P 62 The effect of alternating current stimulation on individual alpha – A rebound phenomenon?

Abstract

Background Non-invasive brain stimulation (NiBS), including transcranial alternating current stimulation (tACS), has shown impressive effects ranging from short changes in neural activity to long lasting recovery maximization following neural injury. Despite these encouraging results, little is known about the mechanism of action. TACS may be in danger of facing the challenges of large effect variability and poor result reproducibility. A better understanding of the underlying neurophysiological mechanisms of tACS” effects on neuronal oscillations should help address these issues. Hypotheses about these mechanisms include neuronal entrainment as well as rebound phenomena. Originally, we assumed a frequency peak shift towards stimulation frequency to occur after AC stimulation as a correlate for entrainment. Therefore, we investigated the alpha band peak before and after 10 Hz retinofugal tACS (rtACS) as well as before, during and after well-established photic driving. Methods To address this question, we stimulated 15 healthy subjects with rtACS at a frequency of 10 Hz while recording EEG. Prior to stimulation, we assessed electrode impedances and phosphene thresholds and recorded a resting state EEG. We applied rtACS in 6 blocks of 30 s stimulation. We assessed spectral alpha power as well as alpha peak frequency (individual alpha) over occipital electrodes during baseline and 30 s after stimulation and compared the two in a repeated-measure ANOVA. Results The alpha peak frequency changed by a mean 0.02 Hz (+/−0.04), not correlated with the distance of the baseline alpha peak to stimulation frequency. A greater change in alpha peak frequency did not correlate with an increase in alpha power. Similar results were found during and after photic driving (0.03 (+/−0.05)). Conclusions Surprisingly, we did not find the expected frequency peak shift. This provides an argument against entrainment and in favor of a rebound effect, which would explain stabilization and enhancement of an intrinsic frequency following a period of inhibition. Still, this does not explain behavioral changes associated with effects during rtACS or the phenomenon of photic driving. As neural oscillators share features of both harmonic and relaxation oscillators, the mechanism of action may be a combination of both effects, where short periods of entrainment are followed by a rebound-like recruitment of similar patterned oscillators.