P189 Transcranial modulation of visually induced gamma power: a concurrent TACS-MEG study

Abstract

Introduction Alpha oscillations (8–12 Hz) are proposed to mediate ‘attentional gating’ by suppressing neuronal processing in task-irrelevant sensory regions, reflected by ‘pulsed-inhibition’ of stimulus-induced gamma-band activity (40–100 Hz). Concurrent TACS-MEG has the potential to reveal TACS entrainment of alpha oscillations at the stimulation site and thus their causal impact on neuronal activity representing stimulus processing. Objectives We aimed to provide evidence that TACS can entrain the phase of alpha oscillations in the visual cortex by assessing its modulatory impact on the power of visual stimulus-induced gamma oscillations via simultaneous MEG recordings. Materials & methods TACS was applied during MEG in intermingled 5.4s trials either in Oz-Cz or FPz-Cz montage. Montage and stimulation frequency were randomized across trials, applying either individual alpha frequency (IAF), IAF−4 Hz, IAF+4 Hz, or no stimulation, resulting in a 2 (Montage) × 3 (Frequency) design plus a ‘NoStim’ condition. Stimulation intensity for both montages was adjusted separately to 95% retinal phosphene threshold. Participants performed a simple visual detection task, attending an inward moving grating, known to induce strong occipital gamma responses. TACS artifacts were reduced via regression methods, LCMV beamformer time-course extraction from the visual cortex, and high-pass filtering. Importantly, the gamma frequency of interest (∼50–60 Hz) was largely unaffected by TACS artifacts. We compared TACS-to-gamma phase-power coupling (PAC) for the respective experimental conditions. Results We found a rhythmic modulation of stimulus-induced gamma power by TACS at IAF (i.e., TACS-gamma phase-amplitude coupling) for the Oz-Cz montage, but not for the FPz-Cz montage which was introduced to control for subthreshold retinal entrainment effects. Conclusion Our findings (i) demonstrate that concurrent TACS-MEG can principally be used to study cross-frequency-coupling between transcranially entrained slower and spontaneous or stimulus-related faster oscillations, (ii) cannot merely result from subthreshold retinal entrainment, and (iii) suggest that alpha oscillations are indeed a possible means for top-down suppression of bottom-up sensory processing.