tDCS in post-stroke aphasia: The role of stimulation parameters, behavioral treatment and patient characteristics

Neurostimulation techniques have been recently adopted in aphasia rehabilitation. In several studies transcranial direct current stimulation (tDCS) was used to enhance treatment effects. The methodology adopted in different studies is characterized by a large variability, as concerns stimulation parameters (e.g., montage type, current intensity, session duration, number and frequency of treatment sessions), participant inclusion criteria (subacute vs chronic, selected vs general aphasia types) and characteristics of associated behavioral treatments (online vs offline treatment, focused on different underlying deficits). Group analyses report on positive results for most of the adopted paradigms. We review the available literature focusing on tDCS in the rehabilitation of stroke-related aphasia, with reference to the current views on tDCS's action mechanisms and on the factors that may influence the effects of stimulation. Even though our understanding of the mechanisms activated by neurostimulation techniques is still limited, available evidence already allows to propose methodological recommendations for studies intending to use tDCS as a treatment adjuvant. Where several options for a specific stimulation parameter seem suitable, we provide information to reach a knowledgeable decision.