Noninvasive brain stimulation in the treatment of aphasia: exploring interhemispheric relationships and their implications for neurorehabilitation

Aphasia is a common consequence of unilateral stroke, typically involving perisylvian regions of the left hemisphere. The course of recovery from aphasia after stroke is variable, and relies on the emergence of neuroplastic changes in language networks. Recent evidence suggests that rehabilitation interventions may facilitate these changes. Functional reorganization of language networks following left-hemisphere stroke and aphasia has been proposed to involve multiple mechanisms, including intrahemispheric recruitment of perilesional left-hemisphere regions and transcallosal interhemispheric interactions between lesioned left-hemisphere language areas and homologous regions in the right hemisphere. Moreover, it is debated whether interhemispheric interactions are beneficial or deleterious to recovering language networks. Transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) are two safe and noninvasive procedures that can be applied clinically to modulate cortical excitability during poststroke language recovery. Intervention with these noninvasive brain stimulation techniques also allows for inferences to be made regarding mechanisms of recovery, including the role of intrahemispheric and interhemispheric interactions. Here we review recent evidence that suggests that TMS and tDCS are promising tools for facilitating language recovery in aphasic patients, and examine evidence that indicates that both right and left hemisphere mechanisms of plasticity are instrumental in aphasia recovery.     

Cortical activation studies in aphasia

Positron emission tomography and functional magnetic resonance imaging are the major techniques of functional brain imaging. Both techniques have been used successfully in studies of the speech-relevant cortex in normal individuals and in aphasic patients with brain lesions. The activation studies basically agree with the classic model of language organization in that the left inferior frontal and superior temporal cortex (Broca’s and Wernicke’s area, respectively) are the pivotal areas of speech processing. Activation studies additionally emphasize that the speech-relevant cortex is a rather widespread network. It also encompasses contributions from other left hemispheric regions and, to some degree, from the contralateral right hemisphere. The studies of aphasic patients point out that the functional preponderance of the left over the right cerebral hemisphere varies between individuals, and that language recovery after stroke depends on the restitution of the speech-relevant network in both brain hemispheres.     

Efficacy and limitation of cognitive rehabilitation]

The "higher brain dysfunction" illustrates various cognitive and behavioral consequences resulted from organic brain damage. Individually-tailored cognitive rehabilitation aims to directly and explicitly ameliorate disability of people with higher brain dysfunction. In this symposium, the efficacy and limitation of cognitive rehabilitation was discussed with particular interest in the two cognitive domains, i.e., language and memory. In the realm of aphasia rehabilitation, two mechanisms have been postulated for language restitution following aphasia: 1) partial recovery of left-hemisphere language-related areas, and 2) activation of their homologous counterparts in the right hemisphere. Although the both hemispheres may eventually contribute for functional reorganization of the language network, recent functional imaging studies of aphasic patients have demonstrated that the residual left hemisphere is primarily important for aphasia recovery. A recently presented hypothesis was described in which suppressing the right hemisphere may lead to better aphasia recovery. It is now widely accepted in the field of memory rehabilitation for individuals with amnesia/dementia that the theoretical framework of "errorless learning" is a guiding principle. Error elimination during learning is essential for favorable outcome of memory training. We should be aware of functional organization of the brain which underlies the efficacy of cognitive rehabilitation.     

Sign language aphasia following right hemisphere damage in a left-hander: a case of reversed cerebral dominance in a deaf signer?

Recent lesion studies have shown that left hemisphere lesions often give rise to frank sign language aphasias in deaf signers, whereas right hemisphere lesions do not, suggesting similar patterns of hemispheric asymmetry for signed and spoken language. We present here a case of a left-handed, deaf, life-long signer who became aphasic after a right-hemisphere lesion. The subject exhibits deficits in sign language comprehension and production typically associated with left hemisphere damaged signers. He also exhibits evidence of local versus global deficits similar to left-hemisphere lesioned hearing patients. This case represents reversed lateralization for sign language and also may represent reversed lateralization for visuo-spatial abilities in a deaf signer.     

Acquisition of a motor skill after left-hemisphere damage

Specialization of function in the left hemisphere of man, as compared with the right, was studied with special reference to the nature of the impairments seen after left-hemisphere damage. Patients with unilateral lesions of the left or the right hemisphere were compared in the acquisition and subsequent performance of a manual skill requiring several hand movements. Patients with left-hemisphere damage were further subdivided into aphasic and non-aphasic groups. Patients with left-hemisphere damage were further subdivided into aphasic and non-aphasic groups. Patients with left-hemisphere damage, whether aphasic or not, were impaired in the acquisition of the task, relative to patients with right hemisphere damage, although aphasics were most severely impaired. Analysis of the errors made during acquisition indicated that perseverative errors and unique errors (unrelated movements) differentiated the left and right groups, but that sequencing errors did not. It was concluded that a major function of the left hemisphere is the control of changes in limb or articulatory posture, and that its complex verbal and praxic functions are derived from such control.     

