GABA and Glutamate in hMT+ Link to Individual Differences in Residual Visual Function After Occipital Stroke

BACKGROUND

Damage to the primary visual cortex following an occipital stroke causes loss of conscious vision in the contralateral hemifield. Yet, some patients retain the ability to detect moving visual stimuli within their blind field. The present study asked whether such individual differences in blind field perception following loss of primary visual cortex could be explained by the concentration of neurotransmitters γ-aminobutyric acid (GABA) and glutamate or activity of the visual motion processing, human middle temporal complex (hMT+).

METHODS

We used magnetic resonance imaging in 19 patients with chronic occipital stroke to measure the concentration of neurotransmitters GABA and glutamate (proton magnetic resonance spectroscopy) and functional activity in hMT+ (functional magnetic resonance imaging). We also tested each participant on a 2-interval forced choice detection task using high-contrast, moving Gabor patches. We then measured and assessed the strength of relationships between participants' residual vision in their blind field and in vivo neurotransmitter concentrations, as well as visually evoked functional magnetic resonance imaging activity in their hMT+. Levels of GABA and glutamate were also measured in a sensorimotor region, which served as a control.

RESULTS

Magnetic resonance spectroscopy-derived GABA and glutamate concentrations in hMT+ (but not sensorimotor cortex) strongly predicted blind-field visual detection abilities. Performance was inversely related to levels of both inhibitory and excitatory neurotransmitters in hMT+ but, surprisingly, did not correlate with visually evoked blood oxygenation level-dependent signal change in this motion-sensitive region.

CONCLUSIONS

Levels of GABA and glutamate in hMT+ appear to provide superior information about motion detection capabilities inside perimetrically defined blind fields compared to blood oxygenation level-dependent signal changes-in essence, serving as biomarkers for the quality of residual visual processing in the blind-field. Whether they also reflect a potential for successful rehabilitation of visual function remains to be determined.

Microstructural Properties of the Cerebellar Peduncles in Children with Developmental Language Disorder

Children with developmental language disorder (DLD) struggle to learn their native language for no apparent reason. While research on the neurobiological underpinnings of the disorder has focused on the role of cortico-striatal systems, little is known about the role of the cerebellum in DLD. Cortico-cerebellar circuits might be involved in the disorder as they contribute to complex sensorimotor skill learning, including the acquisition of spoken language. Here, we used diffusion-weighted imaging data from 77 typically developing and 54 children with DLD and performed probabilistic tractography to identify the cerebellum's white matter tracts: the inferior, middle, and superior cerebellar peduncles. Children with DLD showed lower fractional anisotropy (FA) in the inferior cerebellar peduncles (ICP), fiber tracts that carry motor and sensory input via the inferior olive to the cerebellum. Lower FA in DLD was driven by lower axial diffusivity. Probing this further with more sophisticated modeling of diffusion data, we found higher orientation dispersion but no difference in neurite density in the ICP of DLD. Reduced FA is therefore unlikely to be reflecting microstructural differences in myelination in this tract, rather the organization of axons in these pathways is disrupted. ICP microstructure was not associated with language or motor coordination performance in our sample. We also found no differences in the middle and superior peduncles, the main pathways connecting the cerebellum with the cortex. To conclude, it is not cortico-cerebellar but atypical olivocerebellar white matter connections that characterize DLD and suggest the involvement of the olivocerebellar system in speech acquisition and development.

Generalized models for quantifying laterality using functional transcranial Doppler ultrasound

We consider how analysis of brain lateralization using functional transcranial Doppler ultrasound (fTCD) data can be brought in line with modern statistical methods typically used in functional magnetic resonance imaging (fMRI). Conventionally, a laterality index is computed in fTCD from the difference between the averages of each hemisphere's signal within a period of interest (POI) over a series of trials. We demonstrate use of generalized linear models (GLMs) and generalized additive models (GAM) to analyze data from individual participants in three published studies (N = 154, 73 and 31), and compare this with results from the conventional POI averaging approach, and with laterality assessed using fMRI (N = 31). The GLM approach was based on classic fMRI analysis that includes a hemodynamic response function as a predictor; the GAM approach estimated the response function from the data, including a term for time relative to epoch start (simple GAM), plus a categorical index corresponding to individual epochs (complex GAM). Individual estimates of the fTCD laterality index are similar across all methods, but error of measurement is lowest using complex GAM. Reliable identification of cases of bilateral language appears to be more accurate with complex GAM. We also show that the GAM-based approach can be used to efficiently analyze more complex designs that incorporate interactions between tasks.

Quantitative MRI reveals differences in striatal myelin in children with DLD

Developmental language disorder (DLD) is a common neurodevelopmental disorder characterised by receptive or expressive language difficulties or both. While theoretical frameworks and empirical studies support the idea that there may be neural correlates of DLD in frontostriatal loops, findings are inconsistent across studies. Here, we use a novel semiquantitative imaging protocol - multi-parameter mapping (MPM) - to investigate microstructural neural differences in children with DLD. The MPM protocol allows us to reproducibly map specific indices of tissue microstructure. In 56 typically developing children and 33 children with DLD, we derived maps of (1) longitudinal relaxation rate R1 (1/T1), (2) transverse relaxation rate R2* (1/T2*), and (3) Magnetization Transfer saturation (MTsat). R1 and MTsat predominantly index myelin, while R2* is sensitive to iron content. Children with DLD showed reductions in MTsat values in the caudate nucleus bilaterally, as well as in the left ventral sensorimotor cortex and Heschl's gyrus. They also had globally lower R1 values. No group differences were noted in R2* maps. Differences in MTsat and R1 were coincident in the caudate nucleus bilaterally. These findings support our hypothesis of corticostriatal abnormalities in DLD and indicate abnormal levels of myelin in the dorsal striatum in children with DLD.

Genome-wide analyses of individual differences in quantitatively assessed reading- and language-related skills in up to 34,000 people

The use of spoken and written language is a fundamental human capacity. Individual differences in reading- and language-related skills are influenced by genetic variation, with twin-based heritability estimates of 30 to 80% depending on the trait. The genetic architecture is complex, heterogeneous, and multifactorial, but investigations of contributions of single-nucleotide polymorphisms (SNPs) were thus far underpowered. We present a multicohort genome-wide association study (GWAS) of five traits assessed individually using psychometric measures (word reading, nonword reading, spelling, phoneme awareness, and nonword repetition) in samples of 13,633 to 33,959 participants aged 5 to 26 y. We identified genome-wide significant association with word reading (rs11208009, P = 1.098 × 10-8) at a locus that has not been associated with intelligence or educational attainment. All five reading-/language-related traits showed robust SNP heritability, accounting for 13 to 26% of trait variability. Genomic structural equation modeling revealed a shared genetic factor explaining most of the variation in word/nonword reading, spelling, and phoneme awareness, which only partially overlapped with genetic variation contributing to nonword repetition, intelligence, and educational attainment. A multivariate GWAS of word/nonword reading, spelling, and phoneme awareness maximized power for follow-up investigation. Genetic correlation analysis with neuroimaging traits identified an association with the surface area of the banks of the left superior temporal sulcus, a brain region linked to the processing of spoken and written language. Heritability was enriched for genomic elements regulating gene expression in the fetal brain and in chromosomal regions that are depleted of Neanderthal variants. Together, these results provide avenues for deciphering the biological underpinnings of uniquely human traits.

