Modulation of cortical network activity by transcranial alternating current stimulation.

Editor's Note: These short, critical reviews of recent papers in the Journal, written exclusively by graduate students or postdoctoral fellows, are intended to summarize the important findings of the paper and provide additional insight and commentary. For more information on the format and purpose of the Journal Club, please see Review of Ali et al. In recent years, it has become evident that neuronal oscillations play a key role in human perception and behavior. In particular , synchronized activity in large-scale neuronal networks has been linked to different perceptual properties (Siegel et al., 2012). The ability to directly drive specific cortical rhythms offers a unique opportunity to reproduce functionally relevant os-cillatory signatures and study subsequent behavioral performance (Thut et al., 2012). This approach could further extend the vast amount of correlative evidence and establish a causal link between brain oscillations and distinct cognitive functions. To date, three methods for external neuronal entrainment are available: rhythmic sensory stimulation, rhythmic transcra-nial magnetic stimulation (rTMS), and transcranial alternating current stimulation (tACS). tACS might be a particularly powerful tool, because unlike rTMS it allows continuous stimulation below action-potential threshold without inducing a con-comitant phase reset (Thut et al., 2011). In addition, tACS parameters, such as frequency or amplitude, can easily be varied and stimulation intensity can be kept below skin sensation threshold. The physiological mechanisms of tACS are, however, poorly understood, because of excessive electrical stimulation artifacts in electrophysiological recordings (for review, see Herrmann et al., 2013). In the absence of compelling physiological evidence for neuronal entrainment by tACS, modeling approaches can be beneficial. In a recent report, Ali et al. (2013) simulated large-scale networks of spiking neurons to investigate how weak global perturbations with alternating currents modulate the dynamics of neuronal oscil-latory networks. They then confirmed simulation results by recording multiunit activity in anesthetized ferrets. The initial simulation supports the hypothesis that tACS can entrain neuronal networks in a frequency-specific manner through network resonance (Pikovsky, 2003; Herr-mann et al., 2013). Resonance frequencies are stimulation frequencies at which weak periodic perturbations are particularly effective in entraining the network. Here, the authors demonstrated that a neuronal network with an intrinsic frequency of ϳ3 Hz can successfully be entrained by weak alternating currents of varying strength and frequencies. Crucially, the network was maximally entrained by external stimulation at its intrinsic frequency, but it was also increasingly excitable by stronger currents at adjacent frequencies. This dependency between …