Phase-Locked Noninvasive Brain Stimulation

One of the unique methods to directly impact the electrical activity of the brain is through the use of noninvasive brain stimulation methods. These methods, such as transcranial current stimulation (tCS), are applied in scientific community to provide insights in cognitive processes but also to treat certain health conditions. Although transcranial alternate current stimulation (tACS) is less frequently used than widespread transcranial direct current stimulation (tDCS), it offers the possibility to directly entrain brain activity, leading to short- and potentially long-term physiological and cognitive effects. Together with methods facilitating better sensing and stimulation focus, it enables improved spatio-temporal neuromodulation. One of the crucial aspects of facilitating this is introducing a phase locking mechanism within a closed-loop, encompassing a real-time brain activity readout and targeted stimulation. In this paper we introduce a phase-locked closed-loop tACS framework and demonstrate its working principles. We focus on the phase-locking method as a crucial component and analyse its performance on an artificial setup that includes an in-house developed phantom head and a software framework. The achieved temporal precision is in the order of 5.4° ± 2.31°. The future outlook of the proposed framework is discussed, in particular looking at the research applications and clinical potential.