Adaptive Plasticity in the Healthy Language Network: Implications for Language Recovery after Stroke

Abstract

Across the last three decades, the application of noninvasive brain stimulation (NIBS) has substantially increased the current knowledge of the brain's potential to undergo rapid short-term reorganization on the systems level. A large number of studies applied transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) in the healthy brain to probe the functional relevance and interaction of specific areas for different cognitive processes. NIBS is also increasingly being used to induce adaptive plasticity in motor and cognitive networks and shape cognitive functions. Recently, NIBS has been combined with electrophysiological techniques to modulate neural oscillations of specific cortical networks. In this review, we will discuss recent advances in the use of NIBS to modulate neural activity and effective connectivity in the healthy language network, with a special focus on the combination of NIBS and neuroimaging or electrophysiological approaches. Moreover, we outline how these results can be transferred to the lesioned brain to unravel the dynamics of reorganization processes in poststroke aphasia. We conclude with a critical discussion on the potential of NIBS to facilitate language recovery after stroke and propose a phase-specific model for the application of NIBS in language rehabilitation.

Figure 1

Illustration of different combinations of transcranial magnetic stimulation (TMS) and functional magnetic resonance imaging (fMRI). (a) fMRI can be used to localize target areas for TMS application. Subsequently, TMS is applied to probe the contribution of these regions to a specific task. (b) TMS can also be applied prior to fMRI to probe its lasting neuromodulatory effects on the network level. (c) Simultaneous TMS and fMRI can be used to map the immediate consequences of TMS on brain functions.

Figure 2

TMS-induced suppression of the semantic network during a word decision task in a representative subject. (a) Illustration of the strong remote effects induced by continuous theta burst stimulation (cTBS) given over the left angular gyrus (AG) prior to fMRI. Relative to cTBS over the neighboring supramarginal gyrus (SMG), cTBS of AG inhibited task-related neural activity during semantic decisions in the stimulated area as well as in the left inferior frontal gyrus (IFG) and in temporal regions. (b) Effects of cTBS on the mean reaction times (RTs) of semantic decisions. SEM = standard error of the mean.

Figure 3

Illustration of a phase-specific stimulation approach to promote language recovery after left-hemispheric stroke. Note that the appropriate stimulation protocol might strongly depend on the site and size of the lesion and the individual deficits. (a) In the acute and early subacute phase, an upregulation of homologous right-hemispheric regions with facilitatory noninvasive brain stimulation (NIBS) might promote language recovery. (b) In the late subacute and chronic phase, patients might rather benefit from an inhibition of homologous right-hemispheric regions and an upregulation of ipsilesional regions by NIBS. Grey circles illustrate a stroke-induced lesion.