Objective

            Concurrent application of transcranial alternating current stimulation (tACS) over distant cortical regions appeared to modulate functional connectivity between these regions; however, the precise mechanisms by which functional connectivity is modulated between cortical regions remain unclear. Here, we investigated how dual-site tACS (ds-tACS) applied over bilateral motor cortices (M1s) modulates the functional connectivity between M1s using transcranial magnetic stimulation (TMS) and electroencephalogram (EEG).

Methods

            Using a cross-over sham-controlled triple-blind within-subject design, 35 (24 female, age 18-40 yrs) healthy younger participants received tACS (1.0mA, 20Hz) over bilateral M1s for 20 min. Before and after tACS, functional connectivity between M1s was assessed using imaginary coherence (ImCoh) measure via resting-state EEG and interhemispheric inhibition (IHI) via dual-site TMS protocol.

Findings

The preliminary analyses indicated that ds-tACS modulated interhemispheric inhibition between M1s, whereas no such modulation was found in the sham session. While ImCoh showed no change after the simulation, spectral power analysis showed a significant decrease in beta (13-30 Hz) power was found in the tACS session.

Conclusion

            Based on our preliminary results, we suggest that ds-tACS is capable of modulating IHI between the M1s. This change in IHI is likely associated with a decrease in beta power, indicating that the IHI changes following tACS are unlikely due to the induction of phase coupling in the oscillatory activity of neural populations. Since functional connectivity underlies many clinical symptoms in neurological and psychiatric disorders, these findings are invaluable in developing non-invasive therapeutic interventions that target specific neural networks to alleviate symptoms.