Anodal transcranial direct current stimulation (tDCS) boosts dominant brain oscillations

The biological mechanisms behind the observed behavioural effects of transcranial direct current stimulation (tDCS) are still unclear and have prevented further clinical applications. The widely adopted explanation is that anodal tDCS increases the excitability of stimulated areas in a polarised manner, based on early studies with animals [1] and humans [2]. However, this explanation is at odds with neuroimaging findings [[3]; [4]; [5] ; [6]]. For instance, anodal tDCS was found to increase baseline alpha power [5] (a marker of cortical inhibition), but also to increase gamma power (a marker of excitation) in response to visual stimuli [5]. Additionally, anodal tDCS was found to increase low-frequency oscillations in the underlying tissue without increasing firing rates [6]. Here we suggest a conciliatory explanation that subtle membrane depolarization, caused by weak direct currents (anodal), could make the synchronized neurons more sensitive or responsive to inputs (either excitatory or inhibitory) rather than excitable. A previous study [7] using computational modelling and in-vitro experiments demonstrated that the modulation caused by direct currents is amplified by the network dynamics. Since synchronized neuronal network activity is manifested in brain oscillations - synchronized assemblies are brought about by strong common inputs [8], we predicted that anodal tDCS would boost dominant brain rhythms rather than fast brain rhythms. We also predicted an increase in directed connectivity towards the anodal region, i.e. the area would be more receptive to input connections rather than driving other regions.