Language and nonverbal auditory processing in the occipital cortex of individuals who are congenitally blind due to anophthalmia

Individuals with congenital blindness due to bilateral anophthalmia offer a unique opportunity to examine cross-modal plasticity in the complete absence of any stimulation of the 'visual' pathway even during development in utero. Our previous work has suggested that this complete sensory deafferentation results in different patterns of reorganisation compared with those seen in other early blind populations. Here, we further test the functional specialisation of occipital cortex in six well-studied cases with anophthalmia. Whole brain functional MRI was obtained while these human participants and a group of sighted controls performed two experiments involving phonological and semantic processing of words (verbal experiment) and spatial and identity processing of piano chords (nonverbal experiment). Both experiments were predicted to show a dorsal-ventral difference in activity based on the specific task performed. All tasks evoked activation in occipital cortex in the individuals with anophthalmia but not in the sighted controls. For the verbal experiment, both dorsal and ventral occipital areas were strongly activated by the phonological and semantic tasks in anophthalmia. For the nonverbal experiment, both the spatial and the identity task robustly activated the dorsal occipital area V3a but showed inconsistent activity elsewhere in the occipital lobe. V1 was most strongly activated by the verbal tasks, showing greater activity on the left for the verbal task relative to the nonverbal one. For individual anophthalmic participants, however, activity in V1 was inconsistent across tasks and hemispheres with many participants showing activity levels in the control range, which was not significantly above baseline. Despite the homogeneous nature of the cause of blindness in the anophthalmic group, there remain differences in patterns of activation among the individuals with this condition. Investigation at the case level might further our understanding of how post-natal experiences shape functional reorganisation in deafferented cortex.

Characteristics of articulatory gestures in stuttered speech: A case study using real-time magnetic resonance imaging

INTRODUCTION

Most of the previous articulatory studies of stuttering have focussed on the fluent speech of people who stutter. However, to better understand what causes the actual moments of stuttering, it is necessary to probe articulatory behaviors during stuttered speech. We examined the supralaryngeal articulatory characteristics of stuttered speech using real-time structural magnetic resonance imaging (RT-MRI). We investigated how articulatory gestures differ across stuttered and fluent speech of the same speaker.

METHODS

Vocal tract movements of an adult man who stutters during a pseudoword reading task were recorded using RT-MRI. Four regions of interest (ROIs) were defined on RT-MRI image sequences around the lips, tongue tip, tongue body, and velum. The variation of pixel intensity in each ROI over time provided an estimate of the movement of these four articulators.

RESULTS

All disfluencies occurred on syllable-initial consonants. Three articulatory patterns were identified. Pattern 1 showed smooth gestural formation and release like fluent speech. Patterns 2 and 3 showed delayed release of gestures due to articulator fixation or oscillation respectively. Block and prolongation corresponded to either pattern 1 or 2. Repetition corresponded to pattern 3 or a mix of patterns. Gestures for disfluent consonants typically exhibited a greater constriction than fluent gestures, which was rarely corrected during disfluencies. Gestures for the upcoming vowel were initiated and executed during these consonant disfluencies, achieving a tongue body position similar to the fluent counterpart.

CONCLUSION

Different perceptual types of disfluencies did not necessarily result from distinct articulatory patterns, highlighting the importance of collecting articulatory data of stuttering. Disfluencies on syllable-initial consonants were related to the delayed release and the overshoot of consonant gestures, rather than the delayed initiation of vowel gestures. This suggests that stuttering does not arise from problems with planning the vowel gestures, but rather with releasing the overly constricted consonant gestures.

Elevated iron concentration in putamen and cortical speech motor network in developmental stuttering

Theoretical accounts of developmental stuttering implicate dysfunctional cortico-striatal-thalamo-cortical motor loops through the putamen. However, the analysis of conventional MRI brain scans in individuals who stutter has failed to yield strong support for this theory in terms of reliable differences in the structure or function of the basal ganglia. Here, we performed quantitative mapping of brain tissue, which can be used to measure iron content alongside markers sensitive to myelin and thereby offers particular sensitivity to the measurement of iron-rich structures such as the basal ganglia. Analysis of these quantitative maps in 41 men and women who stutter and 32 individuals who are typically fluent revealed significant group differences in maps of R2*, indicative of higher iron content in individuals who stutter in the left putamen and in left hemisphere cortical regions important for speech motor control. Higher iron levels in brain tissue in individuals who stutter could reflect elevated dopamine levels or lysosomal dysfunction, both of which are implicated in stuttering. This study represents the first use of these quantitative measures in developmental stuttering and provides new evidence of microstructural differences in the basal ganglia and connected frontal cortical regions.

Disruption of speech motor adaptation with repetitive transcranial magnetic stimulation of the articulatory representation in primary motor cortex

When auditory feedback perturbation is introduced in a predictable way over a number of utterances, speakers learn to compensate by adjusting their own productions, a process known as sensorimotor adaptation. Despite multiple lines of evidence indicating the role of primary motor cortex (M1) in motor learning and memory, whether M1 causally contributes to sensorimotor adaptation in the speech domain remains unclear. Here, we aimed to assay whether temporary disruption of the articulatory representation in left M1 by repetitive transcranial magnetic stimulation (rTMS) impairs speech adaptation. To induce sensorimotor adaptation, the frequencies of first formants (F1) were shifted up and played back to participants when they produced “head”, “bed”, and “dead” repeatedly (the learning phase). A low-frequency rTMS train (.6 Hz, subthreshold, 12 min) over either the tongue or the hand representation of M1 (between-subjects design) was applied before participants experienced altered auditory feedback in the learning phase. We found that the group who received rTMS over the hand representation showed the expected compensatory response for the upwards shift in F1 by significantly reducing F1 and increasing the second formant (F2) frequencies in their productions. In contrast, these expected compensatory changes in both F1 and F2 did not occur in the group that received rTMS over the tongue representation. Critically, rTMS (subthreshold) over the tongue representation did not affect vowel production, which was unchanged from baseline. These results provide direct evidence that the articulatory representation in left M1 causally contributes to sensorimotor learning in speech. Furthermore, these results also suggest that M1 is critical to the network supporting a more global adaptation that aims to move the altered speech production closer to a learnt pattern of speech production used to produce another vowel.

Cerebral lateralisation of first and second languages in bilinguals assessed using functional transcranial Doppler ultrasound

Background: Lateralised language processing is a well-established finding in monolinguals. In bilinguals, studies using fMRI have typically found substantial regional overlap between the two languages, though results may be influenced by factors such as proficiency, age of acquisition and exposure to the second language. Few studies have focused specifically on individual differences in brain lateralisation, and those that have suggested reduced lateralisation may characterise representation of the second language (L2) in some bilingual individuals.

