Mechanisms of aphasia recovery after stroke and the role of noninvasive brain stimulation

Abstract

One of the most frequent symptoms of unilateral stroke is aphasia, the impairment or loss of language functions. Over the past few years, behavioral and neuroimaging studies have shown that rehabilitation interventions can promote neuroplastic changes in aphasic patients that may be associated with the improvement of language functions. Following left hemisphere strokes, the functional reorganization of language in aphasic patients has been proposed to involve both intrahemispheric interactions between damaged left hemisphere and perilesional sites and transcallosal interhemispheric interactions between the lesioned left hemisphere language areas and homotopic regions in the right hemisphere. A growing body of evidence for such reorganization comes from studies using transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS), two safe and noninvasive procedures that can be applied clinically to modulate cortical excitability during post-stroke language recovery. We discuss a hierarchical model for the plastic changes in language representation that occur in the setting of dominant hemisphere stroke and aphasia. We further argue that TMS and tDCS are potentially promising tools for enhancing functional recovery of language and for further elucidating mechanisms of plasticity in patients with aphasia.

Copyright © 2011 Elsevier Inc. All rights reserved.

Figures

Figure 1

We submitted activation foci from fMRI and PET studies using the same language tasks on both aphasic patients and control subjects to Activation Likelihood Estimation (ALE) analysis. Control ALE clusters are in blue-green scale, and show left hemisphere language and motor activity. Aphasic ALE clusters are in red-yellow scale, and show bilateral activations with right hemisphere areas that are homotopic to control ALE clusters. ALE maps are overlaid on the standard Colin brain in MNI space, using an FDR q = .01 critical threshold, and minimum cluster size of 100 mm3. (Reproduced with permission from Turkeltaub et al., submitted)

Figure 2

Differing accounts of plasticity in language systems in chronic aphasia. 2a) After unilateral left hemisphere stroke (grey), some language functions may be subserved by recovered lesional areas or recruited perilesional areas (light green). 2b) Right perisylvian areas (light green) may be recruited to subserve some language functions, a process facilitated by decreased transcallosal inhibition of the right hemisphere by the damaged left hemisphere. 2c) By contrast, right hemisphere activity may be deleterious. Released from interhemispheric inhibition, right hemisphere structures (red) may exert increased inhibitory influence on left perisylvian areas, impeding functional recovery of lesional and perilesional areas in the left hemisphere (dark green).