Effects of tACS on alpha oscillations are task-dependent

Transcranial alternating current stimulation (tACS) is a widely used non-invasive technique that aims to modulate neural activity and behaviour. However, the effects of tACS are rather inconsistent and difficult to replicate. In this study, we seek to modulate alpha oscillations and further research on identifying the optimal conditions for observing reliable tACS-induced changes. Specifically, we investigated the effects of 10 Hz stimulation on alpha oscillations compared to 41 Hz stimulation in a within-subject experimental design. We chose a montage with the anode and cathodes spaced farther apart to generate stronger electric fields at the expense of focality. Participants performed an oddball version of the standard vigilance task with a lateralized bias in motor responses, which is likely to engage alpha oscillations as they are thought to play a role in inhibitory control and lateralized attention management. Our findings revealed that the difference in alpha power enhancement between alpha and gamma stimulation was higher for the contralateral electrodes than the ipsilateral ones. This lateralized effect emerged despite our non-lateralized tACS electrode positioning, suggesting it may be due to the lateralized inhibitory motor planning involved in the task. Our results indicate that the effects of tACS on alpha oscillations are task-specific. Consequently, tACS researchers may opt for stronger electric fields, even at the cost of focality, as tasks could help direct tACS effects to specific brain areas.