The effects of transcranial direct-current stimulation on offline visual entrainment: A magnetoencephalography study

Abstract

While a wealth of literature shows changes in behavior and cognition following transcranial direct-current stimulation (tDCS), the underlying neuronal mechanisms remain largely unknown. Neuroimaging studies in general have shown only limited consensus to date, and the few electrophysiological studies have reported mostly null or counterintuitive findings. The goal of the current investigation was to quantify tDCS-induced alterations in the oscillatory dynamics of visual processing. To this end, we performed 20 minutes of either active or sham tDCS using an occipital-frontal electrode configuration while participants received passive visual stimulation. We then recorded magnetoencephalography (MEG) offline during a visual entrainment task using a 15 Hz flicker stimulus. Significant oscillatory responses were imaged in the time-frequency domain using beamforming, and the effects of tDCS on absolute and relative power were quantified. The results indicated significantly increased basal alpha levels in the occipital cortex following anodal tDCS, as well as reduced occipital synchronization at the second harmonic of the stimulus-flicker frequency (i.e., 30 Hz) relative to sham stimulation. Interestingly, basal alpha power and the induced 30 Hz synchronization were negatively correlated. Additionally, we found reduced power in brain regions near the cathode following active tDCS, which were absent in the sham group. Taken together, these results suggest that anodal tDCS of the occipital cortices differentially modulates basal levels and induced activity, and may interfere with the entrainment of neural populations by a visual-flicker stimulus. These findings also demonstrate the importance of electrode configuration, and highlight the deceptively complicated nature of tDCS in the context of neurophysiology.