Functional connectivity between cortical hand motor and language areas during recovery from aphasia

Previous data indicate that in healthy subjects, there is a connectivity between cortical areas for hand movement and language on the left hemisphere. This link is possibly mediated by the so-called mirror neuron system. The present study investigated the functional relationship between linguistic and hand movement processing in patients who were recovering from post-stroke aphasia. The excitability of the right- and left-hand motor cortex during language production in patients who were recovering from post-stroke aphasia and age-matched controls was investigated. As control, phonation was investigated. Hand motor cortex excitability was assessed with Motor Evoked Potentials which were elicited by Transcranial Magnetic Stimulation (TMS). In patients, reading aloud enhanced the excitability of the right hemispheric hand motor cortex, whereas phonation had no effect on hand motor cortex excitability. In the control group, an increased excitability of the left hemispheric hand motor system was found during reading aloud in accordance with previous data. The present data suggest a functional connectivity between regions mediating hand movements and reading. This may indicate that the right hemisphere participates in language processing as far as involved in single word reading in patients recovering from aphasia. The coactivation between cerebral representations of hand movements and language may be used therapeutically for aphasia rehabilitation.     

Opposite hemispheric lateralization effects during speaking and singing at motor cortex, insula and cerebellum

Aside from spoken language, singing represents a second mode of acoustic (auditory-vocal) communication in humans. As a new aspect of brain lateralization, functional magnetic resonance imaging (fMRI) revealed two complementary cerebral networks subserving singing and speaking. Reproduction of a non-lyrical tune elicited activation predominantly in the right motor cortex, the right anterior insula, and the left cerebellum whereas the opposite response pattern emerged during a speech task. In contrast to the hemodynamic responses within motor cortex and cerebellum, activation of the intrasylvian cortex turned out to be bound to overt task performance. These findings corroborate the assumption that the left insula supports the coordination of speech articulation. Similarly, the right insula might mediate temporo-spatial control of vocal tract musculature during overt singing. Both speech and melody production require the integration of sound structure or tonal patterns, respectively, with a speaker's emotions and attitudes. Considering the widespread interconnections with premotor cortex and limbic structures, the insula is especially suited for this task.     

Opposite musical-manual interference in young vs expert musicians

Cerebral lateralization for music has been studied through a music-manual interference paradigm (tapping) in a group of young musicians (seven males and seven females) attending the 1st and 3rd intermediate grades of Udine's "J. Tomadini" State Conservatory of Music and in a group of graduated expert musicians or higher course students during the execution of three distinct tasks (singing notes, whistling a melody and singing a melody). A significant superiority of the right hemisphere (greater degree of interference with the left hand) in these tasks has been found in young musicians, while an opposite left hemisphere superiority (greater degree of interference with the right hand) was evident in the expert musicians. Other differences between sexes and tasks were not significant. The modification of hemispheric specialization occurring during academical musical training are discussed in terms of the role of education in the cerebral organization of superior cognitive functions.     

FMRI reveals functional cortex in a case of inconclusive Wada testing

OBJECTIVES: The intracarotid amobarbital test (Wada test) currently represents the gold standard for preoperative lateralization of hemispheric dominance. Here, we report an epileptic patient with a longstanding extended lesion of the left hemisphere showing absence of motor and speech dysfunction with left carotid amobarbital injection, but tetraplegia and speech arrest with right carotid injection interpreted as a neuroplastic shift of motor and language functions to the right hemisphere. In contrast to the Wada results, motor functional magnetic resonance imaging (fMRI) showed a strong left hemispheric activation with right hand movements. METHODS: Right and left hand motor fMRI was performed. FMRI results and neurophysiological information obtained by motor and sensory evoked potential measurements were compared with the Wada test results. RESULTS: Initial interpretation of neuroplastic shifts of intrinsic left hemisphere functions to the right brain was revised after fMRI results which were confirmed by motor and sensory evoked potentials. CONCLUSION: As motor inactivation usually is thought to be the most robust feature of the Wada test, this case demonstrates that fMRI may reveal residual functional cortex in cases of inconclusive Wada results.     

Localization and characterization of speech arrest during transcranial magnetic stimulation.

