Broca's area damage is necessary but not sufficient to induce after-effects of cathodal tDCS on the unaffected hemisphere in post-stroke aphasia.

Background: The inter-individual variability of behavioral effects after tDCS applied to the unaffected right hemisphere in stroke may be related to factors such as the lesion location.
Objective/hypothesis: We investigated the effect of left Broca's area (BA) damage on picture naming in aphasic patients after cathodal tDCS applied over the right BA.
Methods: We conducted a study using pre-interventional diffusion and resting state functional MRI (rsfMRI) and two cross-over tDCS sessions (TYPE: sham and cathodal) over the right homologous BA in aphasic stroke patients with ischemic lesions involving the left BA (BA+) or other left brain areas (BA-). Picture naming accuracy was assessed after each session. Inter-hemispheric (IH) functional balance was investigated via rsfMRI connectivity maps using the right BA as a seed. Probabilistic tractography was used to study the integrity of language white matter pathways.
Results: tDCS had different effects on picture naming accuracy in BA+ and BA- patients (TYPE × GROUP interaction, F(1,19): 4.6, P: 0.04). All BA- patients except one did not respond to tDCS and demonstrated normal IH balance between the right and left BA when compared to healthy subjects. BA+ patients were improved by tDCS in 36% and had decreased level of functional IH balance. Improvement in picture naming after cathodal tDCS was associated with the integrity of the arcuate fasciculus in BA+ patients.
Conclusions: Behavioral effects of cathodal tDCS on the unaffected right hemisphere differ depending on whether BA and the arcuate fasciculus are damaged. Therefore, IH imbalance could be a direct consequence of anatomical lesions.
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