Transcranial direct current stimulation effects on neural processing in post-stroke aphasia

Abstract

Non-invasive transcranial direct current stimulation (tDCS) can enhance recovery after stroke. However, fundamental knowledge about how tDCS impacts neural processing in the lesioned human brain is currently lacking. In the present study, it was investigated how tDCS modulates brain function in patients with post-stroke language impairment (aphasia). In a cross-over, randomized trial, patients named pictures of common objects during functional magnetic resonance imaging (fMRI). Concurrently, excitatory (anodal-) or sham-tDCS (1 mA, 20 min, or 30 s, respectively) was administered to the left primary motor cortex, a montage with demonstrated potential to improve aphasic language. By choosing stimuli that could reliable be named by the patients, the authors aimed to derive a pure measure of stimulation effects that was independent of treatment or performance effects and to assess how tDCS interacts with the patients' residual language network. Univariate fMRI data analysis revealed reduced activity in domain-general regions mediating high-level cognitive control during anodal-tDCS. Independent component functional network analysis demonstrated selectively increased language network activity and an inter-correlated shift from higher to lower frequency bands, indicative of increased within-network communication. Compared with healthy controls, anodal-tDCS resulted in overall "normalization" of brain function in the patients. These results demonstrate for the first time how tDCS modulates neural processing in stroke patients. Such information is crucial to assure that behavioral treatments targeting specific neural circuits overlap with regions that are modulated by tDCS, thereby maximizing stimulation effects during therapy. Hum Brain Mapp 38:1518-1531, 2017. © 2016 Wiley Periodicals, Inc.

Keywords: aphasia; brain stimulation; functional magnetic resonance imaging; language; stroke.

© 2016 Wiley Periodicals, Inc.

Figures

Figure 1

Lesion overlay plot: Colors indicate the number of patients with lesions in a given area. [Color figure can be viewed at http://wileyonlinelibrary.com]

Figure 2

Univariate analysis: Illustrates activity patterns associated with correct naming trials during sham‐tDCS (blue) and anodal‐tDCS (red). Left (L) and right (R) hemisphere (voxel threshold P < 0.001; cluster P < 0.05 FWE‐corrected). [Color figure can be viewed at http://wileyonlinelibrary.com]

Figure 3

Univariate analysis: (A) Regions showing significant activity reductions in the patients during anodal‐ versus sham‐tDCS. Those regions were used for comparison with data of the healthy control group. (B) Illustrates activity in the three ROIs during anodal‐tDCS and sham‐tDCS in the patients relative to activity of healthy controls that was acquired during the same task but without stimulation. * = significant differences within and between groups. Means and SEMs are reported. [Color figure can be viewed at http://wileyonlinelibrary.com]

Figure 4

Independent component analysis: Illustrates the three networks that were associated with the naming task as identified by independent components analysis. Green = language network; purple = motor network; yellow = visual network. Left (L) and right (R) hemisphere (Bonferroni corrected, P < 0.003). [Color figure can be viewed at http://wileyonlinelibrary.com]

Figure 5

Independent component analysis: Shows selective modulation of mean activity in the language network in the patients during anodal‐ versus sham‐tDCS. * = significant difference between sham‐ and anodal‐tDCS in the patients. Means and SEMs are reported. [Color figure can be viewed at http://wileyonlinelibrary.com]

Figure 6

Independent component analysis: Illustrates the inter‐correlated shift of enhanced lower frequency oscillations and reduced higher frequency oscillations in the patients during anodal‐ versus sham‐tDCS in the language component. * = significant difference between sham‐ and anodal‐tDCS in the patients. Means and SEMs are reported. [Color figure can be viewed at http://wileyonlinelibrary.com]