Transcranial alternating current stimulation attenuates BOLD adaptation and increases functional connectivity

Abstract

Background: 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 [1]. Most studies of tACS focus on its ability to entrain brain oscillations [2], but our behavioral results in humans [3] and extracellular recordings in nonhuman primates [4] support the view that tACS at 10 Hz additionally affects brain function by reducing sensory adaptation. Our primary goal here was to investigate this using BOLD imaging in human subjects. Hypothesis: 10 Hz tACS applied during a motion adaptation paradigm attenuates the neuronal adaptation reflected in the BOLD signal.Methods: We used a motion adaptation paradigm developed to quantify BOLD adaptation [5] and used standard ANOVA analyses to compare the amount of adaptation in blocks during which tACS was applied to the human motion area (hMT+) to blocks without tACS. In addition, we performed an exploratory analysis to investigate whether tACS affected the functional connectivity of hMT+. Results: tACS significantly attenuated BOLD adaptation and increased functional connectivity between the stimulated hMT+ and the rest of the brain, in particular the dorsal attention network (DAN).Conclusion: Weak 10 Hz currents applied to the scalp change the BOLD signal. Within the targeted area, these changes are compatible with the hypothesis that tACS attenuates neuronal adaptation; similar to what we previously reported based on extracellular recordings. In addition, our findings show that tACS results in increased global brain connectivity of the stimulated area.Transcranial alternating current stimulation | fMRI | motion adaptation| BOLD | functional connectivity

Individual differences in neuroanatomy and neurophysiology predict effects of transcranial alternating current stimulation

“…Here, we extend these findings from tDCS research to the application of tACS, such that participants with a higher modeled EF experience the greatest multitasking benefits. These results support recent work demonstrating that the modeled EF from tACS correlates with greater changes in neural activity in humans [68,69] and nonhuman primates [70]. Here, we build on this research to show modeled EF correlates with behavioral performance, despite not observing the hypothesized relationship between performance and neural activity.…”

Transcranial stimulation of alpha oscillations upregulates the default mode network

“…By strengthening oscillatory circuits via spike-timing dependent plasticity (STDP) and long-term potentiation (LTP) at the synapse (36, 37), α-tACS can have lasting effects on long-range BOLD (38-40) and alpha-frequency oscillatory connectivity (27, 29). However, network-level effects have been examined in only a handful α-tACS-fMRI studies, which, using varied stimulation montages and protocols, have yielded ambiguous findings (38-40). We chose to apply HD-tACS for concentrated stimulation of the alpha source in the occipitoparietal cortex, which was demonstrated to amplify not only the local alpha power but also the posterior-to-frontal alpha connectivity (27).…”

Alpha Oscillatory Activity Causally Linked to Working Memory Retention: Insights from Online Phase-corrected Closed-loop Transcranial Alternating Current Stimulation (tACS)

“…Anti-phase tACS disturbed frontoparietal alpha synchronization compared to in-phase tACS. Previous studies have reported the potential impact of tACS on functional connectivity (29,30) and found increases in the strength of alpha synchronization with increasing memory load among the frontoparietal regions known to underlie executive and attentional functions during WM maintenance (31,32), so online tACS effects in frontoparietal alpha synchronization were also assessed. The phase lag index (PLI) was used to describe frontoparietal alpha synchronization considering its reliable estimates of phase synchronization against the presence of volume conduction (33).…”