Neuromodulation using cross-frequency coupling tACS: preliminary data

Abstract Non-invasive brain stimulation (NIBS) techniques are widely used in both clinical and experimental settings because they can modulate and provide information about ongoing cortical activity. It is well known that our brain works in networks and that frequency changes probably sustain communication among different cortical areas. In particular, the cross-frequency coupling (CFC) phenomenon is thought to mediate large-scale integration and communication by synchronizing the discharge frequencies of other neuronal populations in time. Based on the CFC hypothesis, this study aimed to interfere with the spontaneous cortical activity of primary motor cortices (M1) using an innovative transcranial alternating current stimulation (tACS) protocol. Seven healthy subjects underwent two experimental sessions in which we applied either 20 minutes of theta-gamma tACS (θ-γ tACS) or 20 minutes of sham tACS (s-tACS). To assess the aftereffects induced with this protocol, we used transcranial magnetic stimulation (TMS) over left M1 to evaluate cortical changes in different intracortical circuits before and after the intervention. In particular, we tested motor evoked potentials (MEP), Short Intracortical Facilitation (SICF), and Short Intracortical Inhibition-Intracortical Facilitation (SICI-ICF) before and after the stimulation. We expected there to be a shift in the balance between inhibitory and excitatory activity only following the θ-γ tACS stimulation. The main result of the study is that θ-γ tACS exerted an effect on SICI, leading to a lower inhibition after the stimulation (p < 0.05), whereas no changes were found in the MEP and ICF following stimulation (p > 0.05). Importantly we found no statistically significant differences in any of the neurophysiological measures (i.e. MEP, SICF, SICI-ICF) following the sham condition. These preliminary data suggest that combining different frequencies for tACS interventions may effectively induce changes in cortical activity. Understanding the neural underpinnings of such neuromodulation may shed light on the future therapeutic application in clinical settings. Research Category and Technology and Methods Basic Research: 8. Transcranial Alternating Current Stimulation (tACS) Keywords: tACS, TMS, cross-frequency , neurophysiology