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

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.

from Brain and Language

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Observation-execution matching and action inhibition in human primary motor cortex during viewing of speech-related lip movements or listening to speech

One influential theory posits that language has evolved from gestural communication through observation-execution matching processes in the mirror neuron system (MNS). This theory predicts that observation of speech-related lip movements or even listening to speech would result in effector and task specific increase of the excitability of the corresponding motor representations in the primary motor cortex (M1), since actual movement execution is known be effector and task specific. In addition, effector and task specific inhibitory control mechanisms should be important to prevent overt motor activation during observation of speech-related lip movements or listening to speech. We tested these predictions by applying focal transcranial magnetic stimulation to the left M1 of 12 healthy right-handed volunteers and measuring motor evoked potentials (MEPs) and short-interval intracortical inhibition (SICI) in a lip muscle, the right orbicularis oris (OO), versus a hand muscle, the right first dorsal interosseus (FDI). We found that MEP and SICI increased only in the OO but not in the FDI during viewing of speech-related lip movements or listening to speech. These changes were highly task specific because they were absent when lip movements non-related to speech were viewed. Finally, the increase in MEP amplitude in the OO correlated inversely with accuracy of speech perception, i.e. the MEP increase was directly related to task difficulty. The MEP findings support the notion that observation-execution matching is an operating process in the putative human MNS that might have been fundamental for evolution of language. Furthermore, the SICI findings provide evidence that inhibitory mechanisms are recruited to prevent unwanted overt motor activation during action observation.

from Neuropsychologia

Task-dependent differences in corticobulbar excitability of the submental motor projections:implications for neural control of swallowing

It has been suggested that the primary motor cortex plays a substantial role in the neural circuitry that controls swallowing. Although its role in the voluntary oral phase of swallowing is undisputed, its precise role in motor control of the more reflexive, pharyngeal phase of swallowing is unclear. The contribution of the primary motor cortex to the pharyngeal phase of swallowing was examined using transcranial magnetic stimulation (TMS) to evoke motor evoked potentials (MEPs) in the anterior hyomandibular muscle group during either volitional submental muscle contraction or contraction during the pharyngeal phase of both volitionally, and reflexively, initiated swallowing. For each subject, in all three conditions, TMS was triggered when submental surface EMG (sEMG) reached 75% of the mean maximal submental sEMG amplitude measured during 10 volitional swallows. MEPs recorded during volitional submental muscle contraction were elicited in 22 of the 35 healthy subjects examined (63%). Only 16 of these 22 subjects (45.7%) also displayed MEPs recorded during volitional swallowing, but their MEP amplitudes were larger when triggered by submental muscle contraction than when triggered by volitional swallowing. Additionally, only 7 subjects (of 19 tested) showed MEPs triggered by submental muscle contraction during a reflexively triggered pharyngeal swallow. These differences indicate differing levels of net M1 excitability during execution of the investigated tasks, possibly brought about by task-dependent changes in the balance of excitatory and inhibitory neural activity.

from Brain Research Bulletin

Effects of Repeated Volitional Swallowing on the Excitability of Submental Corticobulbar Motor Pathways

The aim of this study was to examine the effects of repeated volitional saliva swallowing on corticobulbar excitability recorded during two muscle preactivation conditions of the submental muscle group. Motor-evoked potentials (MEPs), elicited by transcranial magnetic stimulation (TMS), were assessed in ten healthy volunteers prior to and at 5, 30, 60, and 90 min after 60 volitional saliva swallows (Protocol A). To control for intrinsic fluctuations in corticobulbar excitability during this assessment period, MEPs were also recorded, on a different day, at 30-min intervals across a 2-h period (Protocol B). At each assessment, 15 MEPs were recorded during two submental muscle preactivation conditions: volitional contraction and contraction associated with the pharyngeal phase of volitional swallowing. There were no significant effects of repetitive volitional swallowing or time on MEP measures (p > 0.05). We conclude that volitional saliva swallowing does not have immediate effects on the excitability of corticobulbar projections to the submental musculature during volitionally initiated swallowing motor tasks. These results provide no evidence for use-dependent potentiation of corticobulbar excitability through repetitive saliva swallowing. The lack of effects of time on mean MEP measures supports previous reports of good intrasession reliability of MEPs as a measure of corticobulbar excitability.

