Bifocal high-definition tACS over early sensory regions modulates crossmodal matching: Combined evidence from EEG and tACS/behavioral studies

Abstract

Cross-talk between distant brain regions is an integral part of complex brain dynamics. In order to study such interactions, bifocal high-definition tACS (hd-tACS) represents a promising tool to modulate oscillatory synchrony between distant brain regions in a phase- and frequency-specific manner. Here, we investigated cortico-cortical synchronization in alpha and gamma bands underlying multisensory integration. First, we employed a paradigm on crossmodal matching in an EEG study to show that stimulus-driven benefits of crossmodal congruence are paralleled by decreases in gamma band coherence between early sensory cortices. Second, we used bifocal hd-tACS to target visual and somatosensory regions in a similar crossmodal matching paradigm. In- or anti-phase stimulation between the two regions (0°/180° phase shift) was applied with either 10 or 40 Hz and was controlled by sham. As an additional control, electrical stimulation was applied ipsilateral or contralateral to the presentation of visual-tactile stimuli. As hypothesized based on our EEG data, we found contralateral gamma stimulation to decrease behavioral benefits of crossmodal congruence compared with sham. To our surprise, both in- and anti-phase stimulation showed such an effect. We conclude that 40 Hz stimulation might have synchronized gamma band activity across modalities irrespective of stimulation phase. Ipsilateral stimulation showed no interaction with stimulus congruence. Yet, overall performance was increased by ipsilateral alpha and decreased by ipsilateral gamma stimulation. We suggest that, in the absence of sensory input, tACS modulated general cortical excitation/inhibition and related hemispheric competition. Taken together, we present evidence for the involvement of gamma synchronization in crossmodal matching. Our results further suggest that conventional approaches to decrease functional coupling by anti-phase tACS might not always be effective. Our study design – incorporating both EEG and behavioral studies – also demonstrates how acute effects of tACS can be studied bypassing the unsolved problem of tACS-related artifacts in electrophysiological recordings.