Functional reorganization of language networks for semantics and syntax in chronic stroke: Evidence from MEG

Using magnetoencephalography, we investigated the potential of perilesional and contralesional activity to support language recovery in patients with poststroke aphasia. In healthy young controls, left‐lateralized ventral frontotemporal regions responded to semantic anomalies during sentence comprehension and bilateral dorsal frontoparietal regions responded to syntactic anomalies. Older adults showed more extensive bilateral responses to the syntactic anomalies and less lateralized responses to the semantic anomalies, with decreased activation in the left occipital and parietal regions for both semantic and syntactic anomalies. In aphasic participants, we observed compensatory recruitment in the right hemisphere (RH), which varied depending on the type of linguistic information that was processed. For semantic anomalies, aphasic patients activated some preserved left hemisphere regions adjacent to the lesion, as well as homologous parietal and temporal RH areas. Patients also recruited right inferior and dorsolateral frontal cortex that was not activated in the healthy participants. Responses for syntactic anomalies did not reach significance in patients. Correlation analyses indicated that recruitment of homologous temporoparietal RH areas is associated with better semantic performance, whereas higher accuracy on the syntactic task was related to bilateral superior temporoparietal and right frontal activity. The results suggest that better recovery of semantic processing is associated with a shift to ventral brain regions in the RH. In contrast, preservation of syntactic processing is mediated by dorsal areas, bilaterally, although recovery of syntactic processing tends to be poorer than semantic. Hum Brain Mapp 37:2869–2893, 2016. © 2016 Wiley Periodicals, Inc .     

Selective,Non-lateralized Impairment of Motor Imagery Following Right Parietal Damage

Recent lesion studies have shown that left hemisphere lesions often give rise to frank sign language aphasias in deaf signers, whereas right hemisphere lesions do not, suggesting similar patterns of hemispheric asymmetry for signed and spoken language. We present here a case of a left-handed, deaf, life-long signer who became aphasic after a right-hemisphere lesion. The subject exhibits deficits in sign language comprehension and production typically associated with left hemisphere damaged signers. He also exhibits evidence of local versus global deficits similar to left-hemisphere lesioned hearing patients. This case represents reversed lateralization for sign language and also may represent reversed lateralization for visuo-spatial abilities in a deaf signer.     

Improvement of language functions in a chronic non-fluent post-stroke aphasic patient following bilateral sequential theta burst magnetic stimulation

In chronic non-fluent aphasia patients, inhibition of the intact right hemisphere (RH), by transcranial magnetic stimulation (TMS) or similar methods, can induce improvement in language functions. The supposed mechanism behind this improvement is a release of preserved left hemisphere (LH) language networks from RH transcallosal inhibition. Direct stimulation of the damaged LH can sometimes bring similar results too. Therefore, we developed a novel treatment approach that combined direct LH (Broca’s area (BA)) stimulation, by intermittent theta burst stimulation (TBS), with homologue RH area’s inhibition, by continuous TBS. We present the results of application of 15 daily sessions of the described treatment approach in a right-handed patient with chronic post-stroke non-fluent aphasia. The intervention appeared to improve several language functions, but most notably propositional speech, semantic fluency, short-term verbal memory, and verbal learning. Bilateral TBS modulation of activation of the language-related areas of both hemispheres seems to be a feasible and promising way to induce recovery in chronic aphasic patients. Due to potentially cumulative physiological effects of bilateral stimulation, the improvements may be even greater than following unilateral interventions.     

Right‐hemispheric processing of non‐linguistic word features: Implications for mapping language recovery after stroke

Verbal stimuli often induce right‐hemispheric activation in patients with aphasia after left‐hemispheric stroke. This right‐hemispheric activation is commonly attributed to functional reorganization within the language system. Yet previous evidence suggests that functional activation in right‐hemispheric homologues of classic left‐hemispheric language areas may partly be due to processing nonlinguistic perceptual features of verbal stimuli. We used functional MRI (fMRI) to clarify the role of the right hemisphere in the perception of nonlinguistic word features in healthy individuals. Participants made perceptual, semantic, or phonological decisions on the same set of auditorily and visually presented word stimuli. Perceptual decisions required judgements about stimulus‐inherent changes in font size (visual modality) or fundamental frequency contour (auditory modality). The semantic judgement required subjects to decide whether a stimulus is natural or man‐made; the phonologic decision required a decision on whether a stimulus contains two or three syllables. Compared to phonologic or semantic decision, nonlinguistic perceptual decisions resulted in a stronger right‐hemispheric activation. Specifically, the right inferior frontal gyrus (IFG), an area previously suggested to support language recovery after left‐hemispheric stroke, displayed modality‐independent activation during perceptual processing of word stimuli. Our findings indicate that activation of the right hemisphere during language tasks may, in some instances, be driven by a “nonlinguistic perceptual processing” mode that focuses on nonlinguistic word features. This raises the possibility that stronger activation of right inferior frontal areas during language tasks in aphasic patients with left‐hemispheric stroke may at least partially reflect increased attentional focus on nonlinguistic perceptual aspects of language. Hum Brain Mapp, 2013. © 2012 Wiley Periodicals, Inc.     

Language prosody and the right hemisphere

Abstract

Groups of right hemisphere damaged, left hemisphere damaged aphasic and normal control subjects were assessed on a series of tests designed to examine discrimination and production of different aspects of linguistic prosody. These included lexical stress, intonation, emphatic stress, lexical stress in sentence contexts, language identification using prosodic cues, and prosody in discourse. The right hemisphere damaged subjects were significantly impaired on all of the tests as compared to the control subjects. Furthermore, they were impaired as compared to the left hemisphere damaged aphasic subjects on some of the tests. The results support the notion that the right hemisphere has an important role in the processing of linguistic prosody.