Discordant Alpha-Band Transcranial Alternating Current Stimulation Affects Cortico-Cortical and Cortico-Cerebellar Connectivity.

Synchronization of oscillatory brain activity is believed to play a critical role in linking distributed neuronal populations into transient functional networks. Alpha-band alternating current stimulation (tACS) was applied over bilateral parietal cortex in a double-blind sham-controlled study to test the notion that widespread alpha mediates causal relationships in the gamma-band both within local neuronal populations and also across distant brain regions. Causal relationships of oscillatory alpha- and gamma-band activity were characterized during performance of a visual global/local attention task. Nonfocal and nonphase-locked tACS, discordant with endogenous oscillatory activity, was hypothesized to induce a performance deficit and differences in network-level causal relationships between both cortical and subcortical brain regions. Although modulation of fronto-parieto-cerebellar causal relationships was observed following stimulation, there was no evidence for a behavioral deficit. We propose that olivo-cerebellar circuits may have responded to the discordant tACS-induced currents as if they were "error signals" in the context of ongoing functional alpha-band brain dynamics. Compensatory cerebellar activity may have contributed to the lack of behavioral deficits and to differences in causal relationships observed following stimulation. Understanding a potential compensatory mechanism involving short-term plasticity in the cerebellum may be critical to developing potential clinical applications of tACS, particularly for disorders such as autism that are characterized by both atypical cortical and cerebellar dynamics.