As changes in neural oscillations have been found in all major neurological diseases (Buzsáki and Watson
2012), it is of vital importance that researchers and clinicians have a method of modulating such oscillations to both research and potentially treat neurological conditions (Zich et al.
2017). Whilst tDCS is unable to target specific frequencies of oscillations, as previously mentioned, it is able to alter excitability and therefore neuronal activity in particular brain regions (Nitsche and Paulus,
2000). Interestingly, tDCS also decreases GABA in resting-state networks (Bachtiar et al.
2015), which has in turn been linked to changes in resting-state connectivity (Bachtiar et al.
2015; Stagg et al.
2014). Indeed, a number of studies demonstrate that tDCS is capable of altering connectivity. For example, Keeser et al. (
2011) investigated whether tDCS can alter resting-state network connectivity by exposing participants to real or sham stimulation in two different sessions during which the anode was placed over the left dorsal lateral prefrontal cortex (DLPFC) and the cathode over the right supraorbital region. Participants received 20 min of 2 mA real or sham stimulation, and functional magnetic resonance imaging (fMRI) resting-state data were recorded before and after stimulation. When compared with sham, real tDCS participants showed significant changes in regional brain connectivity in the default mode network and frontal–parietal networks. These results clearly demonstrate the ability of tDCS to modulate resting-state connectivity. Further, Polanía et al. (
2012) examined the differential effects of anodal and cathodal stimulation delivered over the motor cortex and demonstrated an increase within corticostriatal and thalamocortical circuits in response to anodal stimulation and a decrease in connectivity in response to cathodal stimulation, confirming the different effects of the two electrodes on connectivity. In another similar study, Polanía et al. (
2011) demonstrated that tDCS can also modulate connectivity by not only increasing communication between areas related to the task, but also by reducing communication between other areas. In addition, they showed that changes in connectivity were higher during the motor task than at rest. Task-related connectivity changes have been demonstrated in a wide variety of other tasks, including risk taking (Weber et al.
2014), on a sensorimotor rhythm brain computer interface task (Baxter et al.
2017), a smoking cue reactivity task (Yang et al.
2017) and during speech (Holland et al.
2016). Examination of glutamatergic neurotransmission during anodal tDCS using proton magnetic resonance spectroscopy (MRS) found that glutamate and glutamine concentrations (Glx) were increased under the anodal electrode, and individual differences in Glx predicted network connectivity (Hunter et al.
2015) and remote effects on brain regions that were not directly beneath the electrodes (Hone-Blanchet et al.
2016).