P186 Prefrontal transcranial direct current stimulation induces polarity-specific changes in resting state electroencephalographic gamma activity

Abstract

Question The mechanisms underlying transcranial direct current stimulation (tDCS) induced changes of cortical excitability and arousal remain to be fully characterized. In the context of our larger study on the effects of bifrontal tDCS on overnight sleep, we analyzed tDCS-modulated resting state wake EEG data for a better understanding of tDCS’s effects on cortical arousal during wakefulness. Methods We applied a standardized tDCS protocol (1 mA/electrode, 2 × 9 min cathodal, 2 × 13 min anodal, 2 × 11 min sham, with 20 min inter-stimulation interval each) prior to nighttime sleep bilaterally to the prefrontal cortex (electrodes 5 × 7 cm, FP1/FP2; return electrodes 10 × 10 cm, P3/P4 ) of 19 healthy subjects in a randomized crossover design. We conducted 5 min resting state wake EEGs prior to stimulation (T0), after the stimulation (T1) and on the following morning (T2). In a first step of analysis for EEG power differences between conditions at a single frequency, we calculated false discovery rate (FDR) corrected significances, using the Benjamini-Hochberg step-up procedure to assess the significance of the multiple tests. In a second exploratory step, we used uncorrected ANOVAs to further explore tDCS effects on EEG power spectra. Results The FDR-corrected approach did not reveal any significant condition effect. ANOVAs comparing T1 vs. T0 demonstrated a significant decrease in EEG gamma power after cathodal stimulation. A significant main effect for the factor Condition was observed for frequencies of the gamma range. ANOVAs comparing T2 vs. T0 demonstrated a significant increase in EEG gamma power after anodal stimulation and also a significant main effect for the factor Condition for frequencies of the gamma band. Conclusions Exploratory resting state EEG analyses suggested polarity-specific changes in cortical arousal as indexed by resting state EEG power in the gamma frequency range. Cortical gamma activity is considered to emerge from synchronous activity of fast-spiking inhibitory neurons in the cortex and has been linked to the integration of temporally correlated neural events as a prerequisite for attentive wakefulness. It is plausible that depolarization of cortical structures after anodal stimulation facilitates fast-spiking and EEG gamma activity, with reverse effects after cathodal stimulation.