Methods: In Study 1, we used functional transcranial Doppler sonography (FTCD) to measure cerebral lateralisation in both languages in high proficiency bilinguals who varied in age of acquisition (AoA) of L2. They had German (N = 14) or French (N = 10) as their first language (L1) and English as their second language. FTCD was used to measure task-dependent blood flow velocity changes in the left and right middle cerebral arteries during phonological word generation cued by single letters. Language history measures and handedness were assessed through self-report. Study 2 followed a similar format with 25 Japanese (L1) /English (L2) bilinguals, with proficiency in their second language ranging from basic to advanced, using phonological and semantic word generation tasks with overt speech production.

Results: In Study 1, participants were significantly left lateralised for both L1 and L2, with a high correlation (r = .70) in the size of laterality indices for L1 and L2. In Study 2, again there was good agreement between LIs for the two languages (r = .77 for both word generation tasks). There was no evidence in either study of an effect of age of acquisition, though the sample sizes were too small to detect any but large effects. 

Conclusion: In proficient bilinguals, there is strong concordance for cerebral lateralisation of first and second language as assessed by a verbal fluency task.

Cerebral lateralisation of first and second languages in bilinguals assessed using functional transcranial Doppler ultrasound

Background: Lateralised language processing is a well-established finding in monolinguals. In bilinguals, studies using fMRI have typically found substantial regional overlap between the two languages, though results may be influenced by factors such as proficiency, age of acquisition and exposure to the second language. Few studies have focused specifically on individual differences in brain lateralisation, and those that have suggested reduced lateralisation may characterise representation of the second language (L2) in some bilingual individuals. Methods: In Study 1, we used functional transcranial Doppler sonography (FTCD) to measure cerebral lateralisation in both languages in high proficiency bilinguals who varied in age of acquisition (AoA) of L2. They had German (N = 14) or French (N = 10) as their first language (L1) and English as their second language. FTCD was used to measure task-dependent blood flow velocity changes in the left and right middle cerebral arteries during phonological word generation cued by single letters. Language history measures and handedness were assessed through self-report. Study 2 followed a similar format with 25 Japanese (L1) /English (L2) bilinguals, with proficiency in their second language ranging from basic to advanced, using phonological and semantic word generation tasks with overt speech production. Results: In Study 1, participants were significantly left lateralised for both L1 and L2, with a high correlation (r = .70) in the size of laterality indices for L1 and L2. In Study 2, again there was good agreement between LIs for the two languages (r = .77 for both word generation tasks). There was no evidence in either study of an effect of age of acquisition, though the sample sizes were too small to detect any but large effects.  Conclusion: In proficient bilinguals, there is strong concordance for cerebral lateralisation of first and second language as assessed by a verbal fluency task.

Speech Movement Variability in People Who Stutter: A Vocal Tract Magnetic Resonance Imaging Study

Purpose People who stutter (PWS) have more unstable speech motor systems than people who are typically fluent (PWTF). Here, we used real-time magnetic resonance imaging (MRI) of the vocal tract to assess variability and duration of movements of different articulators in PWS and PWTF during fluent speech production. Method The vocal tracts of 28 adults with moderate to severe stuttering and 20 PWTF were scanned using MRI while repeating simple and complex pseudowords. Midsagittal images of the vocal tract from lips to larynx were reconstructed at 33.3 frames per second. For each participant, we measured the variability and duration of movements across multiple repetitions of the pseudowords in three selected articulators: the lips, tongue body, and velum. Results PWS showed significantly greater speech movement variability than PWTF during fluent repetitions of pseudowords. The group difference was most evident for measurements of lip aperture using these stimuli, as reported previously, but here, we report that movements of the tongue body and velum were also affected during the same utterances. Variability was not affected by phonological complexity. Speech movement variability was unrelated to stuttering severity within the PWS group. PWS also showed longer speech movement durations relative to PWTF for fluent repetitions of multisyllabic pseudowords, and this group difference was even more evident as complexity increased. Conclusions Using real-time MRI of the vocal tract, we found that PWS produced more variable movements than PWTF even during fluent productions of simple pseudowords. PWS also took longer to produce multisyllabic words relative to PWTF, particularly when words were more complex. This indicates general, trait-level differences in the control of the articulators between PWS and PWTF. Supplemental Material https://doi.org/10.23641/asha.14782092.

Investigating the effects of handedness on the consistency of lateralization for speech production and semantic processing tasks using functional transcranial Doppler sonography

The left hemisphere is dominant for language in most people, but lateralization strength varies between different tasks and individuals. A large body of literature has shown that handedness is associated with lateralization: left handers have weaker language lateralization on average, and a greater incidence of atypical (right hemisphere) lateralization; but typically, these studies have relied on a single measure of language lateralization. Here we consider the relationships between lateralization for two different language tasks. We investigated the influence of handedness on lateralization using functional transcranial Doppler sonography (fTCD), using an existing dataset (N = 151 adults, 21 left handed). We compared a speech production task (word generation) and a semantic association task. We demonstrated stronger left-lateralization for word generation than semantic association; and a moderate correlation between laterality indices for the two tasks (r = 0.59). Laterality indices were stronger for right than left handers, and left handers were more likely than right handers to have atypical (right hemisphere) lateralization or inconsistent lateralization between the two tasks. These results add to our knowledge of individual differences in lateralization and support the view that language lateralization is multifactorial rather than unitary.

Morphological and functional variability in central and subcentral motor cortex of the human brain

There is a long-established link between anatomy and function in the somatomotor system in the mammalian cerebral cortex. The morphology of the central sulcus is predictive of the location of functional activation peaks relating to movement of different effectors in individuals. By contrast, morphological variation in the subcentral region and its relationship to function is, as yet, unknown. Investigating the subcentral region is particularly important in the context of speech, since control of the larynx during human speech production is related to activity in this region. Here, we examined the relationship between morphology in the central and subcentral region and the location of functional activity during movement of the hand, lips, tongue, and larynx at the individual participant level. We provide a systematic description of the sulcal patterns of the subcentral and adjacent opercular cortex, including the inter-individual variability in sulcal morphology. We show that, in the majority of participants, the anterior subcentral sulcus is not continuous, but consists of two distinct segments. A robust relationship between morphology of the central and subcentral sulcal segments and movement of different effectors is demonstrated. Inter-individual variability of underlying anatomy might thus explain previous inconsistent findings, in particular regarding the ventral larynx area in subcentral cortex. A surface registration based on sulcal labels indicated that such anatomical information can improve the alignment of functional data for group studies.

Functional organisation for verb generation in children with developmental language disorder

Developmental language disorder (DLD) is characterised by difficulties in learning one's native language for no apparent reason. These language difficulties occur in 7% of children and are known to limit future academic and social achievement. Our understanding of the brain abnormalities associated with DLD is limited. Here, we used a simple four-minute verb generation task (children saw a picture of an object and were instructed to say an action that goes with that object) to test children between the ages of 10-15 years (DLD N = 50, typically developing N = 67). We also tested 26 children with poor language ability who did not meet our criteria for DLD. Contrary to our registered predictions, we found that children with DLD did not have (i) reduced activity in language relevant regions such as the left inferior frontal cortex; (ii) dysfunctional striatal activity during overt production; or (iii) a reduction in left-lateralised activity in frontal cortex. Indeed, performance of this simple language task evoked activity in children with DLD in the same regions and to a similar level as in typically developing children. Consistent with previous reports, we found sub-threshold group differences in the left inferior frontal gyrus and caudate nuclei, but only when analysis was limited to a subsample of the DLD group (N = 14) who had the poorest performance on the task. Additionally, we used a two-factor model to capture variation in all children studied (N = 143) on a range of neuropsychological tests and found that these language and verbal memory factors correlated with activity in different brain regions. Our findings indicate a lack of support for some neurological models of atypical language learning, such as the procedural deficit hypothesis or the atypical lateralization hypothesis, at least when using simple language tasks that children can perform. These results also emphasise the importance of controlling for and monitoring task performance.

