In-vivo phase-dependent enhancement and suppression of brain oscillations by transcranial alternating current stimulation (tACS)

Transcranial alternating current stimulation (tACS) can influence human perception and behavior, with recent evidence also suggesting its potential impact in a clinical setting, but its underlying mechanisms are poorly understood. Behavioral and indirect physiological evidence indicates that phase-dependent constructive and destructive interference between the applied electric field and ongoing brain oscillations may play an important role, but direct in-vivo validation was infeasible because stimulation artifacts impeded such assessment. Here, we overcame this limitation and provide direct evidence for phase-dependent enhancement and suppression of ongoing brain oscillations during amplitude-modulated tACS (AM-tACS). We found that AM-tACS enhanced and suppressed targeted brain oscillations by 11.7 ± 5.14% and 10.1 ± 4.07% respectively. These results not only provide direct evidence for constructive and destructive interference as a key mechanism of AM-tACS but suggest superiority of phase-locked (closed-loop) AM-tACS over conventional (open-loop) AM-tACS to purposefully enhance or suppress brain oscillations.