tDCS over the motor cortex improves lexical retrieval of action words in post-stroke aphasia

One-third of stroke survivors worldwide suffer from aphasia. Speech and language therapy (SLT) is considered effective in treating aphasia, but due to time constraints, improvements are often limited. Non-invasive brain stimulation is a promising adjuvant strategy to facilitate SLT. However, stroke might render "classical" language regions ineffective as stimulation sites. Recent work showed the effectiveness of motor-cortex stimulation together with intensive naming therapy to improve outcomes in aphasia (Meinzer et al., 2016). Although, this study highlights the involvement of the motor cortex, the functional aspects by which it influences language are yet unclear. In this study, we focus on the role of motor cortex in language, investigating its functional involvement in access to specific lexico-semantic (object- vs. action related) information in post-stroke aphasia. To this end, we tested effects of anodal tDCS to the left motor cortex on lexical retrieval in 16 patients with post-stroke aphasia in a sham-controlled, double-blind study design. Critical stimuli were action and object words, and pseudowords. Participants performed a lexical-decision task, deciding whether stimuli were words or pseudowords. Anodal tDCS improved accuracy in lexical decision, especially for words with action-related content and for pseudowords with an "action-like" ending (t15 = 2.65, p = 0.036), but not for words with object-related content and pseudowords with "object-like" characteristics. We show as a proof-of-principle that the motor cortex may play a specific role in access to lexical-semantic content. Thus, motor-cortex stimulation may strengthen content-specific word-to-semantic concept associations during language treatment in post-stroke aphasia.

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