Mapping Human Laryngeal Motor Cortex during Vocalization

The representations of the articulators involved in human speech production are organized somatotopically in primary motor cortex. The neural representation of the larynx, however, remains debated. Both a dorsal and a ventral larynx representation have been previously described. It is unknown, however, whether both representations are located in primary motor cortex. Here, we mapped the motor representations of the human larynx using functional magnetic resonance imaging and characterized the cortical microstructure underlying the activated regions. We isolated brain activity related to laryngeal activity during vocalization while controlling for breathing. We also mapped the articulators (the lips and tongue) and the hand area. We found two separate activations during vocalization-a dorsal and a ventral larynx representation. Structural and quantitative neuroimaging revealed that myelin content and cortical thickness underlying the dorsal, but not the ventral larynx representation, are similar to those of other primary motor representations. This finding confirms that the dorsal larynx representation is located in primary motor cortex and that the ventral one is not. We further speculate that the location of the ventral larynx representation is in premotor cortex, as seen in other primates. It remains unclear, however, whether and how these two representations differentially contribute to laryngeal motor control.

Failure of tDCS to modulate motor excitability and speech motor learning

Transcranial direct current stimulation (tDCS) modulates cortical excitability in a polarity-specific way and, when used in combination with a behavioural task, it can alter performance. TDCS has the potential, therefore, for use as an adjunct to therapies designed to treat disorders affecting speech, including, but not limited to acquired aphasias and developmental stuttering. For this reason, it is important to conduct studies evaluating its effectiveness and the parameters optimal for stimulation. Here, we aimed to evaluate the effects of bi-hemispheric tDCS over speech motor cortex on performance of a complex speech motor learning task, namely the repetition of tongue twisters. A previous study in older participants showed that tDCS could modulate performance on a similar task. To further understand the effects of tDCS, we also measured the excitability of the speech motor cortex before and after stimulation. Three groups of 20 healthy young controls received: (i) anodal tDCS to the left IFG/LipM1 and cathodal tDCS to the right hemisphere homologue; or (ii) cathodal tDCS over the left and anodal over the right; or (iii) sham stimulation. Participants heard and repeated novel tongue twisters and matched simple sentences before, during and 10 min after the stimulation. One mA tDCS was delivered concurrent with task performance for 13 min. Motor excitability was measured using transcranial magnetic stimulation to elicit motor-evoked potentials in the lip before and immediately after tDCS. The study was double-blind, randomized, and sham-controlled; the design and analysis were pre-registered. Performance on the task improved from baseline to after stimulation but was not significantly modulated by tDCS. Similarly, a small decrease in motor excitability was seen in all three stimulation groups but did not differ among them and was unrelated to task performance. Bayesian analyses provide substantial evidence in support of the null hypotheses in both cases, namely that tongue twister performance and motor excitability were not affected by tDCS. We discuss our findings in the context of the previous positive results for a similar task. We conclude that tDCS may be most effective when brain function is sub-optimal due to age-related declines or pathology. Further study is required to determine why tDCS failed to modulate excitability in the speech motor cortex in the expected ways.

Evasion of host defenses by intracellular Staphylococcus aureus

Staphylococcus aureus is one of the leading causes of hospital and community-acquired infections worldwide. The increasing occurrence of antibiotic resistant strains and the high rates of recurrent staphylococcal infections have placed several treatment challenges on healthcare systems. In recent years, it has become evident that S. aureus is a facultative intracellular pathogen, able to invade and survive in a range of cell types. The ability to survive intracellularly provides this pathogen with yet another way to evade antibiotics and immune responses during infection. Intracellular S. aureus have been strongly linked to several recurrent infections, including severe bone infections and septicemias. S. aureus is armed with an array of virulence factors as well as an intricate network of regulators that enable it to survive, replicate and escape from a number of immune and nonimmune host cells. It is able to successfully manipulate host cell pathways and use it as a niche to multiply, disseminate, as well as persist during an infection. This bacterium is also known to adapt to the intracellular environment by forming small colony variants, which are metabolically inactive. In this review we will discuss the clinical evidence, the molecular pathways involved in S. aureus intracellular persistence, and new treatment strategies for targeting intracellular S. aureus.

Cross-species cortical alignment identifies different types of anatomical reorganization in the primate temporal lobe

Evolutionary adaptations of temporo-parietal cortex are considered to be a critical specialization of the human brain. Cortical adaptations, however, can affect different aspects of brain architecture, including local expansion of the cortical sheet or changes in connectivity between cortical areas. We distinguish different types of changes in brain architecture using a computational neuroanatomy approach. We investigate the extent to which between-species alignment, based on cortical myelin, can predict changes in connectivity patterns across macaque, chimpanzee, and human. We show that expansion and relocation of brain areas can predict terminations of several white matter tracts in temporo-parietal cortex, including the middle and superior longitudinal fasciculus, but not the arcuate fasciculus. This demonstrates that the arcuate fasciculus underwent additional evolutionary modifications affecting the temporal lobe connectivity pattern. This approach can flexibly be extended to include other features of cortical organization and other species, allowing direct tests of comparative hypotheses of brain organization.

Neocerebellar Crus I Abnormalities Associated with a Speech and Language Disorder Due to a Mutation in FOXP2

Bilateral volume reduction in the caudate nucleus has been established as a prominent brain abnormality associated with a FOXP2 mutation in affected members of the 'KE family', who present with developmental orofacial and verbal dyspraxia in conjunction with pervasive language deficits. Despite the gene's early and prominent expression in the cerebellum and the evidence for reciprocal cerebellum-basal ganglia connectivity, very little is known about cerebellar abnormalities in affected KE members. Using cerebellum-specific voxel-based morphometry (VBM) and volumetry, we provide converging evidence from subsets of affected KE members scanned at three time points for grey matter (GM) volume reduction bilaterally in neocerebellar lobule VIIa Crus I compared with unaffected members and unrelated controls. We also show that right Crus I volume correlates with left and total caudate nucleus volumes in affected KE members, and that right and total Crus I volumes predict the performance of affected members in non-word repetition and non-verbal orofacial praxis. Crus I also shows bilateral hypo-activation in functional MRI in the affected KE members relative to controls during non-word repetition. The association of Crus I with key aspects of the behavioural phenotype of this FOXP2 point mutation is consistent with recent evidence of cerebellar involvement in complex motor sequencing. For the first time, specific cerebello-basal ganglia loops are implicated in the execution of complex oromotor sequences needed for human speech.