OBJECTIVE: To determine the anatomic and physiologic localization of speech arrest induced by repetitive transcranial magnetic stimulation (rTMS), and to examine the relationship of speech arrest to language function. METHODS: Ten normal, right-handed volunteers were tested in a battery of language tasks during rTMS. Four underwent mapping of speech arrest on a 1 cm grid over the left frontal region. Compound motor action potentials from the right face and hand were mapped onto the same grid. Mean positions for speech arrest and muscle activation were identified in two subjects on 3-dimensional MRI. RESULTS: All subjects had lateralized arrest of spontaneous speech and reading aloud during rTMS over the left posterior-inferior frontal region. Writing, comprehension, repetition, naming, oral praxis, and singing were relatively spared (P < .05). Stimulation on the right during singing abolished melody in two subjects, but minimally affected speech production. The area of speech arrest overlay the caudal portion of the left precentral gyrus, congruous with the region where stimulation produced movement of the right face. CONCLUSIONS: The site of magnetic speech arrest appears to be the facial motor cortex. Its characteristics differ from those of classic aphasias, and include a prominent dissociation among different types of speech output.     

Lateralization of language and spatial functions in left-handed populations.

The level of performance in the execution of a tactual-linguistic task was studied in two left-handed populations, one with language lateralized to the left and the other with language lateralized to the right hemisphere, according to Kimura's dichotic digit test. The results showed that left-handers with speech most likely lateralized to the left hemisphere performed significantly better in the tactual-linguistic task with the left hand, while the ones with speech probably lateralized to the right hemisphere, did better with the right hand. The data suggest that integration of functions asymmetrically represented is facilitated by their differential lateralization.     

Left hemisphere motor facilitation in response to manual action sounds

Previous studies indicate that the motor areas of both hemispheres are active when observing actions. Here we explored how the motor areas of each hemisphere respond to the sounds associated with actions. We used transcranial magnetic stimulation (TMS) to measure motor corticospinal excitability of hand muscles while listening to sounds. Sounds associated with bimanual actions produced greater motor corticospinal excitability than sounds associated with leg movements or control sounds. This facilitation was exclusively lateralized to the left hemisphere, the dominant hemisphere for language. These results are consistent with the hypothesis that action coding may be a precursor of language.     

Motor cortex hand area and speech: implications for the development of language

Recently a growing body of evidence has suggested that a functional link exists between the hand motor area of the language dominant hemisphere and the regions subserving language processing. We examined the excitability of the hand motor area and the leg motor area during reading aloud and during non-verbal oral movements using transcranial magnetic stimulation (TMS). During reading aloud, but not before or afterwards, excitability was increased in the hand motor area of the dominant hemisphere. This reading effect was found to be independent of the duration of speech. No such effect could be found in the contralateral hemisphere. The excitability of the leg area of the motor cortex remained unchanged during reading aloud. The excitability during non-verbal oral movements was slightly increased in both hemispheres. Our results are consistent with previous findings and may indicate a specific functional connection between the hand motor area and the cortical language network.     

Hemispheric control of speech expression in aphasia. A mouth asymmetry study

Clinical observation of aphasic patients often shows a dissociation between impaired propositional speech and preserved automatic speech. The question of differing hemispheric control between these speech tasks was examined by measuring asymmetry in facial muscle activity. In spite of right-sided facial palsy, greater opening of the right side of the mouth was seen for spontaneous speech, repetition, and word list generation, suggesting that the damaged left hemisphere still controlled motor output in these propositional tasks. However, greater opening of the left side of the mouth for serial speech and singing indicated at least a relatively greater role of the right hemisphere in these "automatic" tasks.     

