Decomposing the effects of α-tACS on brain oscillations and aperiodic 1/f activity

Background Aftereffects of transcranial alternating current stimulation (tACS) are usually analyzed with a focus on the individual frequency band, thereby neglecting broadband spectral components. Recently, it was shown that the broadband spectrum, which exhibits a 1/f-like characteristic, is functionally relevant. Objective/Hypothesis The goal of this study was a spectral parameterization of brain activity into oscillatory alpha activity and aperiodic 1/f components before and after tACS and sham stimulation. It was expected that the broadband spectrum will not be differentially influenced by 20-min of tACS at individual α-frequency (IAF) in comparison to sham. Additionally, it was expected that the tACS aftereffect on the α-band can still be observed, even when controlling for 1/f activity differences. Methods We performed a re-analysis of a recently published resting-state tACS-magnetoencephalography (MEG) data set. Parameterization of the frequency spectrum was computed with the fitting-oscillations-and-one-over-F (FOOOF) algorithm. The intercept as well as the slope parameter of the aperiodic 1/f fit was extracted. Data was analyzed in sensor space with a focus on magnetometers. Comparison of changes in α-band power and 1/f activity was performed with non-parametric cluster-based random permutation tests. Results The tACS aftereffect survived the 1/f-correction. The previously observed natural rise in alpha oscillations over time independent of experimental conditions could not be replicated. However, differences in aperiodic parameters over time were observed. Especially, the intercept parameter increased from pre to post stimulation to a similar degree in both conditions. Conclusion It is imperative to correct for the aperiodic 1/f spectral component when analyzing aftereffects of brain stimulation on brain oscillations.