Robust Sensorimotor Learning during Variable Sentence-Level Speech

Sensorimotor learning has been studied by altering the sound of the voice in real time as speech is produced. In response to voice alterations, learned changes in production reduce the perceived auditory error and persist for some time after the alteration is removed [1-5]. The results of such experiments have led to the development of prominent models of speech production. This work proposes that the control of speech relies on forward models to predict sensory outcomes of movements, and errors in these predictions drive sensorimotor learning [5-7]. However, sensorimotor learning in speech has only been observed following intensive training on a handful of discrete words or perceptually similar sentences. Stereotyped production does not capture the complex sensorimotor demands of fluid, real-world speech [8-11]. It remains unknown whether talkers predict the sensory consequences of variable sentence production to allow rapid and precise updating of speech motor plans when sensory prediction errors are encountered. Here, we used real-time alterations of speech feedback to test for sensorimotor learning during the production of 50 sentences that varied markedly in length, vocabulary, and grammar. Following baseline production, all vowels were simultaneously altered and played back through headphones in near real time. Robust feedforward changes in sentence production were observed that, on average, precisely countered the direction of the alteration. These changes occurred in every participant and transferred to the production of single words with varying vowel sounds. The results show that to maintain accurate sentence production, the brain actively predicts the auditory consequences of variable sentence-level speech.

The influence of evaluative right/wrong feedback on phonological and semantic processes in word learning

Feedback is typically incorporated in word learning paradigms, in both research studies and commercial language learning apps. While the common-sense view is that feedback is helpful during learning, relatively little empirical evidence exists about the role of feedback in spoken vocabulary learning. Some work has suggested that long-term word learning is not enhanced by the presence of feedback, and that words are best learned implicitly. It is also plausible that feedback might have differential effects when learners focus on learning semantic facts, or when they focus on learning a new phonological sequence of sounds. In this study, we assess how providing evaluative (right/wrong) feedback on a spoken response influences two different components of vocabulary learning, the learning of a new phonological form, and the learning of a semantic property of the phonological form. We find that receiving evaluative feedback improves retention of phonological forms, but not of semantic facts.

Separation of trait and state in stuttering

Stuttering is a disorder in which the smooth flow of speech is interrupted. People who stutter show structural and functional abnormalities in the speech and motor system. It is unclear whether functional differences reflect general traits of the disorder or are specifically related to the dysfluent speech state. We used a hierarchical approach to separate state and trait effects within stuttering. We collected sparse-sampled functional MRI during two overt speech tasks (sentence reading and picture description) in 17 people who stutter and 16 fluent controls. Separate analyses identified indicators of: (1) general traits of people who stutter; (2) frequency of dysfluent speech states in subgroups of people who stutter; and (3) the differences between fluent and dysfluent states in people who stutter. We found that reduced activation of left auditory cortex, inferior frontal cortex bilaterally, and medial cerebellum were general traits that distinguished fluent speech in people who stutter from that of controls. The stuttering subgroup with higher frequency of dysfluent states during scanning (n = 9) had reduced activation in the right subcortical grey matter, left temporo-occipital cortex, the cingulate cortex, and medial parieto-occipital cortex relative to the subgroup who were more fluent (n = 8). Finally, during dysfluent states relative to fluent ones, there was greater activation of inferior frontal and premotor cortex extending into the frontal operculum, bilaterally. The above differences were seen across both tasks. Subcortical state effects differed according to the task. Overall, our data emphasise the independence of trait and state effects in stuttering.

Facilitation of motor excitability during listening to spoken sentences is not modulated by noise or semantic coherence

Comprehending speech can be particularly challenging in a noisy environment and in the absence of semantic context. It has been proposed that the articulatory motor system would be recruited especially in difficult listening conditions. However, it remains unknown how signal-to-noise ratio (SNR) and semantic context affect the recruitment of the articulatory motor system when listening to continuous speech. The aim of the present study was to address the hypothesis that involvement of the articulatory motor cortex increases when the intelligibility and clarity of the spoken sentences decreases, because of noise and the lack of semantic context. We applied Transcranial Magnetic Stimulation (TMS) to the lip and hand representations in the primary motor cortex and measured motor evoked potentials from the lip and hand muscles, respectively, to evaluate motor excitability when young adults listened to sentences. In Experiment 1, we found that the excitability of the lip motor cortex was facilitated during listening to both semantically anomalous and coherent sentences in noise relative to non-speech baselines, but neither SNR nor semantic context modulated the facilitation. In Experiment 2, we replicated these findings and found no difference in the excitability of the lip motor cortex between sentences in noise and clear sentences without noise. Thus, our results show that the articulatory motor cortex is involved in speech processing even in optimal and ecologically valid listening conditions and that its involvement is not modulated by the intelligibility and clarity of speech.

Transcranial direct current stimulation over left inferior frontal cortex improves speech fluency in adults who stutter

See Crinion (doi:10.1093/brain/awy075) for a scientific commentary on this article.Stuttering is a neurodevelopmental condition affecting 5% of children, and persisting in 1% of adults. Promoting lasting fluency improvement in adults who stutter is a particular challenge. Novel interventions to improve outcomes are of value, therefore. Previous work in patients with acquired motor and language disorders reported enhanced benefits of behavioural therapies when paired with transcranial direct current stimulation. Here, we report the results of the first trial investigating whether transcranial direct current stimulation can improve speech fluency in adults who stutter. We predicted that applying anodal stimulation to the left inferior frontal cortex during speech production with temporary fluency inducers would result in longer-lasting fluency improvements. Thirty male adults who stutter completed a randomized, double-blind, controlled trial of anodal transcranial direct current stimulation over left inferior frontal cortex. Fifteen participants received 20 min of 1-mA stimulation on five consecutive days while speech fluency was temporarily induced using choral and metronome-timed speech. The other 15 participants received the same speech fluency intervention with sham stimulation. Speech fluency during reading and conversation was assessed at baseline, before and after the stimulation on each day of the 5-day intervention, and at 1 and 6 weeks after the end of the intervention. Anodal stimulation combined with speech fluency training significantly reduced the percentage of disfluent speech measured 1 week after the intervention compared with fluency intervention alone. At 6 weeks after the intervention, this improvement was maintained during reading but not during conversation. Outcome scores at both post-intervention time points on a clinical assessment tool (the Stuttering Severity Instrument, version 4) also showed significant improvement in the group receiving transcranial direct current stimulation compared with the sham group, in whom fluency was unchanged from baseline. We conclude that transcranial direct current stimulation combined with behavioural fluency intervention can improve fluency in adults who stutter. Transcranial direct current stimulation thereby offers a potentially useful adjunct to future speech therapy interventions for this population, for whom fluency therapy outcomes are currently limited.