from Dysphagia

Repetitive transcranial magnetic stimulation (rTMS) for treatment of chronic tinnitus

Conclusion
These results support the potential of rTMS as a new therapeutic tool for the treatment of chronic tinnitus. Because this study was performed with a small sample size and showed high interindividual variability in treatment effects, further development of the technique is needed before it can be recommended for clinical applications.

from Auris Nasus Larynx

Effects of olfactory and gustatory stimuli on neural excitability for swallowing

ABDUL WAHAB, N., R. JONES AND M. L. HUCKABEE. Effects of olfactory and gustatory stimuli on neural excitability for swallowing. PHYSIOL BEHAV 00(0) 000-000, 2010.—This project evaluated the effects of olfactory and gustatory stimuli on the amplitude and latency of motor-evoked potentials (MEPs) from the submental muscles when evoked by transcranial magnetic stimulation (TMS). Sixteen healthy volunteers (8 males; age range 19-43) participated in the study. Lemon concentrate at 100% and diluted in water to 25% were presented separately as odor and tastant stimuli. Tap water was used as control. 15 trials of TMS-evoked MEPs triggered by volitional contraction of the submental muscles and volitional swallowing were measured at baseline, during control condition, during stimulus presentation, and immediately, 30-, 60-, and 90-min poststimulation for each of the four stimulus presentations. Experiments were repeated using the combined odor and tastant concentrations that most influenced the MEP independently. Differences in MEP amplitude measured during swallowing were seen at 30-, 60-, and 90-min poststimulation for simultaneous olfactory and gustatory stimulation as opposed to no differences seen at any point for stimuli presented separately. This study has shown that combined odor and tastant stimulation (i.e., flavor) can increase MEP amplitude during swallowing and that this enhancement of MEP can persist for at least 90 min following stimulation. As increased MEP amplitude has been associated with improved swallowing performance, a follow-up study is underway to determine the biomechanical changes produced by altered MEPs to facilitate translation of these data to clinical dysphagia management.<p><p>from <a href=”http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T0P-511K3WK-2&_user=108452&_coverDate=09%2F16%2F2010&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_acct=C000059732&_version=1&_urlVersion=0&_userid=108452&md5=b9c8165706a57b93ef664f175efd7e15&searchtype=a”><em>Psychology and Behavior</em></a></p>

Neural Control of Cross-language Asymmetry in the Bilingual Brain

Most bilinguals understand their second language more slowly than their first. This behavioral asymmetry may arise from the perceptual, phonological, lexicosemantic, or strategic components of bilingual word processing. However, little is known about the neural source of such language dominance and how it is regulated in the bilingual brain. Using functional magnetic resonance imaging, we found that unconscious neural priming in bilingual word recognition is language nonselective in the left midfusiform gyrus but exhibits a preference for the dominant language in the left posterior middle temporal gyrus (MTG). These early-stage components of reading were located slightly upstream of the left midlateral MTG, which exhibited enhanced response during a conscious switch of language. Effective connectivity analysis revealed that this language switch is triggered by reentrant signals from inferior frontal cortex and not by bottom–up signals from occipitotemporal cortex. We further confirmed that magnetic stimulation of the same inferior frontal region interferes with conscious language control but does not disrupt unconscious priming by masked words. Collectively, our results demonstrate that the neural bottleneck in the bilingual brain is a cross-language asymmetry of form–meaning association in inferolateral temporal cortex, which is overcome by a top–down cognitive control for implementing a task schema in each language.

from Cerebral Cortex

The right posterior inferior frontal gyrus contributes to phonological word decisions in the healthy brain: evidence from dual-site TMS

