Effects of Transcranial Direct Current Stimulation (tDCS) on Brain Organization and Naming in Aphasic Patients.

Aphasia, Stroke

tDCS, Functional connectivity, EEG

High-Definition Transcranial Direct Current Stimulation (HD-tDCS) allows to induce, in a non-invasive way, a transient inhibitory or excitatory neuromodulation of a given cerebral region and to obtain a very focused cortical effect. Previous studies using HD-tDCS have shown the effectiveness of this stimulation technique for enhancing language recovery in patients with aphasia. However, language processes are not determined solely by local neural activity at a single site, but rather by the interaction between neural networks. This is because a large cortical network is involved in language processes and, therefore, the same language disorder may result from lesions at different locations in this network. The investigators hypothesize that anodal HD-tDCS will enhance neural interactions between language areas and, thereby, improve language processing and word learning. The investigators propose to carry out a study on chronic aphasic patients involving HD-tDCS of the Broca region (left inferior frontal gyrus) combined with a verb learning task.

Functional connectivity (FC) represents a means of analyzing functional interactions in the brain network by assessing the statistical dependence of neural activity between brain areas. The brain maintains a high level of interactions between brain areas even in the resting state, when it is not explicitly involved in a specific task. Resting-state interactions occur mainly in the so-called alpha frequency band (8-13 Hz) . Moreover, these resting-state interactions are crucial for correct task performance and learning. Indeed, healthy subjects with high resting-state alpha-band FC levels are better able to perform tasks and learn. For example, subjects with high levels of FC between Broca's area and the rest of the brain perform better on language production tasks and show greater training gains when learning new words. In addition, patients with brain damage due to stroke show a reduction in resting-state alpha-band FC, which correlates with the severity of neurological deficits. In particular, a loss of FC of structurally preserved inferior frontal areas was associated with more severe aphasia. Conversely, high-levels of FC between the inferior frontal brain areas and the rest of the brain during the first weeks after stroke was associated with better recovery from aphasia, probably reflecting a reorganization of neural connections as a neural mechanism involved in plasticity. Resting-state FC is therefore an interesting neural target for therapy, which could help improve recovery from aphasia. In a previous study on healthy subjects, the investigators have indeed observed that HD-tDCS applied to Broca's area improves the ability for new-verb learning, i.e., the correct naming of action pictures. This improvement in learning correlated with greater increase in FC between Broca's and Wernicke's area. The investigators now carry out a study on chronic aphasic patients involving HD-tDCS of the Broca region (left inferior frontal gyrus) combined with a verb learning task.

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