Cortico-cerebellar Networks Drive Sensorimotor Learning in Speech

The motor cortex and cerebellum are thought to be critical for learning and maintaining motor behaviors. Here we use transcranial direct current stimulation (tDCS) to test the role of the motor cortex and cerebellum in sensorimotor learning in speech. During productions of “head,” “bed,” and “dead,” the first formant of the vowel sound was altered in real time toward the first formant of the vowel sound in “had,” “bad,” and “dad.” Compensatory changes in first and second formant production were used as a measure of motor adaptation. tDCS to either the motor cortex or the cerebellum improved sensorimotor learning in speech compared with sham stimulation ( n = 20 in each group). However, in the case of cerebellar tDCS, production changes were restricted to the source of the acoustical error (i.e., the first formant). Motor cortex tDCS drove production changes that offset errors in the first formant, but unlike cerebellar tDCS, adaptive changes in the second formant also occurred. The results suggest that motor cortex and cerebellar tDCS have both shared and dissociable effects on motor adaptation. The study provides initial causal evidence in speech production that the motor cortex and the cerebellum support different aspects of sensorimotor learning. We propose that motor cortex tDCS drives sensorimotor learning toward previously learned patterns of movement, whereas cerebellar tDCS focuses sensorimotor learning on error correction.

Planum temporale asymmetry in people who stutter

PURPOSE

Previous studies have reported that the planum temporale - a language-related structure that normally shows a leftward asymmetry - had reduced asymmetry in people who stutter (PWS) and reversed asymmetry in those with severe stuttering. These findings are consistent with the theory that altered language lateralization may be a cause or consequence of stuttering. Here, we re-examined these findings in a larger sample of PWS.

METHODS

We evaluated planum temporale asymmetry in structural MRI scans obtained from 67 PWS and 63 age-matched controls using: 1) manual measurements of the surface area; 2) voxel-based morphometry to automatically calculate grey matter density. We examined the influences of gender, age, and stuttering severity on planum temporale asymmetry.

RESULTS

The size of the planum temporale and its asymmetry were not different in PWS compared with Controls using either the manual or the automated method. Both groups showed a significant leftwards asymmetry on average (about one-third of PWS and Controls showed rightward asymmetry). Importantly, and contrary to previous reports, the degree of asymmetry was not related to stuttering severity. In the manual measurements, women who stutter had a tendency towards rightwards asymmetry but men who stutter showed the same degree of leftwards asymmetry as male Controls. In the automated measurements, Controls showed a significant increase in leftwards asymmetry with age but this relationship was not observed in PWS.

CONCLUSIONS

We conclude that reduced planum temporale asymmetry is not a prominent feature of the brain in PWS and that the asymmetry is unrelated to stuttering severity.

The effect of recall, reproduction, and restudy on word learning: a pre-registered study

Background

Certain manipulations, such as testing oneself on newly learned word associations (recall), or the act of repeating a word during training (reproduction), can lead to better learning and retention relative to simply providing more exposure to the word (restudy). Such benefit has been observed for written words. Here, we test how these training manipulations affect learning of words presented aurally, when participants are required to produce these novel phonological forms in a recall task.

Methods

Participants (36 English-speaking adults) learned 27 pseudowords, which were paired with 27 unfamiliar pictures. They were given cued recall practice for 9 of the words, reproduction practice for another set of 9 words, and the remaining 9 words were restudied. Participants were tested on their recognition (3-alternative forced choice) and recall (saying the pseudoword in response to a picture) of these items immediately after training, and a week after training. Our hypotheses were that reproduction and restudy practice would lead to better learning immediately after training, but that cued recall practice would lead to better retention in the long term.

Results

In all three conditions, recognition performance was extremely high immediately after training, and a week following training, indicating that participants had acquired associations between the novel pictures and novel words. In addition, recognition and cued recall performance was better immediately after training relative to a week later, confirming that participants forgot some words over time. However, results in the cued recall task did not support our hypotheses. Immediately after training, participants showed an advantage for cued Recall over the Restudy condition, but not over the Reproduce condition. Furthermore, there was no boost for the cued Recall condition over time relative to the other two conditions. Results from a Bayesian analysis also supported this null finding. Nonetheless, we found a clear effect of word length, with shorter words being better learned than longer words, indicating that our method was sufficiently sensitive to detect an impact of condition on learning.

Conclusions

Our primary hypothesis about training conditions conferring specific advantages for production of novel words presented aurally, especially over long intervals, was not supported by this data. Although there may be practical reasons for preferring a particular method for training expressive vocabulary, no difference in effectiveness was detected when presenting words aurally: reproducing, recalling or restudying a word led to the same production accuracy.

Investigating the feasibility of using transcranial direct current stimulation to enhance fluency in people who stutter

Developmental stuttering is a disorder of speech fluency affecting 1% of the adult population. Long-term reductions in stuttering are difficult for adults to achieve with behavioural therapies. We investigated whether a single session of transcranial direct current stimulation (TDCS) could improve fluency in people who stutter (PWS). In separate sessions, either anodal TDCS (1mA for 20min) or sham stimulation was applied over the left inferior frontal cortex while PWS read sentences aloud. Fluency was induced during the stimulation period by using choral speech, that is, participants read in unison with another speaker. Stuttering frequency during sentence reading, paragraph reading and conversation was measured at baseline and at two outcome time points: immediately after the stimulation period and 1h later. Stuttering was reduced significantly at both outcome time points for the sentence-reading task, presumably due to practice, but not during the paragraph reading or conversation tasks. None of the outcome measures were significantly modulated by anodal TDCS. Although the results of this single-session study showed no significant TDCS-induced improvements in fluency, there were some indications that further research is warranted. We discuss factors that we believe may have obscured the expected positive effects of TDCS on fluency, such as heterogeneity in stuttering severity for the sample and variations across sessions. Consideration of such factors may inform future studies aimed at determining the potential of TDCS in the treatment of developmental stuttering.

Patterns of Individual Variation in Visual Pathway Structure and Function in the Sighted and Blind

Many structural and functional brain alterations accompany blindness, with substantial individual variation in these effects. In normally sighted people, there is correlated individual variation in some visual pathway structures. Here we examined if the changes in brain anatomy produced by blindness alter the patterns of anatomical variation found in the sighted. We derived eight measures of central visual pathway anatomy from a structural image of the brain from 59 sighted and 53 blind people. These measures showed highly significant differences in mean size between the sighted and blind cohorts. When we examined the measurements across individuals within each group we found three clusters of correlated variation, with V1 surface area and pericalcarine volume linked, and independent of the thickness of V1 cortex. These two clusters were in turn relatively independent of the volumes of the optic chiasm and lateral geniculate nucleus. This same pattern of variation in visual pathway anatomy was found in the sighted and the blind. Anatomical changes within these clusters were graded by the timing of onset of blindness, with those subjects with a post-natal onset of blindness having alterations in brain anatomy that were intermediate to those seen in the sighted and congenitally blind. Many of the blind and sighted subjects also contributed functional MRI measures of cross-modal responses within visual cortex, and a diffusion tensor imaging measure of fractional anisotropy within the optic radiations and the splenium of the corpus callosum. We again found group differences between the blind and sighted in these measures. The previously identified clusters of anatomical variation were also found to be differentially related to these additional measures: across subjects, V1 cortical thickness was related to cross-modal activation, and the volume of the optic chiasm and lateral geniculate was related to fractional anisotropy in the visual pathway. Our findings show that several of the structural and functional effects of blindness may be reduced to a smaller set of dimensions. It also seems that the changes in the brain that accompany blindness are on a continuum with normal variation found in the sighted.

