Left posterior-dorsal area 44 couples with parietal areas to promote speech fluency while right area 44 activity promotes the stopping of motor responses

Area 44 is a cytoarchitectonically distinct portion of Broca's region. Parallel and overlapping large-scale networks couple with this region thereby orchestrating heterogeneous language, cognitive and motor functions. In the context of stuttering, area 44 frequently comes into focus because structural and physiological irregularities affect developmental trajectories, stuttering severity, persistency, and etiology. A remarkable phenomenon accompanying stuttering is the preserved ability to sing. Speaking and singing are connatural behaviours recruiting largely overlapping brain networks including left and right area 44. Analysing which potential subregions of area 44 are malfunctioning in adults who stutter, and what effectively suppresses stuttering during singing, may provide a better understanding of the coordination and reorganization of large-scale brain networks dedicated to speaking and singing in general. We used fMRI to investigate functionally distinct subregions of area 44 during imagery of speaking and imaginary of humming a melody in 15 dextral males who stutter and 17 matched control participants. Our results are fourfold. First, stuttering was specifically linked to a reduced activation of left posterior-dorsal area 44, a subregion that is involved in speech production, including phonological word processing, pitch processing, working memory processes, sequencing, motor planning, pseudoword learning, and action inhibition. Second, left posterior-area-44-to-parietal functional coupling was deficient in stuttering. Third, despite the preserved ability to sing, males who stutter showed bilaterally a reduced activation of area 44 when imagine humming a melody, suggesting that this fluency-enhancing condition seems to bypass posterior-dorsal area 44 to achieve fluency. Fourth, time courses of the posterior subregions in area 44 showed delayed peak activations in the right hemisphere in both groups, possibly signaling the offset response. Because these offset response-related activations in the right hemisphere were comparably large in males who stutter, our data suggest a hyperactive mechanism to stop speech motor responses and thus possibly reflect a pathomechanism, which, until now, has been neglected. Overall, the current results confirmed a recently described co-activation based parcellation supporting the idea of functionally distinct subregions of left area 44

Stuttering in Cinema

The image that the seventh art offers about disability is fundamental in the collective perception of disability. This paper studies, in the form of textual analysis, two films that address the disability of stuttering and whose productions are twenty-two years apart: A Fish Called Wanda (Charles Crichton, 1988) and The King's Speech (Tom Hooper, 2010). Both productions are fundamental to understand how in the last decades the representation of the collective of people who stutter in cinema has changed. We begin with an exposition of the main works on what stuttering is, followed by the impact of the associated stigma that it generates in those affected, continuing with the textual analysis of both pieces to finally discuss and conclude what this change in the collective imaginary means for people who stutter, highlighting the benefits in terms of social inclusion and dignification in society of a realistic and correct representation of the disability of stuttering in film.La imagen que el séptimo arte ofrece sobre la discapacidad es fundamental en la percepción colectiva que se tiene de la misma En este trabajo se estudia, en forma de análisis textual, dos películas que abordan la discapacidad de la tartamudez y a cuyas producciones las separan veintidós años de diferencia: Un pez llamado Wanda (A Fish Called Wanda, Charles Crichton, 1988) y El discurso del rey (The King's Speech, Tom Hooper, 2010). Ambas producciones son fundamentales para entender cómo en las últimas décadas la representación del colectivo de las personas con tartamudez en el cine ha cambiado. Para ello se comenzará con una exposición de los principales trabajos sobre qué es la tartamudez, seguido del impacto del estigma asociada a ella que genera en sus afectados, prosiguiendo con el análisis textual de ambas piezas para finalmente discutir y concluir qué ha supuesto para las personas tartamudas este cambio en el imaginario colectivo, poniendo de manifiesto los beneficios en materia de inclusión social y dignificación en la sociedad de una representación realista y correcta de la discapacidad de la tartamudez en el medio cinematográfico

The frontal aslant tract (FAT) and its role in speech, language and executive function

In this review, we examine the structural connectivity of a recently-identified fiber pathway, the frontal aslant tract (FAT), and explore its function. We first review structural connectivity studies using tract-tracing methods in non-human primates, and diffusion-weighted imaging and electrostimulation in humans. These studies suggest a monosynaptic connection exists between the lateral inferior frontal gyrus and the pre-supplementary and supplementary motor areas of the medial superior frontal gyrus. This connection is termed the FAT. We then review research on the left FAT's putative role in supporting speech and language function, with particular focus on speech initiation, stuttering and verbal fluency. Next, we review research on the right FAT's putative role supporting executive function, namely inhibitory control and conflict monitoring for action. We summarize the extant body of empirical work by suggesting that the FAT plays a domain general role in the planning, timing, and coordination of sequential motor movements through the resolution of competition among potential motor plans. However, we also propose some domain specialization across the hemispheres. On the left hemisphere, the circuit is proposed to be specialized for speech actions. On the right hemisphere, the circuit is proposed to be specialized for general action control of the organism, especially in the visuo-spatial domain. We close the review with a discussion of the clinical significance of the FAT, and suggestions for further research on the pathway

