The aim of the study is to investigate whether intensive speech-language therapy (SLT) combined with anodal transcranial direct current stimulation (tDCS) leads to better communication performance than SLT combined with placebo stimulation (using sham-tDCS).

Intensive speech-language therapy (SLT) can promote recovery from chronic post-stroke aphasia, but effect sizes are moderate. This highlights the pressing need to explore adjunct strategies, such as transcranial direct current stimulation (tDCS), to enhance training effectiveness. Recently, the investigators provided evidence from a single-center randomized controlled trial (RCT) suggesting that anodal-tDCS of the left primary motor cortex (M1) improves naming and communication ability in chronic post-stroke aphasia, with medium-to-large effect sizes. However, prior to integration into clinical routine, a multi-center RCT with adequate power, duration, and outcomes relevant to everyday life is required, which is the goal of the present study. After trial completion, a workshop with relevant stakeholders will ensure transfer into best-practice guidelines.

Inclusion Criteria:

• left-hemisphere cortical or subcortical stroke with first-ever aphasic symptoms
• at least 6 months post-onset of stroke;
• aphasia, as determined by the Aachen Aphasia Test (AAT);
• 13 moderate-to-severe word finding difficulties (maximum of 75% correct items on a computerized naming task at baseline);
• at least 1 correct reaction on the first part of the AAT subscale Token Test (ensuring basic comprehension skills);
• at least 1 point on the communicative task of the AAT subscale Spontaneous Speech (ensuring basic communication abilities);
• German as first language;
• intact left-hemisphere "hand knob" without right prefrontal lesions for placement of tDCS electrodes, as confirmed by magnetic resonance imaging or computer tomography scans.

Exclusion Criteria:

• contraindications for tDCS (e.g., cardiac pacemaker, history of seizures, implanted metal inside the head);
• more than one clinically apparent stroke with aphasic symptoms;
• other severe neurological diseases (e.g., brain tumor, and subdural hematoma);
• epilepsy with seizures during the last 12 months prior to study start and/or intake of sedating antiepileptic drugs (barbiturates and benzodiazepines),
• history of severe alcohol or drug abuse;
• current severe depression;
• current psychosis or other relevant psychiatric condition;
• very severe apraxia of speech, as revealed by Hierarchical Word Lists;
• severe non-verbal cognitive deficits, as indicated by the Corsi Block-Tapping Task;
• severe uncontrolled medical problems;
• severely impaired vision or hearing that prevents patients from engaging in intensive SLT;
• changes in centrally active drugs within 2 weeks prior to study inclusion.