Transcranial alternating current Stimulation (tACS) induced aftereffects in the individual Alpha frequency band: an EEG/fNIRS study

• Applicant: Professor Dr. Michael Doppelmayr
• Subject Area: Clinical Neurology; Neurosurgery and Neuroradiology
• Term: from 2018 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 398325528

Transcranial alternating current stimulation (tACS) is a highly interesting, new method to modulate brain rhythms. Electrodes are placed on the head and a minimal alternating current (1mA) within a selected frequency is sent through the brain. As far as reported this method can be used in humans without any risk. However, it may alter or modulate brain rhythms and concurrent the discharge pattern of the neurons in the targeted area. The most important brain oszillation in humans is the alpha rhythm with a frequency of about 10 Hz. Because alpha frequency can vary between subjects, for stimulation and analysis the individually adjusted alpha frequency (IAF) is used.Recent studies indicated that a) IAF adjusted tACS can enhance the amplitudes of the electroencephalogram (EEG) in this frequency for up to 90 minutes (Kasten et al.2016) b) if an implicit motor learning paradigm as a “serial reaction time task” (SRTT) is executed during tACS, this can increase learning performance (Pollok et al. 2015).Based on these findings we will perform a study including a 20 minute tACS adjusted to the IAF. We will also include a SRTT and we will analyze the effects for six hours after the stimulation. Forty male participants, double-blind divided in a stimulation (Stim) and a sham stimulation (Sham) group, will voluntarily take part in the experiment. We will focus on the effects of IAF-tACS on the EEG amplitudes, on learning performance and on the blood oxygenation in the brain, as assessed by functional near infrared spectroscopy (fNIRS). Three main questions will be answered:i) It has been reported that IAF-tACS can enhance the EEG amplitudes in this frequency range for up to 90 minutes. In a pilot study we have observed increased amplitudes for more than three hours after the stimulation. Thus, we will analyze the duration of this after effect for up to six hours after the tACS.ii) implicit motor learning paradigms as the SRTT yield better results if trained with accompanying tACS. If this positive effect is due to the tACS induced enhanced amplitudes, learning effectivity should be increased too, as long as the amplitudes are enhanced. Thus, the participants will perform the SRTT repeatedly after the tACS (assuming there are enlevated amplitudes) and we will analyze the learning efficiency.iii) Up to now there is no single report about whether the increase of the EEG amplitudes during tACS is correlated to an accompanying increased or decreased brain activity. Thus, we will record brain blood oxygenation by the means of fNIRS, as a measure of brain activity. fNIRS will be recorded before, during and after tACS.Taken together these are three highly relevant topics that will be answered with this study. The results are of high importance both, for the understanding of the basic mechanisms of tACS as well as for potential therapeutic applications.

• DFG Programme: Research Grants