Speech and song: The role of the cerebellum

An exploration into cerebellar activity during the perception and production of speech and song may elucidate general underlying cerebellar functions. Recently, the cerebellum has been hypothesized to be involved with sharpening sensory input, temporal coordination and processing of motor articulation and perception, as well as instantiation of internal models that simulate the input-output characteristics of a specific system. Sung language and spoken language share many common features (physiology for articulation and perception as well as phonology, phonotactics, syntax, and semantics of the underlying language), although they differ in certain vocal and prosodic aspects. A review of the literature on perception and production of singing and speech reveals considerable overlap in the lateral aspect of the VI lobule of the posterior cerebellum, a region known to somatotopically represent the lips and tongue. This region may instantiate internal models of vocal tract articulation that simulate well learned phonological and/or segmental articulatory—auditory/orosensory mappings utilized for both speech and singing. Recent results show tendencies for left cerebellar hemispheric specialization for processing of singing and right specialization for processing of speech, both in the VI lobule of the cerebellum, inferior to that found for representing both speech and singing. Given the crossed pattern of cerebellar-cortical anatomical connectivity the findings are consistent with the hypothesis that the right cerebellum differentially processes high pass filtered information (segmental properties) and the left cerebellum differentially processes low pass filtered information (prosodic, melodic properties). Further research is necessary to examine these hypotheses and their alternatives directly.     

Hemispheric asymmetry of ipsilateral motor cortex activation during unimanual motor tasks: further evidence for motor dominance.

OBJECTIVES: To test to which extent the increase in ipsilateral motor cortex excitability during unimanual motor tasks shows hemispheric asymmetry. METHODS: Six right-handed healthy subjects performed one of several motor tasks of different complexity (including rest) with one hand (task hand) while the other hand (non-task hand) was relaxed. Focal transcranial magnetic stimulation was applied to the motor cortex ipsilateral to the task hand and the amplitude of the motor evoked potential (MEP) in the non-task hand was measured. In one session, the task hand was the right hand, in the other session it was the left hand. The effects of motor task and side of the task hand were analyzed. Spinal motoneuron excitability was assessed using F-wave measurements. RESULTS: Motor tasks, in particular complex finger sequences, resulted in an increase in MEP amplitude in the non-task hand. This increase was significantly less when the right hand rather than the left hand was the task hand. This difference was seen only in muscles homologous to primary task muscles. The asymmetry could not be explained by changes in F-wave amplitudes. CONCLUSIONS: Hemispheric asymmetry of ipsilateral motor cortex activation either supports the idea that, in right handers, the left motor cortex is more active in ipsilateral hand movements, or alternatively, that the left motor cortex exerts more effective inhibitory control over the right motor cortex than vice versa. We suggest that hemispheric asymmetry of ipsilateral motor cortex activation is one property of motor dominance of the left motor cortex.     

Lateralized dual-task performance: the effects of spatial and muscular repositioning

In three experiments subjects were required to perform different manual tapping tasks using either the left or right hand while concurrently performing a speech task. The major independent variables examined were muscular effort and the spatial and muscular repositioning components of the tapping tasks. In all speech conditions right hand performance was disrupted more than left hand performance. The degree of this lateralized disruption did not alter as a function of variation in either muscular effort or the repositioning requirements of the manual task. However, muscular repositioning activity, unlike other components of the motor task, was affected bilaterally by the addition of the speech task. Overall the results suggest that the dual-task procedure can be used to assess two types of "lateralized" interference effects; one relating to right hand performance, the other to left hemisphere performance.     

Anticipation and execution of a simple reading task enhance corticospinal excitability.

OBJECTIVE: Electromyographic responses (EMG) evoked in the right hand by transcranial magnetic stimulation (TMS) of the left motor cortex are enhanced during continuous reading. This enhancement is the result of increased excitability of the motor cortex. We proposed that anticipation and reading of single words would also enhance corticospinal excitability. We studied the temporal course of corticospinal excitability changes following left and right hemisphere TMS. METHODS: Ten normal volunteers were studied. A warning stimulus (S1) was followed by an imperative stimulus (S2) whereupon a word was presented. Subjects responded by reading the word aloud or reading it silently. In other conditions, no word was displayed and the subjects responded to S2 by saying the word 'Cat', pursing their lips, or doing nothing. EMG was recorded over the contralateral hand following a TMS pulse over the motor cortex during and after the S1-S2 period. RESULTS: Enhancement of EMG amplitudes was significantly greater following left hemisphere TMS. The enhancement in the S1-S2 period and that following S2 had a time course similar to several event-related brain potentials. CONCLUSIONS: There may be a common mechanism underlying both corticospinal excitability and the contingent negative variation, readiness potential and N400.     

