Post-Stroke rehabilitation in chronic aphasia: the combined effect of tDCS and speech therapy
The role of the left and the right hemispheres in recovery from chronic post-stroke aphasia remains a debate topic. The majority of studies point to the beneficial role of the right-hemispheric functional recruitment. However, analysis of literature allowed us to formulate an alternative hypothesis: the left hemisphere in aphasic patients might have a latent functional resource beneficial for the recovery. In our study we attempt to reactivate this resource via application of anodal tDCS over Broca's area in patients with a chronic non-fluent aphasia. This procedure is combined with intensive daily language practice based on the so-called Constraint-induced aphasia therapy developed by Professor Friedemann Pulvermüller, the head of the Brain and Language Laboratory at the Free University of Berlin. The study is carried out using a double blind randomized sham-controlled design. To evaluate the functional improvement, we use MEG to measure brain responses during perception of speech in the lexico-phonological mismatch negativity (MMN) task before and after treatment. We then compared patients’ results with the results obtained in the age-matched healthy control subjects. To reach an acceptable signal-to-noise ratio for MMN responses we apply a special noise bootstrapping procedure with an individual computation of the noise levels. We found that the power of MMN response in the left hemisphere was significantly higher in the patients than in the healthy control participants. Moreover, variance of the response power increased significantly after treatment. The increase in the response power correlates with the behavioral improvement measured using the verb generation task given before and after therapy. Our results point to the compensatory role of the left hemisphere in the chronic stage of aphasia observed in this group of patients. They also highlight the role of individual variability in recovery of the language function, suggesting presence of considerable individual differences in recovery potential.
This research is carried out in cooperation with the National Research University "Higher School of Economics" (Malyutina S.A., Shtyrov Yu.U.) and the Center for the Center for the Speech Pathology and Neurorehabilitation.