Nachweis von Unterschieden in der neuronalen Aktivierung des Gehirns durch Artikulation unterschiedlicher Silbenfolgen

1.1 Zusammenfassung in deutscher Sprache Hintergrund: Zur Generierung von Sprache sind viele verschiedene Strukturen des menschlichen Körpers, sogenannte Artikulatoren, erforderlich, für deren Einsatz eine neuronale Aktivierung notwendig ist. Diese findet ihren Ursprung als elektrischer Reiz im Gehirn, welcher zu den jeweiligen Artikulatoren weitergeleitet wird, um dort eine Aktion, beispielsweise eine Bewegung, auszulösen. Ziel dieser Arbeit war es, herauszufinden, ob die Aussprache von verschiedenen Silbenfolgen durch die spezielle Bewegung einzelner Artikulatoren unterschiedliche neuronale Aktivität im Gehirn hervorruft und weiter, ob sich diese unterschiedliche Aktivität anhand fMRI-Aufnahmen bildlich und somit nachweisbar darstellen lässt. Methoden: Für die Messungen wurden 43 Probanden weiblichen und männlichen Geschlechts im Alter zwischen 21 und 55 Jahren (mittleres Alter 27,3 Jahre) im MRT untersucht. Es wurden fMRI- Aufnahmen während der lauten Aussprache von drei verschiedenen CV- Silbenfolgen - /'dadada/, /'nanana/ und /'tatata/ - angefertigt, welche in vorgegebener Reihenfolge wiedergegeben wurden. Zusätzlich wurden in gleicher Anzahl Aufnahmen angefertigt, während derer keine Produktion von Sprache gefordert war. Diese Aufnahmen dienten sämtlichen Berechnungen als Baseline. Die erzeugten Aufnahmen wurden mit dem Programm SPM8 statistisch ausgewertet. Die Auswertung erfolgte zunächst einzeln für jede Silbenfolge, um herauszufinden, in welchen Gehirnregionen Aktivitäten zu finden sind. Anschließend wurden die Aktivitäten der einzelnen Silbenfolgen mittels ROIAnalyse miteinander verglichen, um zu überprüfen, ob sich nachweisbare Unterschiede hinsichtlich der neuronalen Aktivierung ergeben. Ergebnisse: Verschiedenheiten in der neuronalen Aktivität für die Artikulation unterschiedlicher Silbenfolgen ließen sich bildlich, sowohl in kortikalen als auch in nichtkortikalen Gehirnregionen, darstellen. Für /'dadada/ ergab sich eine geringere neuronale Aktivität im Vergleich zu den beiden anderen Silbenfolgen, Abstufungen hinsichtlich des Ausmaßes an neuronaler Aktivität zwischen /'nanana/ und /'tatata/ konnte nicht getroffen werden. Unterschiede ließen sich aber auch für diese Kontraste nachweisen. Schlussfolgerung: Es konnte gezeigt werden, dass sich die durch die Aussprache verschiedener Silbenfolgen hervorgerufenen unterschiedlichen neuronalen Aktivitäten im Gehirn bildlich nachweisen lassen. Nun sind weitere Untersuchungen nötig, um den komplexen Verschaltungen im Gehirn, gerade hinsichtlich Produktion von Sprache, weiter auf den Grund gehen zu können. Diese könnten in Verbindung mit dem in dieser Arbeit verwendeten Studiendesign dazu beitragen, ein besseres Verständnis zu komplexen Krankheitsbildern, beispielsweise bei Patienten mit Zustand nach Apoplexien mit Aphasien, neurodegenerativen Erkrankungen oder Stottern zu erlangen.1.2 Abstract Background: The act of generating speech involves many different structures of the human body, i.e. the so-called articulators, which require neuronal activation. This activation originates from within the brain as an electrical stimulus, which is transmitted to particular articulators in order to trigger an action in terms of a physical motion. It was the topic and aim of the present study to examine whether the production of different kinds of sequences of syllables result in different neural activities by using diverse articulators and, furthermore, whether those different activities are visually detectable by using fMRI. Methods: In total, 43 female and male subjects aged between 21 and 55 years old (average age 27,3 years) took part in the study. The fMRI recordings were made while the subjects pronounced three different sequences of syllables in the format of CV - skeleton - /'dadada/, /'nanana/ and /'tatata/ - loudly and in prescribed order. Furthermore, an identical number of recordings was made without loud pronunciation as the subjects were instructed to be silent. These recordings served as baseline for the calculations. The statistical evaluation was made using the program SPM8: firstly, each sequence of syllables was evaluated by itself to detect active brain regions for that specific speech production; secondly, the activities during the production of the sequences of syllables were compared with each other and evaluated via ROI - analysis in order to assess whether there are any verifiable differences between them. Results: Differences in the neural activity while pronouncing the sequences of syllables could be depicted both in cortical and non-cortical brain regions. The neuronal activity while pronouncing /'dadada/ was significantly lower compared to the other sequences of syllables. Whereas there were detectable differences for the two other contrasts as well, it was impossible to establish gradations between /'nanana/ and /'tatata/ regarding neuronal activity. Conclusion: The study has been shown that different neural activity caused by the pronunciation of different sequences of syllables is visually verifiable via fMRI. At this point, further studies are required in order to illuminate these complex brain networks, particularly those involved in speech production. Subsequent findings in conjunction with this study might be able to contribute to a better understanding of complex diseases like conditions after stroke combined with aphasia, neurodegenerative diseases, or stutter

