A panorama of the effectiveness of tDCS in aphasic anomia under a brain-state-dependent stimulation perspective

Effects of transcranial direct current stimulation protocols that control for typical parameters of polarity, brain hemisphere and brain site of stimulation on aphasic anomia are inconsistent in the literature. It has been suggested that controlling for typical parameters is insufficient for the effectiveness of tDCS without controlling for factors that lead to crucial differences in the baseline brain activity of the subjects who receive brain stimulation. This meta-analysis aimed to assess the effectiveness of typical parameters in tDCS protocols used for the treatment of aphasic anomia. Specifically, we discussed the potential role that the main brain-state factors explored in the literature to date had for the results. Overall, meta-analytical outcomes enabled inferences on the underlying interaction between typical parameters and brain-state factors to be made. Furthermore, we found hard evidence that the time of stimulation and time of task administration supports the recommended simultaneous administration of direct current and task for older populations. We advocate that brain-state factors are considered during the planning stages of tDCS protocols for rehabilitation studies. The conclusions relate specifically to tDCS mechanisms in naming and aphasia, but they may have potential applications more generally to other cognitive functions. Therefore, we further recommend that brain stimulation research in general takes the brain-state-dependent stimulation approach into account.