Neural basis of understanding communicative actions: Changes associated with knowing the actor's intention and the meanings of the actions

People can communicate by using hand actions, e.g., signs. Understanding communicative actions requires that the observer knows that the actor has an intention to communicate and the meanings of the actions. Here, we investigated how this prior knowledge affects processing of observed actions. We used functional MRI to determine changes in action processing when non-signers were told that the observed actions are communicative (i.e., signs) and learned the meanings of half of the actions. Processing of hand actions activated the left and right inferior frontal gyrus (IFG, BA 44 and 45) when the communicative intention of the actor was known, even when the meanings of the actions remained unknown. These regions were not active when the observers did not know about the communicative nature of the hand actions. These findings suggest that the left and right IFG play a role in understanding the intention of the actor, but do not process visuospatial features of the communicative actions. Knowing the meanings of the hand actions further enhanced activity in the anterior part of the IFG (BA 45), the inferior parietal lobule and posterior inferior and middle temporal gyri in the left hemisphere. These left-hemisphere language regions could provide a link between meanings and observed actions. In sum, the findings provide evidence for the segregation of the networks involved in the neural processing of visuospatial features of communicative hand actions and those involved in understanding the actor’s intention and the meanings of the actions.

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Participants observed hand actions before and after learning that they are signs.

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Learning-induced changes in brain activity measured using fMRI.

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No activity in mirror neuron system when actions were not known to be communicative.

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Knowing the actor’s intention to communicate activated IFG and IPL.

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Knowing meanings of the actions increased activity in left IFG (BA 45), IPL and MTG.

Neurochemical changes in the pericalcarine cortex in congenital blindness attributable to bilateral anophthalmia

Congenital blindness leads to large-scale functional and structural reorganization in the occipital cortex, but relatively little is known about the neurochemical changes underlying this cross-modal plasticity. To investigate the effect of complete and early visual deafferentation on the concentration of metabolites in the pericalcarine cortex, (1)H magnetic resonance spectroscopy was performed in 14 sighted subjects and 5 subjects with bilateral anophthalmia, a condition in which both eyes fail to develop. In the pericalcarine cortex, where primary visual cortex is normally located, the proportion of gray matter was significantly greater, and levels of choline, glutamate, glutamine, myo-inositol, and total creatine were elevated in anophthalmic relative to sighted subjects. Anophthalmia had no effect on the structure or neurochemistry of a sensorimotor cortex control region. More gray matter, combined with high levels of choline and myo-inositol, resembles the profile of the cortex at birth and suggests that the lack of visual input from the eyes might have delayed or arrested the maturation of this cortical region. High levels of choline and glutamate/glutamine are consistent with enhanced excitatory circuits in the anophthalmic occipital cortex, which could reflect a shift toward enhanced plasticity or sensitivity that could in turn mediate or unmask cross-modal responses. Finally, it is possible that the change in function of the occipital cortex results in biochemical profiles that resemble those of auditory, language, or somatosensory cortex.

The neurological underpinnings of cluttering: Some initial findings

BACKGROUND

Cluttering is a fluency disorder characterised by overly rapid or jerky speech patterns that compromise intelligibility. The neural correlates of cluttering are unknown but theoretical accounts implicate the basal ganglia and medial prefrontal cortex. Dysfunction in these brain areas would be consistent with difficulties in selection and control of speech motor programs that are characteristic of speech disfluencies in cluttering. There is a surprising lack of investigation into this disorder using modern imaging techniques. Here, we used functional MRI to investigate the neural correlates of cluttering.

METHOD

We scanned 17 adults who clutter and 17 normally fluent control speakers matched for age and sex. Brain activity was recorded using sparse-sampling functional MRI while participants viewed scenes and either (i) produced overt speech describing the scene or (ii) read out loud a sentence provided that described the scene. Speech was recorded and analysed off line. Differences in brain activity for each condition compared to a silent resting baseline and between conditions were analysed for each group separately (cluster-forming threshold Z>3.1, extent p<0.05, corrected) and then these differences were further compared between the two groups (voxel threshold p<0.01, extent>30 voxels, uncorrected).

RESULTS

In both conditions, the patterns of activation in adults who clutter and control speakers were strikingly similar, particularly at the cortical level. Direct group comparisons revealed greater activity in adults who clutter compared to control speakers in the lateral premotor cortex bilaterally and, as predicted, on the medial surface (pre-supplementary motor area). Subcortically, adults who clutter showed greater activity than control speakers in the basal ganglia. Specifically, the caudate nucleus and putamen were overactive in adults who clutter for the comparison of picture description with sentence reading. In addition, adults who clutter had reduced activity relative to control speakers in the lateral anterior cerebellum bilaterally. Eleven of the 17 adults who clutter also stuttered. This comorbid diagnosis of stuttering was found to contribute to the abnormal overactivity seen in the group of adults who clutter in the right ventral premotor cortex and right anterior cingulate cortex. In the remaining areas of abnormal activity seen in adults who clutter compared to controls, the subgroup who clutter and stutter did not differ from the subgroup who clutter but do not stutter.

CONCLUSIONS

Our findings were in good agreement with theoretical predictions regarding the neural correlates of cluttering. We found evidence for abnormal function in the basal ganglia and their cortical output target, the medial prefrontal cortex. The findings are discussed in relation to models of cluttering that point to problems with motor control of speech.

EDUCATIONAL OBJECTIVES

This paper reports findings on the neural correlates seen in adults who clutter, and offers hypotheses as to how these might map onto the behaviours seen amongst those who clutter. Readers will be able to (a) identify the structures that are implicated in the disorder of cluttering, (b) understand arguments relating these structures to the behavioural expression of the disorder, (c) understand some of the complexities in interpreting data pertaining to recovery from cluttering, (d) understand where future efforts in research into the neurological correlates of cluttering should be focussed.

Subcortical functional reorganization due to early blindness

Lack of visual input early in life results in occipital cortical responses to auditory and tactile stimuli. However, it remains unclear whether cross-modal plasticity also occurs in subcortical pathways. With the use of functional magnetic resonance imaging, auditory responses were compared across individuals with congenital anophthalmia (absence of eyes), those with early onset (in the first few years of life) blindness, and normally sighted individuals. We find that the superior colliculus, a "visual" subcortical structure, is recruited by the auditory system in congenital and early onset blindness. Additionally, auditory subcortical responses to monaural stimuli were altered as a result of blindness. Specifically, responses in the auditory thalamus were equally strong to contralateral and ipsilateral stimulation in both groups of blind subjects, whereas sighted controls showed stronger responses to contralateral stimulation. These findings suggest that early blindness results in substantial reorganization of subcortical auditory responses.

