Discordant Alpha-band Transcranial Alternating Current Stimulation (tACS) 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. 10-Hz 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. Non-focal and non-phase-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 were 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 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.