Effects of alternating current stimulation on the healthy and diseased brain

Abstract

Cognitive and neurological dysfunctions can severely impact a patient's daily activities. In addition to medical treatment, non-invasive transcranial alternating current stimulation (tACS) has been proposed as a therapeutic technique to improve the functional state of the brain. Although during the last years tACS was applied in numerous studies to improve motor, somatosensory, visual and higher order cognitive functions, our knowledge is still limited regarding the mechanisms as to which type of ACS can affect cortical functions and altered neuronal oscillations seem to be the key mechanism. Because alternating current send pulses to the brain at predetermined frequencies, the online- and after-effects of ACS strongly depend on the stimulation parameters so that "optimal" ACS paradigms could be achieved. This is of interest not only for neuroscience research but also for clinical practice. In this study, we summarize recent findings on ACS-effects under both normal conditions and in brain diseases.

Keywords: EEG; oscillation; synchronization; transcranial alternating current stimulation; transorbital alternating current stimulation.

Figure 1

Examples of stimulation electrode placements and neural oscillation. (A) Transcranial ACS: electrodes were placed over the parieto-occipital region [P09 (target) and PO10 (reference), according to the 10–10 system]. (B) Transorbital ACS: four stimulation electrodes were positioned at or near the eyeballs (with eyes closed), and one electrode was positioned at the occipital pole as the reference electrode. (C) The model prediction of neural oscillation and neural firing patterns induced by ACS (adapted from Zaehle et al., ; Gall et al., ; Battleday et al., 2014).