There is consensus that the left hemisphere plays a dominant role in language processing, but functional imaging studies have shown that the right as well as the left posterior inferior frontal gyri (pIFG) are activated when healthy right-handed individuals make phonological word decisions. Here we used online transcranial magnetic stimulation (TMS) to examine the functional relevance of the right pIFG for auditory and visual phonological decisions. Healthy right-handed individuals made phonological or semantic word judgements on the same set of auditorily and visually presented words while they received stereotactically guided TMS over the left, right or bilateral pIFG (n= 14) or the anterior left, right or bilateral IFG (n= 14). TMS started 100 ms after word onset and consisted of four stimuli given at a rate of 10 Hz and intensity of 90% of active motor threshold. Compared to TMS of aIFG, TMS of pIFG impaired reaction times and accuracy of phonological but not semantic decisions for visually and auditorily presented words. TMS over left, right or bilateral pIFG disrupted phonological processing to a similar degree. In a follow-up experiment, the intensity threshold for delaying phonological judgements was identical for unilateral TMS of left and right pIFG. These findings indicate that an intact function of right pIFG is necessary for accurate and efficient phonological decisions in the healthy brain with no evidence that the left and right pIFG can compensate for one another during online TMS. Our findings motivate detailed studies of phonological processing in patients with acute and chronic damage of the right pIFG.

from Neuropsychologia

Differential Effects of Neuromuscular Electrical Stimulation Parameters on Submental Motor-Evoked Potentials

Conclusions. Changes in corticobulbar excitability induced by NMES of the submental muscle group are frequency and dose dependent and only occur after NMES triggered by volitional swallowing. Underlying neural mechanisms are discussed.

from Neurorehabilitation and Neural Repair

Interaction of sound and sight during action perception: Evidence for shared modality-dependent action representations

These findings speak in favour of ‘shared’ action representations in the human motor system that are evoked in a ‘modality-dependent’ way, i.e., they are elicited most robustly by the simultaneous presentation of congruent auditory and visual stimuli. Multimodality in the perception of hand movements bears functional similarities to speech perception, suggesting that multimodal convergence is a generic feature of the mirror system which applies to action perception in general.

from Neuropsychologia

Interaction of sound and sight during action perception: Evidence for shared modality-dependent action representations

These findings speak in favour of ‘shared’ action representations in the human motor system that are evoked in a ‘modality-dependent’ way, i.e., they are elicited most robustly by the simultaneous presentation of congruent auditory and visual stimuli. Multimodality in the perception of hand movements bears functional similarities to speech perception, suggesting that multimodal convergence is a generic feature of the mirror system which applies to action perception in general.

from Neuropsychologia

The lateral-occipital and the inferior-frontal cortex play different roles during the naming of visually presented objects

We reasoned that if an area is devoted to processing only the visual features of objects, then transcranial magnetic stimulation (TMS) applied to this area in either hemisphere would affect the naming of objects presented in contralateral but not ipsilateral space. In contrast, if an area is involved in language, then one might expect to see effects of TMS when applied over the left but not the right hemisphere, regardless whether objects are in contralateral or ipsilateral space. Our experiments reveal two important findings. First, TMS delivered to the lateral-occipital complex (LOC), a visual-form area, affected the naming of objects presented in contralateral but not ipsilateral space, independent of which hemisphere was stimulated. In two additional experiments, when participants named the color of objects or made judgments about the size of stimuli as shown physically on a computer screen, TMS over the contralateral LOC did not affect color naming but did affect the participants’ ability to make size judgments. Second, TMS delivered to the left but not the right posterior inferior-frontal gyrus (pIFG) affected the naming of objects irrespective of whether objects were presented in contralateral or ipsilateral space. In a separate experiment, when participants were asked to either read or categorize words, TMS over the left but not the right pIFG affected word categorization but not word reading. On the basis of these findings, we propose that when people name visually-presented objects, LOC processes the visual form of objects while the left pIFG processes the semantics of objects. Hum Brain Mapp, 2009. © 2009 Wiley-Liss, Inc.

