P 12. Transcranial alternating current stimulation enhances endogenous alpha for 30 min only for moderate alpha levels

Abstract

Transcranial alternating current stimulation (tACS) has been proven to successfully influence endogenous brain oscillations in a frequency specific manner ( Zaehle et al., 2010 ). Besides effects on the electrophysiology, tACS is also able to influence perception and behavior ( Neuling et al., 2012 ). After tACS, the endogenous power of the stimulated frequency is enhanced when compared a pre-stimulation period, however, little is known about the duration of this effect and which parameters contribute to this effect. Our goal was to discern how long endogenous brain oscillations are enhanced post tACS and how pre-stimulation power of the oscillation modulates this effect. Two experiments were conducted whereby subjects had their eyes either open or closed. Experimental procedure is demonstrated in Fig. 1 . Participants were stimulated with their individual alpha frequency (IAF) for 20 min or received sham stimulation. Stimulation electrode positions were chosen in order to effect occipital alpha ( Neuling et al., 2012 ). Five minutes before (pre) and 30 min after stimulation (post), the electroencephalogram (EEG) was recorded. During the experiment, subjects performed a simple auditory detection task to ensure vigilance ( Fig. 1 C). In the eyes closed experiment, no effects on the oscillatory power could be demonstrated ( Fig. 2 AI). After stimulation, both in the stimulation and in the sham group, IAF power was not significantly different than pre-stimulation levels ( Fig. 2 BI). Likewise, no significant effects in the entire post period could be revealed ( Fig. 2 CI). Contrary to the results in the eyed closed experiment, an effect on the oscillatory power, limited to the alpha range, was found in the eyes open experiment ( Fig. 2 AII). The power increase from the pre-to the post stimulation period was significant in the stimulated group, but not in the sham group ( Fig. 2 BII). This enhancement effect was significant over the entire30 min post-stimulation recording ( Fig. 2 CII). If the endogenous alpha oscillations are high, stimulation with tACS fails to increase alpha power due to ceiling effects. The neuronal network activity is unaffected by external oscillations, at least in terms of oscillatory power. Endogenous oscillations exhibiting lower power are more prone to effects of tACS. Here, tACS is able to enhance endogenous oscillations over a long duration. It might be speculated that the enhancement effect would last even longer than 30 min. The results of our study illustrate the feasibility of tACS as a tool for non-invasive brain stimulation and demonstrate the potential for therapeutic application to balance altered brain oscillations.