TDCS and Aphasia Therapy in the Chronic Phase After Stroke

Aphasia is present in about one third of all stroke patients in the chronic phase. The first few months after stroke, considerable spontaneous recovery is initiated, including neuronal plasticity and reorganization processes. Language recovery in aphasic stroke patients involves reorganization of brain functions. Longitudinal fMRI studies reveal that the right hemisphere shows increased activity at different times in the recovery process, but in the long-term is correlated with poorer performance. Left re-lateralization, if possible, seems to be the most effective in restoring language function. For a large subgroup of patients, aphasia therapy is not sufficient to resolve language deficits and not all patients are capable to endure intensive aphasia therapy. Therefore, non-invasive techniques (NIBS) such as transcranial direct current stimulation (tDCS) are currently explored as an add-on treatment to improve or accelerate therapy outcomes. tDCS is a painless and safe stimulation tool that modulates cortical excitability through weak polarizing currents (1 mA - 2 mA) between two electrodes. These weak currents are thought to induce a subthreshold shift of resting membrane potentials towards depolarization or hyperpolarization. The effects of stimulation depend on the polarity of the applied current relative to the axonal orientation. It has been found that tDCS not only triggers immediate aftereffects, but also long-lasting effects that persist beyond the stimulation time, even for up to 12 months. It was suggested that long-term potentiation (LTP) and long-term depression (LTD) might be responsible for these long-term effects, however the precise physiologic mechanisms of action are not yet fully understood.