Hemispheric mechanisms of motor speech

Abstract

A perceptual rating scale was used to evaluate the articulatory impairment of patients with suprabulbar, vascular lesions of the right (RHD, 35 patients) or the left (LHD, 35 patients) hemisphere. Both groups were matched for age, time elapsed since onset, lesion volume and lesion location. The amount and type of motor speech impairment differed significantly, resulting in hemispheric profiles of articulation. Slurring and rate reduction were the prominent features of left hemisphere dysarthria and dysprosody of the right. Verbal apraxia was found in six patients of the LHD group. Intrahemispheric correlations between dysarthria, lesion volume, and several hemispheric variables (orofacial apraxia, aphasia severity and visuospatial functions) did not exceed the 0–6 level. Most lesions causing hemispheric dysarthria were located in the basal ganglia, the cortical sensorimotor and opercular region. The observed lesion distribution and speech patterns implicate distinct hemispheric mechanisms of motor speech: bilateral control is suggested for basic motor speech functions, whereas supramodal and spatio-temporal planning mechanisms are hemisphere specific. Perceptual dysarthria rating is a valuable clinical method to differentiate motor speech impairment.     

Localisation des centres du langage par l'analyse quantifiée de l'EEG (versus test de Wada) dans l'épilepsie temporale chez le droitier

Spectral analysis of the EEG alpha rhythm was studied in nine temporal epileptic right-handed patients in order to predict localization of the speech area. We studied the variation of the spectral power of the alpha rhythm during an activation paradigm previously validated in normal right-handed subjects. Significant alpha power decreases in the left hemisphere during writing with the right hand (as compared to resting) and/or significant alpha power decreases in the right hemisphere during left-hand recognition and classification of cardboard objects (as compared to resting) were considered as consistent with left-hemisphere dominance for language. The results of EEG spectral analysis were compared with those of the Wada test. The left hemisphere was dominant for language according to the Wada test in eight subjects and the right hemisphere in one subject. Six patients had a significant alpha power reduction in the hemisphere concerned during lateralized cognitive tasks, consistent with language localization in the left hemisphere according to the Wada test. The three remaining patients had no significant EEG spectral power variations. A significant decrease of alpha power in the active hemisphere during cerebral activation seems statistically related to left-hemispheric dominance for language in right-handed subjects (hemispheric specialization). However, the localization of the speech area using this electrophysiological method does not appear clinically relevant for a case-by-case decision in individual patients.     

Changes of muscle excitability of the hand contralateral to a task performing one: a transcranial magnetic stimulation study]

It used to be considered that unilateral movements of distal limb parts are associated only with contralateral motor cortical activity. Recent neuroimaging studies, however, suggest that the motor cortex ipsilateral to a task-performing hand is also activated, and that motor patterns in one hand affect the degree of the activity of the ipsilateral motor cortex. If so, muscles of the hand contralateral to a task-performing one may change those excitability depending on types of tasks. We studied eight subjects who performed three different finger tasks by one hand: (a) pinch, (b) sequential finger opposition, and (c) tactile discrimination. Transcranial magnetic stimulation was delivered by a figure eight coil over the hemisphere ipsilateral to a task-performing hand. Motor evoked potentials and background electromyographic activities were recorded from the opponens pollicis muscle contralateral to the stimulated hemisphere. On average, the motor evoked potentials were larger during tactile discrimination task than those at rest in either hand (p < 0.01). Background electromyographic activities in the left hand increased significantly during right hand tactile discrimination task (p < 0.01), whilst those in the right hand did not change during the left hand performance (p > 0.05). These findings suggest the followings: (1) the hand muscle contralateral to a task performing one changes its excitability depending on types of tasks; and (2) increment of excitability of the left hand muscle associated with right hand tactile discrimination is greater than that of the right hand one in association with the same task by the left hand, thus supporting the idea that there is a functional asymmetry between the right and left motor cortex in respect of motor performance.     

