This literature review is aimed to explore the main technical characteristics of both transcranial direct current stimulation (tDCS) and transcranial alternating current stimulation (tACS) using the latest research on both healthy and impaired subjects. These techniques have no oﬃcial standards developed yet. Our intent is to underline the main properties and problems linked with the application of those techniques which show diverse, and sometimes even opposite, results depending mainly on electrode positioning and underlying brain activity.
Among diﬀerent impairments that can aﬀect standard brain functions, we choose to focus primarily on stroke, because it is one of the most prevalent and severe disability worldwide . It is known that after a cerebrovascular accident, reorganization of neural tissues takes place . If the ischemic event occurs on the motor area and it is severe enough to block the spontaneous neural reorganization, it could lead to paresis or even paralysis of one or more body parts .
In order to ameliorate stroke rehabilitation, diﬀerent approaches have been carried out. Over the last decade, within the ﬁeld of functional rehabilitation, transcranial current stimulation (tCS) has garnered considerable attention. It is assumed to improve, above other, motor functions in both healthy and stroke individuals , , .
There are three diﬀerent types of tCS: transcranial direct current stimulation (tDCS), transcranial alternating current stimulation (tACS) and random noise stimulation (tRNS). All of them are non-invasive and involve low intensity current induction into the brain. Some studies have investi
gated the physiological basis of tDCS and tACS in order to get the picture of standard pattern that can be used for future research , .
This paper is oriented towards a broad audience who wants to understand the basic mechanisms of tDCS and tACS techniques. The main parameters of each type of stimulation and the implications related to its application on healthy subjects, stroke patients and individuals with unusual brain oscillations are discussed.