Phase-specific manipulation of neural oscillations by transcranial alternating current stimulation

Abstract Oscillatory phase has been proposed as a key parameter defining the spatiotemporal structure of neural activity. To enhance our understanding of brain rhythms, phase-specific modulation of oscillations by transcranial alternating current stimulation (tACS) emerged as a promising approach. However, the effectiveness of tACS is still critically debated. Here, we investigated the phase-specificity of tACS effects on visually evoked steady state responses (SSRs). We observed that the phase shift between flicker and tACS modulates evoked SSR amplitudes. Neural sources of phase-specific effects were localized in the parieto-occipital cortex within flicker-aligned regions. Importantly, tACS effects were stronger in subjects with weaker locking of neural responses to flicker phase. Overall, our data provide evidence for a phase-specific modulation of oscillations by tACS, since SSR amplitude changes can only result from phase-dependent interactions with applied electric fields. This finding corroborates the physiological efficacy of tACS and highlights its potential for controlled modulations of brain signals.