Manipulation of Oscillatory Phase by Transcranial Alternating Current Stimulation

Oscillatory phase has been proposed as key parameter defining the spatiotemporal structure of neural activity. Phase alignment of oscillations by transcranial alternating current stimulation (tACS) may offer a unique opportunity to enhance our understanding of brain rhythms and to alter brain function. However, the precise mechanism and effectiveness of tACS are still critically debated. Here, we investigated the phase-specificity of tACS effects on visually evoked steady state responses (SSR) measured by electroencephalography (EEG). Using an intermittent electrical stimulation protocol, we assessed the influence of tACS on SSR amplitude as a function of phase shift between rhythmic sensory and electrical stimulation in the interval immediately following tACS. Visual flicker was delivered at six different phase angles relative to the tACS cycle. Participants were presented with flicker and high-definition tACS over the occipital cortex at 10 Hz in two sessions using active or sham stimulation. We observed that the phase shift between flicker and tACS modulates evoked SSR amplitudes. The amplitude change over phase shift conditions was significant for both general and sinusoidal modulation measures. 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 lower phase locking between EEG and flicker. Overall, our data provide evidence for phase alignment of brain activity by tACS, since the change in SSR amplitude can only result from phase-specific interactions with the applied electric fields. This finding corroborates the physiological efficacy of tACS and highlights its potential for controlled modulation of brain signals.