Balbuzie evolutiva: neuromodulazione come trattamento riabilitativo innovativo per comprendere e migliorare le disfluenze

La balbuzie evolutiva è un disturbo del ritmo dell’eloquio caratterizzato da ripetizioni e prolungamenti di sillabe e suoni, oltre che da esitazioni e pause. La balbuzie insorge nella prima infanzia: nella maggior parte dei casi si assiste ad una remissione (che può essere spontanea o facilitata da interventi terapeutici), ma il disturbo può persistere nell’età adulta, rimanendo evidente in circa l’1% della popolazione totale. Sono state proposte numerose teorie circa l’eziologia della balbuzie evolutiva, ma le basi neurobiologiche restano oscure: vista l’interazione di fattori genetici, epigenetici e ambientali durante lo sviluppo delle strutture del sistema nervoso centrale, la balbuzie evolutiva deve essere considerata un disturbo multifattoriale del neurosviluppo. Sebbene esistano vari approcci terapeutici volti al trattamento di tale condizione, non è ancora disponibile una terapia risolutiva. Grazie a studi di neuroimaging e tecniche di stimolazione cerebrale non invasiva, la balbuzie evolutiva viene ad oggi considerata un disturbo funzionale della corretta programmazione di sequenze motorie complesse, come per esempio quelle relative al linguaggio. L’identificazione di marker neurali del disturbo suggerisce la presenza di interazioni anomale tra network cerebrali coinvolti in compiti motori, elaborazione del linguaggio e processi cognitivi: queste evidenze, anche in assenza di compiti linguistici, hanno permesso di ipotizzare che le disfluenze possano essere il sintomo manifesto di un disturbo motorio più generale. Agendo in modo diretto sulla funzionalità di questi circuiti (tramite l’utilizzo di tecniche di neuromodulazione cerebrale non invasiva), potrebbe essere possibile migliorare la fluenza dell’eloquio (interagendo direttamente con il funzionamento del tessuto cerebrale coinvolto), potenziando così l’effetto degli interventi comportamentali ad oggi già disponibili. In più, tale trattamento non convenzionale sarebbe utile non solo per migliorare la fluenza linguistica in ottica riabilitativa, ma potrebbe fornire nuove evidenze sui meccanismi patofisiologici della balbuzie.Developmental stuttering is a disturbance of the speech rhythm, characterized by repetitions and prolongations of syllables and sounds, as well as by blocks and hesitations. Stuttering occurs in early childhood: in most cases there is a remission (either spontaneously or facilitated by therapeutic interventions), but the disorder may persist in adulthood (about 1% of the total population). Many theories about the etiology of developmental stuttering have been proposed, but its neurobiological causes are still unknown: interaction of genetic, epigenetic and environmental factors is evident, acting on the development of central nervous system structures. As a consequence, developmental stuttering has to be considered a multifactorial and neurodevelopmental disorder. Although there are various therapeutic and behavioral approaches aimed at treating this disorder, no decisive rehabilitation solution is still available. Thanks to neuroimaging and non-invasive brain stimulation, developmental stuttering is now considered as a functional disorder of the correct planning of complex motor sequences (e.g. speech). The identification of neural markers of the disturbance suggests the presence of anomalous interactions between brain networks involved in motor planning, speech processing and cognition: their impaired activity, also in absence of speech tasks, allows us to hypothesize that dysfluencies may be the overt symptom of a more general motor disorder. Acting on the functionality of these defective brain circuits (by using non-invasive brain stimulation techniques), it would be possible to improve speech fluency (directly interacting with the functioning of the impaired brain tissue), thus enhancing effects of the already available behavioral interventions. This unconventional treatment will be useful not only to ameliorate speech fluency in rehabilitative interventions but will also provide further suggestions about the pathophysiology of developmental stuttering

