Gamma oscillatory transcranial direct current stimulation of motor cortex enhances corticospinal excitability and brain connectivity in healthy individuals

Cortical excitability, the tendency of neurons to respond to various stimuli, is impaired in most neuropsychiatric conditions. Non-invasive brain stimulation can exert therapeutic effects by modulating the cortical excitability. Transcranial direct current stimulation (tDCS) and transcranial alternating current stimulation (tACS) have shown promise in various neuropsychiatric disorders, including improving cognitive abilities and motor function following stroke. Oscillatory transcranial direct current stimulation (otDCS), as a novel stimulation paradigm, combines tDCS and tACS to simultaneously regulate neuronal membrane potentials and oscillatory rhythms. This combination may produce more significant effects on neurons. To investigate this, participants received the following stimuli for 20 min on different days: (i) 2 mA 40 Hz otDCS, (ii) 2 mA 40 Hz tACS, (iii) 2 mA tDCS, and (iv) sham stimulation. Motor evoked potentials (MEPs) and transcranial magnetic stimulation combined with electroencephalography (TMS-EEG) were assessed both before and after stimulation. The increase in MEPs amplitudes was most pronounced under otDCS conditions compared with tACS and tDCS. Furthermore, analysis of TMS-EEG data revealed that changes in time-varying brain network patterns were most pronounced after otDCS, manifesting as enhanced brain-wide information connectivity. Our results indicate that gamma otDCS has significant potential for regulating cortical excitability and activating brain networks.