Transcranial alternating current stimulation attenuates BOLD adaptation and increases functional connectivity

Abstract

Transcranial alternating current stimulation (tACS) is used as a non-invasive tool for cognitive enhancement and clinical applications. The physiological effects of tACS, however, are complex and poorly understood (Liu et al. 2018). Most studies of tACS focus on its ability to entrain brain oscillations (Herrmann et al. 2013), but our behavioral results in humans (Kar and Krekelberg 2014a) and extracellular recordings in nonhuman primates (Kar et al. 2017) support the view that tACS at 10 Hz additionally affects brain function by reducing sensory adaptation. Our primary goal here was to test this hypothesis using BOLD imaging in human subjects. Using a motion adaptation paradigm developed to quantify BOLD adaptation (Huk et al. 2001) and concurrent fMRI and tACS, we found that tACS significantly attenuated adaptation in the human motion area (hMT+). In addition, an exploratory analysis showed that tACS increased functional connectivity between the stimulated hMT+ and the rest of the brain, in particular the dorsal attention network. We conclude that weak 10 Hz currents applied to the scalp affect both local and global measures of brain activity.New and NoteworthyConcurrent transcranial alternating current stimulation (tACS) and fMRI show that tACS affects the human brain by attenuating adaptation and increasing functional connectivity. This work is important for our basic understanding of what tACS does, but also for therapeutic applications, which need insight into the full range of ways in which tACS affects the brain.