Syoichi Tashiro ^1,2*

• ¹ School of Medicine, Keio University, Tokyo, Japan
• ² Kyorin University, Mitaka, Tōkyō, Japan

Stroke is a leading cause of disability and induces an enormous burden of care as well as economic loss. While various neurorehabilitation methods, including those utilizing physical modalities, are used, the degree of recovery is not favorable in the chronic phase.Non-invasive transcranial brain stimulation (NTBS) has attracted wide attention as it modifies brain state and plasticity via electrical or magnetic stimulation of the brain presenting functional impairment. Transcranial electric current stimulation has the advantages of simplicity and higher practicality than magnetic stimulation. Although transcranial direct current stimulation (tDCS) has been investigated for decades, its clinical application has been limited by variability in efficacy. On the other hand, investigations on transcranial alternating current stimulation (tACS) have only recently begun. It has been suggested that tACS exerts its effect via the entrainment phenomenon to synchronize the brain's endogenous rhythm with the frequency of the exogenous AC current applied transcranially. However, it is only recently that tACS studies were initiated in stroke patients. This editorial introduces the current status of the clinical application of tACS in stroke treatment.Only two studies have been published on the treatment of hemiparesis. Wu et al. Targeting other symptoms of stroke, Xu et al. studied visual field restoration secondary to tACS-tDCS and assessed functional connectivity using functional MRI in patients with occipital stroke. Following O1/2-Fpz tDCS at 1.0 mA, tACS was applied using an Fpz-upper arm montage at frequencies incremented from 1 to 30 Hz at 1.5 mA for 20 min. While there were responders and non-responders, the combinatorial treatment enhanced functional connectivity between the occipital and temporal lobes in the intact hemisphere and decreased low-frequency coherence between the damaged and intact occipital brain areas in responders.In addition to the treatment mentioned above, some researchers have studied the effect of tACS on stroke patients from more experimental aspects. Naros et al.investigated the effect of conditioning with 20 Hz tACS at 1.1 mA targeting the lesioned motor cortex. They compared two conditions, in which tACS was applied continuously before or intermittently during the motor imagery task. They found that intermittent tACS Several researchers have introduced the tACS in clinical settings to target stroke patients, but the target symptoms and stimulation conditions are still highly variable, and the reported effects are still not remarkable. It is noteworthy that some research group combine the other electrical stimulation to the periphery to support voluntary motor output. While some studies have used amplitude-modulated tACS comprising carrier and modulation frequencies, its physiological effects should also be clarified. Therefore, further studies are warranted.