from Human Brain Mapping

A mediating role of the premotor cortex in phoneme segmentation

Abstract
Consistent with a functional role of the motor system in speech perception, disturbing the activity of the left ventral premotor cortex by means of repetitive transcranial magnetic stimulation (rTMS) has been shown to impair auditory identification of syllables that were masked with white noise. However, whether this region is crucial for speech perception under normal listening conditions remains debated. To directly test this hypothesis, we applied rTMS to the left ventral premotor cortex and participants performed auditory speech tasks involving the same set of syllables but differing in the use of phonemic segmentation processes. Compared to sham stimulation, rTMS applied over the ventral premotor cortex resulted in slower phoneme discrimination requiring phonemic segmentation. No effect was observed in phoneme identification and syllable discrimination tasks that could be performed without need for phonemic segmentation. The findings demonstrate a mediating role of the ventral premotor cortex in speech segmentation under normal listening conditions and are interpreted in relation to theories assuming a link between perception and action in the human speech processing system.

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from Brain and Language

Poststroke Dysphagia Rehabilitation by Repetitive Transcranial Magnetic Stimulation: A Noncontrolled Pilot Study

Abstract Poststroke dysphagia is frequent and significantly increases patient mortality. In two thirds of cases there is a spontaneous improvement in a few weeks, but in the other third, oropharyngeal dysphagia persists. Repetitive transcranial magnetic stimulation (rTMS) is known to excite or inhibit cortical neurons, depending on stimulation frequency. The aim of this noncontrolled pilot study was to assess the feasibility and the effects of 1-Hz rTMS, known to have an inhibitory effect, on poststroke dysphagia. Seven patients (3 females, age = 65 ± 10 years), with poststroke dysphagia due to hemispheric or subhemispheric stroke more than 6 months earlier (56 ± 50 months) diagnosed by videofluoroscopy, participated in the study. rTMS at 1 Hz was applied for 20 min per day every day for 5 days to the healthy hemisphere to decrease transcallosal inhibition. The evaluation was performed using the dysphagia handicap index and videofluoroscopy. The dysphagia handicap index demonstrated that the patients had mild oropharyngeal dysphagia. Initially, the score was 43 ± 9 of a possible 120 which decreased to 30 ± 7 (p < 0.05) after rTMS. After rTMS, there was an improvement of swallowing coordination, with a decrease in swallow reaction time for liquids (p = 0.0506) and paste (p < 0.01), although oral transit time, pharyngeal transit time, and laryngeal closure duration were not modified. Aspiration score significantly decreased for liquids (p < 0.05) and residue score decreased for paste (p < 0.05). This pilot study demonstrated that rTMS is feasible in poststroke dysphagia and improves swallowing coordination. Our results now need to be confirmed by a randomized controlled study with a larger patient population.

from Dysphagia

Actions, Words, and Numbers: A Motor Contribution to Semantic Processing?

from Current Directions in Psychological Science

ABSTRACT—Recent findings in neuroscience challenge the view that the motor system is exclusively dedicated to the control of actions, and it has been suggested that it may contribute critically to conceptual processes such as those involved in language and number representation. The aim of this review is to address this issue by illustrating some interactions between the motor system and the processing of words and numbers. First, we detail functional brain imaging studies suggesting that motor circuits may be recruited to represent the meaning of action-related words. Second, we summarize a series of experiments demonstrating some interference between the size of grip used to grasp objects and the magnitude processing of words or numbers. Third, we report data suggestive of a common representation of numbers and finger movements in the adult brain, a possible trace of the finger-counting strategies used in childhood. Altogether, these studies indicate that the motor system interacts with several aspects of word and number representations. Future research should determine whether these findings reflect a causal role of the motor system in the organization of semantic knowledge.