, and inter-individual differences will become larger. Our study highlights the importance of noting the presence of multiple alpha components with relatively small differences in the peak frequency for experimental design and analysis.