The use of non-invasive brain stimulation techniques to facilitate recovery from post-stroke aphasia

Abstract

Aphasia is a common symptom after left hemispheric stroke. Neuroimaging techniques over the last 10-15 years have described two general trends: Patients with small left hemisphere strokes tend to recruit perilesional areas, while patients with large left hemisphere lesions recruit mainly homotopic regions in the right hemisphere. Non-invasive brain stimulation techniques such as transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) have been employed to facilitate recovery by stimulating lesional and contralesional regions. The majority of these brain stimulation studies have attempted to block homotopic regions in the right posterior inferior frontal gyrus (IFG) to affect a presumed disinhibited right IFG (triangular portion). Other studies have used anodal or excitatory tDCS to stimulate the contralesional (right) fronto-temporal region or parts of the intact left IFG and perilesional regions to improve speech-motor output. It remains unclear whether the interhemispheric disinhibition model, which is the basis for motor cortex stimulation studies, also applies to the language system. Future studies could address a number of issues, including: the effect of lesion location on current density distribution, timing of the intervention with regard to stroke onset, whether brain stimulation should be combined with behavioral therapy, and whether multiple brain sites should be stimulated. A better understanding of the predictors of recovery from natural outcome studies would also help to inform study design, and the selection of clinically meaningful outcome measures in future studies.

Figures

Fig. 1

Diffusion Tensor Imaging derived fiber tracts using probabilistic fiber tracking implemented in FSL (http://www.fmrib.ox.ac.uk/fsl). Tracts are superimposed onto an axial slice of a 3D-reconstructed brain. The left side of image shows left hemisphere (neurological view). A shows the two components of the arcuate fasciculus connecting the superior/middle posterior temporal gyrus (pSTG/pMTG) with the posterior inferior frontal gyrus (pIFGop and pIFGtri). The two components of the AF are separated into those fibers that connect with the opercular portion (red, pIFGop) and the triangular portion (green, pIFGtri) of the posterior IFG. The opercular portion approximately corresponds to Brodmann area 44 and the triangular portion corresponds to Brodmann area 45 (Amunts et al 2010). The transcallosal connections between BA 44 (light blue) and BA 45 (dark blue) are shown as well. B shows the right hemisphere AF and the slightly reduced transcallosal fiber tracts in a chronic patient with a large left hemisphere stroke (lesion marked in black) and nonfluent aphasia