Evidence of minimal cerebral asymmetries for the processing of English words and American sign language in the congenitally deaf

Previous investigators reported that auditory association cortex is usually larger within the left than within the right hemisphere, even in the newborn. They suggested that this asymmetry could play a causal role in the predominant tendency for language processes to lateralize in the left rather than in the right hemisphere. For congenitally deaf persons, however, the role of auditory association area asymmetry presumably would be nullified. This question was addressed by comparing the performances of congenitally deaf college students and hearing persons on lateralized tachistoscopic tasks thought to reflect hemispheric language and spatial processing efficiencies. The results indicated that indicated that congenitally deaf Ss showed minimal half-field asymmetries for both English word stimuli and for American Sign Language stimuli (drawings of hand configurations representing letters and words for the deaf Ss). Results were interpreted as supporting auditory experience as a major determinant of cerebral functional asymmetries and as contradictory to clinical reports that had suggested that the cerebral organization of “communicative” functions were entirely comparable in deaf and hearing persons.     

Lateralization of motor excitability during observation of bimanual signs

Viewing another person's hand actions enhances excitability in an observer's left and right primary motor (M1) cortex. We aimed to determine whether viewing communicative hand actions alters this bilateral sensorimotor resonance. Using single-pulse transcranial magnetic stimulation (TMS), we measured excitability in the left and right M1 while right-handed non-signing participants observed bimanual communicative hand actions, i.e., meaningful signs in British Sign Language. TMS-induced motor evoked potentials were recorded from hand muscles during sign observation before and after teaching the participants to associate meanings with half of the signs. Before this teaching, when participants did not know that the presented hand actions were signs, excitability of left and right M1 was modulated equally. After learning the meanings of half the signs, excitability of the left, but not right, M1 was significantly enhanced. This left-lateralized enhancement of M1 excitability occurred during observation of signs with known and unknown meanings. The findings suggest that awareness of the communicative nature of another person's hand actions strengthens sensorimotor resonance in the left M1 cortex and alters hemispheric balance during action observation.     

Lateralization of unimanual and bimanual motor imagery

Most studies of motor imagery have examined motor cortex function during imagery of dominant hand movement. The aim of this study was to examine the modulation of excitability in the dominant and non-dominant corticomotor pathways during kinesthetic motor imagery of unimanual and bimanual movement. Transcranial magnetic stimulation (TMS) was applied over the contralateral motor cortex (M1) to elicit motor-evoked potentials (MEPs) in the abductor pollicis brevis (APB) and abductor digiti minimi (ADM) muscles of each hand, in two separate sessions. Transcutaneous electrical stimuli were also delivered to the median nerve at each wrist, to elicit F-waves from APB. Fifteen right-handed volunteers imagined unimanual and bimanual phasic thumb movements, paced with a 1-Hz auditory metronome. Stimuli were delivered at rest, and either 50 ms before (ON phase), or 450 ms after (OFF phase), the metronome beeps. Significant MEP amplitude facilitation occurred only in right APB, during the ON phase of motor imagery of the right hand and both hands. Significant temporal modulation of right APB MEP amplitude was observed during motor imagery of right, left and bimanual performance. F-wave persistence and amplitude were unaffected by imagery. These results demonstrate that the motor imagery is lateralized to the left (dominant) hemisphere, which is engaged by imagery of each hand separately, and bimanual imagery. This finding has implications for the use of motor imagery in rehabilitation.     

Symmetry detection in typically and atypically speech lateralized individuals: A visual half-field study

Visuospatial functions are typically lateralized to the right cerebral hemisphere, giving rise to a left visual field advantage in visual half-field tasks. In a first study we investigated whether this is also true for symmetry detection off fixation. Twenty right-handed participants with left hemisphere speech dominance took part in a visual half-field experiment requiring them to judge the symmetry of 2-dimensional figures made by joining rectangles in symmetrical or asymmetrical ways. As expected, a significant left visual field advantage was observed for the symmetrical figures. In a second study, we replicated the study with 37 left-handed participants and left hemisphere speech dominance. We again found a left visual field advantage. Finally, in a third study, we included 17 participants with known right hemisphere dominance for speech (speech dominance had been identified with fMRI in an earlier study; Van der Haegen, Cai, Seurinck, & Brysbaert, 2011). Around half of these individuals showed a reversed pattern, i.e. a right visual half-field advantage for symmetric figures while the other half replicated the left visual-field advantage. These findings suggest that symmetry detection is indeed a cognitive function lateralized to the right hemisphere for the majority of the population. The data of the participants with atypical speech dominance are more in line with the idea that language and visuospatial functions are lateralized in opposite brain hemispheres than with the idea that different functions lateralize independently, although there seems to be more variability in this group.