Theories of inter-hemispheric interactions in aphasia: the role of tDCS in rehabilitation of post-stroke aphasia

• ¹ University of Pennsylvania, Neurology, United States

Mounting data from behavioral and neuroimaging studies have shown that the process of recovery from aphasia is largely driven by the reorganization of brain networks related to language. Evidence implicates a variety of potential mechanisms in this reorganization, some of which involve substantive changes in brain functional activity within and between cerebral hemispheres. These changes include intrahemispheric recruitment of perilesional left-hemisphere regions and transcallosal interhemispheric interactions between lesioned left-hemisphere language areas and homologous regions in the right hemisphere. With respect to the role of the right hemisphere, it is debated whether interhemispheric interactions are beneficial or deleterious to recovering language networks. Recent years have also seen the emergence of noninvasive brain stimulation techniques such as transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) as potential novel treatments for post-stroke aphasia. Because these techniques are predicated on either focal excitation or inhibition of brain areas, characterization of the functional roles of the left and right hemispheres and transcallosal interactions in aphasia recovery is of central importance to the development and refinement of stimulation-based therapies. However, most treatment studies involving noninvasive brain stimulation in aphasia have tacitly accepted the interhemispheric inhibition model, in which right hemisphere activity interferes with language recovery that is mediated by left hemisphere perisylvian regions. Based on this account, many studies in aphasia involving TMS and tDCS have adopted one of two approaches consistent with the model: left hemisphere excitation or right hemisphere inhibition. In this presentation, we will review both clinical and cognitive neuroscience evidence that elucidates different hemispheric mechanisms that influence recovery from aphasia after stroke. Based on our prior findings using functional imaging meta-analysis and TMS in patients with aphasia1-2, we will make the case that these mechanisms are not mutually exclusive, and that multiple patterns of plastic change in language systems are observed across and even within individuals with aphasia. We will then the review the growing body of literature in which tDCS has been explored as a treatment for aphasia and argue that the heterogeneity of stimulation approaches speaks to the influence of multiple mechanisms of language plasticity. Moreover, based on our initial findings in a pilot study of tDCS in persons with chronic nonfluent aphasia3, we will argue that current and future approaches to aphasia treatment with tDCS should focus on identifying the influence of different mechanisms of aphasia recovery at the individual level. Future studies in this area will need to determine the relationship between clinical features such as lesion size, lesion distribution, and aphasia symptomotology and the hemispheric neuroplastic mechanisms that individual patients will preferentially employ to recover language ability, in order to optimally modulate these mechanisms using brain stimulation.