Brain volumes as predictors of tDCS effects in primary progressive aphasia

Abstract

The current study aims to determine the brain areas critical for response to anodal transcranial direct current stimulation (tDCS) in PPA. Anodal tDCS and sham were administered over the left inferior frontal gyrus (IFG), combined with written naming/spelling therapy. Thirty people with PPA were included in this study, and assessed immediately, 2 weeks, and 2 months post-therapy. We identified anatomical areas whose volumes significantly predicted the additional tDCS effects. For trained words, the volumes of the left Angular Gyrus and left Posterior Cingulate Cortex predicted the additional tDCS gain. For untrained words, the volumes of the left Middle Frontal Gyrus, left Supramarginal Gyrus, and right Posterior Cingulate Cortex predicted the additional tDCS gain. These findings show that areas involved in language, attention and working memory contribute to the maintenance and generalization of stimulation effects. The findings highlight that tDCS possibly affects areas anatomically or functionally connected to stimulation targets.

Keywords: Intervention; Language rehabilitation; PPA; Prediction of treatment outcomes; Spelling; Writing; tDCS.

Fig. 1.

t-scores brain volume comparison between the tDCS and Sham group of participants.

Fig. 2.

Study design and model of current distribution for stimulation to the IFG. Panel A: study design. The grey-shaded area corresponds to the data included in the present study. Panel B: model of current distribution for used stimulation montage (image courtesy: Dr. Marom Bikson).

Fig. 3.

Effects of tDCS on behavioral score change. Change from baseline on behavioral scores is presented in the y axis, and each post-therapy time-point is presented on the x-axis. Scores are presented in red for the tDCS group and in blue for the Sham group. Panel A: tDCS effect for trained words. Panel B: tDCS effects for untrained words.

Fig. 4.

Predictors of percent change in spelling accuracy: trained words (immediately after therapy). Brain regions showing significant interactions between stimulation and volumetric data in predicting change in letter accuracy are presented. Panel A represents R² increase obtained when adding each ROI to the model containing adjustments for pre-therapy scores and Global Atrophy. Panels B and C contain scatter plots of behavioral score (that is, % letter accuracy) change in relation to the pre-therapy assessment (on the y axis) versus brain volume. In each scatter plot, the y axis is the behavioral score change adjusted using regression coefficients, that is, accounting for the remaining variables included in the model. Red represents points for patients in the tDCS group, and blue for the Sham group. Participants with different PPA variants are represented with different symbols: ● lvPPA; ▲ nfvPPA; ■ svPPA.

Fig. 5.

Predictors of percent change in spelling accuracy: untrained words (2 months after therapy). Brain regions showing significant interactions between stimulation and volumetric data in predicting change in letter accuracy are presented. Panel A represents R2 increase obtained when adding each ROI to the model containing adjustments for pre-therapy scores and Global Atrophy. Panels B to D contain scatter plots of behavioral score (that is, % letter accuracy) change in relation to the pre-therapy assessment (on the y axis) versus brain volume. In each scatter plot, the y axis is the behavioral score change adjusted using regression coefficients, that is, accounting for the remaining variables included in the model. Red represents points for patients in the tDCS group, and blue for the Sham group. Participants with different PPA variants are represented with different symbols: ● lvPPA; ▲ nfvPPA; ■ svPPA.