Transcranial alternating current stimulation It remains unclear how exactly weak electric fields can modulate the mesoscopic and macroscopic dynamics of cortical networks. However, one inroad to our understanding resulted from the application of a conceptual approach that originated in physics. Given the often periodic (ie, rhythmic) structure of cortical network activity, Inhibitors,research,lifescience,medical the intuitive choice of stimulation waveform is one that is equally rhythmic. This reasoning has led to the early use of transcranial alternate current stimulation (t ACS), which has shown very interesting neurobiological effects

despite a lack of understanding regarding if and how such periodic stimulation interacts with the intrinsic oscillators of cortical networks. The most prominent, paradigm-changing study employed tACS (in combination with a DC offset, technically so-tDCS) to enhance slow oscillatory activity (<1 Hz, the so-called slow oscillation, Inhibitors,research,lifescience,medical originally described in studies on cats by Steriade and colleagues)41,42 during

slow-wave sleep.43 It has long been hypothesized for a long time that slowwave sleep is crucial for sleep-dependent learning and memory, and it Inhibitors,research,lifescience,medical was thus a very significant Bcr-Abl inhibitor finding that tACS at 0.75 Hz (the stimulation signal also included a DC bias) enhanced memory consolidation in healthyhuman study participants. However, although the EEG confirmed an enhancement of slow rhythmic activity, the stimulation was not quite as specific in its effects, since it also Inhibitors,research,lifescience,medical enhanced activity signatures in higher frequency bands (sleep spindles, 10-16 Hz) that have also been associated with learning and memory.44,45 Interestingly, the same authors did not find a similar effect Inhibitors,research,lifescience,medical for the same stimulation paradigm in awake subjects,46 suggesting that the state of the brain may contribute to the response to stimulation (and its behavioral outcomes). Nevertheless, this study provided very strong motivation for the subsequent use of tACS to manipulate cortical oscillations, with the hope for a frequency-specific, noninvasive stimulation

modality. Tolmetin Indeed, a-band tACS selectively enhanced a oscillations in occipital cortex47 and differed in its effect on spontaneous EEG activity depending on the brain state as defined by whether the subjects’ eyes were open or closed.48 Additionally, tACS has been demonstrated to alter visual detection performance.49 Tactile sensations were elicited by tACS over the primary somatosensory cortex, but only for α and high-γ stimulation frequencies.50 Interestingly, stimulation in the α-band modulated γ-oscillations in the motor system, suggesting that stimulation of a given frequency band can also affect other frequency bands and therefore provide a counterargument to the idea of frequency-specific stimulation effects.