Induction and stabilization of brain oscillations in delta frequency by phase-synchronized rTMS and tACS

Abstract Delta frequency is the predominant oscillation in the slow-wave, non-rapid-eye-movement (NREM) sleep stage. This sleep stage is also known to have significance for cognitive processes, particularly for the consolidation of declarative memory. However, the respective causalities still remain to be elucidated due to the limitation of techniques for externally modulating neuronal oscillations in delta frequency. In the present study, we aimed to develop a non-invasive stimulation protocol to induce and stabilize delta activity in the human which is qualitatively more efficient than conventional brain stimulation methods. The effects of the phase-synchronized repetitive transcranial magnetic stimulation (rTMS) and transcranial alternating current stimulation (tACS) in the delta frequency (0.75Hz) were tested by EEG. Changes in the power spectral density of 0.75 Hz following the bilateral, phase-synchronized rTMS aligned to the tACS peak and trough were compared with those after rTMS alone, tACS alone and sham rTMS and tACS stimulation. 21 healthy participants took part in a repeated-measures design. Resting-state EEG in eyes-open (EO) and eyes-closed (EC) was recorded before (baseline), immediately (1-min) and every 10 min for up to 60 min after the stimulation. The results showed 0.75 Hz resting-state EEG increased significantly with both peak- and trough- synchronized rTMS+tACS immediately after the stimulation in the EO condition. Increased delta power was still observed at later timepoints. The results suggest the potential of phase-synchronized rTMS and tACS to be a powerful tool in the research and clinical domain. It may help to investigate the causal relevance of delta frequency on sleep and memory consolidation, and restore pathologically altered brain oscillations. Research Category and Technology and Methods Basic Research: 15. Electroencephalography (EEG) Keywords: rTMS, tACS, EEG, oscillations