Persistent changes in cortical, subcortical and network-level dynamics induced by 10-Hz tACS applied over bilateral parietal cortex: a MEG study

Abstract

Previous studies of the effects of tACS on brain dynamics report entrainment at the stimulation frequency, modulation of phase-phase and phase-amplitude relationships, and non-linear effects such as the generation of harmonic and subharmonic oscillatory activity. The majority of these investigations characterized the on-line effects of tACS in regions proximal to the stimulation electrodes. We utilized a 306-channel MEG array to characterize persistent changes in brain dynamics induced by alpha-band tACS. Eight adults with no neurological or mental disorders received 10-Hz stimulation over bilateral parietal cortex (EEG sites P3/P4; 1 mA; 20 min verum, 30 s sham). Participants performed a global/local visual attention task during and after stimulation. Spectro-temporal (TF) and spectral Granger causality measures were utilized to characterize changes in brain dynamics for cortical and subcortical nodes in the Default Mode Network (DMN) and Rich Club. Although there were frequency-, epoch- and task-related effects, verum vs. sham stimulation tended to upregulate gamma-band activity in the DMN nodes of left middle temporal and inferior parietal cortex, bilateral lateral occipital and medial orbitofrontal and bilateral crus I/II of cerebellum. Downregulation occurred primarily in the Rich Club nodes of right precuneus and right superior frontal cortex. There was little evidence for down-regulation within superior parietal cortex and hippocampus. A spectral Granger causality analysis revealed a complex dependence on brain region and task. Interestingly, all significant changes observed at 10, 20, 30, 40 and 70 Hz were consistently either increases or decreases in causality between ordered pairs of nodes. Causality was increased (decreased) following tACS for posterior cortical nodes in the DNM (Rich Club) networks. In contrast, posterior-to-frontal causal relationships increased for nodes in both networks. Changes in cortico-cerebellar relationships occurred only during attention to local features of visual images. The influence of hippocampus onto cortical nodes decreased for both tasks.