Discrimination of speech and non-speech sounds following theta-burst stimulation of the motor cortex

Perceiving speech engages parts of the motor system involved in speech production. The role of the motor cortex in speech perception has been demonstrated using low-frequency repetitive transcranial magnetic stimulation (rTMS) to suppress motor excitability in the lip representation and disrupt discrimination of lip-articulated speech sounds (Möttönen and Watkins, 2009). Another form of rTMS, continuous theta-burst stimulation (cTBS), can produce longer-lasting disruptive effects following a brief train of stimulation. We investigated the effects of cTBS on motor excitability and discrimination of speech and non-speech sounds. cTBS was applied for 40 s over either the hand or the lip representation of motor cortex. Motor-evoked potentials recorded from the lip and hand muscles in response to single pulses of TMS revealed no measurable change in motor excitability due to cTBS. This failure to replicate previous findings may reflect the unreliability of measurements of motor excitability related to inter-individual variability. We also measured the effects of cTBS on a listener's ability to discriminate: (1) lip-articulated speech sounds from sounds not articulated by the lips ("ba" vs. "da"); (2) two speech sounds not articulated by the lips ("ga" vs. "da"); and (3) non-speech sounds produced by the hands ("claps" vs. "clicks"). Discrimination of lip-articulated speech sounds was impaired between 20 and 35 min after cTBS over the lip motor representation. Specifically, discrimination of across-category ba-da sounds presented with an 800-ms inter-stimulus interval was reduced to chance level performance. This effect was absent for speech sounds that do not require the lips for articulation and non-speech sounds. Stimulation over the hand motor representation did not affect discrimination of speech or non-speech sounds. These findings show that stimulation of the lip motor representation disrupts discrimination of speech sounds in an articulatory feature-specific way.

Stimulating the lip motor cortex with transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) has proven to be a useful tool in investigating the role of the articulatory motor cortex in speech perception. Researchers have used single-pulse and repetitive TMS to stimulate the lip representation in the motor cortex. The excitability of the lip motor representation can be investigated by applying single TMS pulses over this cortical area and recording TMS-induced motor evoked potentials (MEPs) via electrodes attached to the lip muscles (electromyography; EMG). Larger MEPs reflect increased cortical excitability. Studies have shown that excitability increases during listening to speech as well as during viewing speech-related movements. TMS can be used also to disrupt the lip motor representation. A 15-min train of low-frequency sub-threshold repetitive stimulation has been shown to suppress motor excitability for a further 15-20 min. This TMS-induced disruption of the motor lip representation impairs subsequent performance in demanding speech perception tasks and modulates auditory-cortex responses to speech sounds. These findings are consistent with the suggestion that the motor cortex contributes to speech perception. This article describes how to localize the lip representation in the motor cortex and how to define the appropriate stimulation intensity for carrying out both single-pulse and repetitive TMS experiments.

Disrupted white matter in language and motor tracts in developmental stuttering

White matter tracts connecting areas involved in speech and motor control were examined using diffusion-tensor imaging in a sample of people who stutter (n=29) who were heterogeneous with respect to age, sex, handedness and stuttering severity. The goals were to replicate previous findings in developmental stuttering and to extend our knowledge by evaluating the relationship between white matter differences in people who stutter and factors such as age, sex, handedness and stuttering severity. We replicated previous findings that showed reduced integrity in white matter underlying ventral premotor cortex, cerebral peduncles and posterior corpus callosum in people who stutter relative to controls. Tractography analysis additionally revealed significantly reduced white matter integrity in the arcuate fasciculus bilaterally and the left corticospinal tract and significantly reduced connectivity within the left corticobulbar tract in people who stutter. Region-of-interest analyses revealed reduced white matter integrity in people who stutter in the three pairs of cerebellar peduncles that carry the afferent and efferent fibers of the cerebellum. Within the group of people who stutter, the higher the stuttering severity index, the lower the white matter integrity in the left angular gyrus, but the greater the white matter connectivity in the left corticobulbar tract. Also, in people who stutter, handedness and age predicted the integrity of the corticospinal tract and peduncles, respectively. Further studies are needed to determine which of these white matter differences relate to the neural basis of stuttering and which reflect experience-dependent plasticity.

Age of language learning shapes brain structure: a cortical thickness study of bilingual and monolingual individuals

We examined the effects of learning a second language (L2) on brain structure. Cortical thickness was measured in the MRI datasets of 22 monolinguals and 66 bilinguals. Some bilingual subjects had learned both languages simultaneously (0-3 years) while some had learned their L2 after achieving proficiency in their first language during either early (4-7 years) or late childhood (8-13 years). Later acquisition of L2 was associated with significantly thicker cortex in the left inferior frontal gyrus (IFG) and thinner cortex in the right IFG. These effects were seen in the group comparisons of monolinguals, simultaneous bilinguals and early and late bilinguals. Within the bilingual group, significant correlations between age of acquisition of L2 and cortical thickness were seen in the same regions: cortical thickness correlated with age of acquisition positively in the left IFG and negatively in the right IFG. Interestingly, the monolinguals and simultaneous bilinguals did not differ in cortical thickness in any region. Our results show that learning a second language after gaining proficiency in the first language modifies brain structure in an age-dependent manner whereas simultaneous acquisition of two languages has no additional effect on brain development.

Motor excitability during visual perception of known and unknown spoken languages

It is possible to comprehend speech and discriminate languages by viewing a speaker's articulatory movements. Transcranial magnetic stimulation studies have shown that viewing speech enhances excitability in the articulatory motor cortex. Here, we investigated the specificity of this enhanced motor excitability in native and non-native speakers of English. Both groups were able to discriminate between speech movements related to a known (i.e., English) and unknown (i.e., Hebrew) language. The motor excitability was higher during observation of a known language than an unknown language or non-speech mouth movements, suggesting that motor resonance is enhanced specifically during observation of mouth movements that convey linguistic information. Surprisingly, however, the excitability was equally high during observation of a static face. Moreover, the motor excitability did not differ between native and non-native speakers. These findings suggest that the articulatory motor cortex processes several kinds of visual cues during speech communication.

Using TMS to study the role of the articulatory motor system in speech perception

Background: The ability to communicate using speech is a remarkable skill, which requires precise coordination of articulatory movements and decoding of complex acoustic signals. According to the traditional view, speech production and perception rely on motor and auditory brain areas, respectively. However, there is growing evidence that auditory-motor circuits support both speech production and perception.Aims: In this article we provide a review of how transcranial magnetic stimulation (TMS) has been used to investigate the excitability of the motor system during listening to speech and the contribution of the motor system to performance in various speech perception tasks. We also discuss how TMS can be used in combination with brain-imaging techniques to study interactions between motor and auditory systems during speech perception.Main contribution: TMS has proven to be a powerful tool to investigate the role of the articulatory motor system in speech perception.Conclusions: TMS studies have provided support for the view that the motor structures that control the movements of the articulators contribute not only to speech production but also to speech perception.

Auditory-motor processing of speech sounds

The motor regions that control movements of the articulators activate during listening to speech and contribute to performance in demanding speech recognition and discrimination tasks. Whether the articulatory motor cortex modulates auditory processing of speech sounds is unknown. Here, we aimed to determine whether the articulatory motor cortex affects the auditory mechanisms underlying discrimination of speech sounds in the absence of demanding speech tasks. Using electroencephalography, we recorded responses to changes in sound sequences, while participants watched a silent video. We also disrupted the lip or the hand representation in left motor cortex using transcranial magnetic stimulation. Disruption of the lip representation suppressed responses to changes in speech sounds, but not piano tones. In contrast, disruption of the hand representation had no effect on responses to changes in speech sounds. These findings show that disruptions within, but not outside, the articulatory motor cortex impair automatic auditory discrimination of speech sounds. The findings provide evidence for the importance of auditory-motor processes in efficient neural analysis of speech sounds.