Aproximación traslacional a la neurofisiología y conducta de la tartamudez

La tartamudez o disfemia es un trastorno del habla generalmente en la parte espontánea y proposicional, es decir cuando se utiliza para comunicarse en situaciones cotidianas. Se caracteriza por repeticiones, prolongaciones o interrupciones durante el discurso normal fluido y conlleva un gran sufrimiento personal provocando un deterioro notable en la calidad de vida. El objetivo de esta tesis es aportar una aproximación traslacional de la patología. Primero se han estudiado los mecanismos de la principal técnica neuromodulatoria (estimulación magnética transcraneal) que se propone como mayor potencial terapéutico de la tartamudez (estudio I). Segundo, se han estudiado los mecanismos de la tartamudez y los cambios en la excitabilidad cortical respecto a controles, así como la predisposición a sufrir trastornos de deglución (estudio II). Tercero, se han abordado los mecanismos cognitivos que refuerzan los comportamientos secundarios negativos de la patología (estudio III) y cuarto se han puesto en práctica en forma de dos casos clínicos los abordajes terapéuticos planteados (estudio IV)..

Investigating the contribution of the right hemisphere to language processing in the damaged and healthy brain

Acquired language disorders after stroke are strongly associated with left hemisphere damage. When language difficulties are observed after right hemisphere damage, patients are commonly considered to have atypical functional anatomy (i.e. crossed aphasia). On the other hand, fMRI studies have reported right hemisphere activation when neurologically-normal participants perform language tasks, and have shown that the right hemisphere contributes to recovery of language function after left hemisphere damage. In this thesis I investigated (i) the degree to which language difficulties after right hemisphere stroke can reflect disruption to typical functional anatomy and (ii) how the damaged areas contribute to normal language processing. In Study 1 (Chapter 3), I investigated a group of patients with unilateral strokes that damaged either the right or the left hemisphere. The most frequently impaired language task was auditory sentence-to-picture matching after right hemisphere strokes, and spoken picture description after left hemisphere strokes. In 9 right hemisphere stroke patients, performance on the auditory sentence-to-picture matching task was selectively impaired and could not be explained by poor perceptual (visual or auditory) or linguistic processing (semantic, phonological or syntactic). I therefore hypothesised that the behavioural difficulties experienced by those patients arose as a consequence of impaired non-linguistic executive functions that are needed to support language processes. In Study 2 (Chapter 4), I investigated the lesions of the 9 patients with selective deficits in the auditory sentence-to-picture matching task, and found that they had significantly more damage to subcortical regions and parts of the superior longitudinal fasciculus impinging on the right inferior frontal sulcus compared to other right hemisphere stroke patients who were not impaired on the sentence comprehension task. Having identified these regions, their function (e.g. linguistic or executive) can be investigated using functional neuroimaging in neurologically-normal participants. In Study 3 (Chapter 5), I used fMRI to investigate whether any parts of the right hemisphere regions associated with impaired sentence comprehension, in Study 2, were activated when neurologically-normal participants performed similar language tasks to those administered to right hemisphere stroke patients in Study 1 (including the auditory sentence-to-picture matching task). I found that, within the brain areas derived from Study 2, the right inferior frontal sulcus and right mediodorsal thalamus were normally activated by auditory sentence-to-picture matching but there was no evidence that these regions were exclusively performing linguistic functions. In Study 4 (Chapter 6), I investigated the contribution of the identified regions further by using a new fMRI study of one-back matching tasks that varied demands on semantic and non-semantic working memory. By systematically integrating neuropsychological, lesion and fMRI data, I conclude that the right inferior frontal cortex and right mediodorsal thalamus contribute to non-semantic working memory capacity that is needed to accurately perform a range of language functions. This account helps to explain why auditory sentence-to-picture matching impairments occur after right hemisphere damage

Longitudinal structural and functional brain changes associated with stuttering improvement by therapy or brain lesion

Stuttering is a speech fluency disorder which is affecting motor speech production and communication in the daily life of persons who stutter (PWS). The involuntarily occurring core symptoms of stuttering are sound and syllable repetitions, sound prolongations and speech blocks (Bloodstein & Ratner, 2008). In addition to these core symptoms, secondary accompanying symptoms like movements of limbs, neck and head as well as facial grimaces can appear (Guitar & McCauley, 2010). PWS repeatedly experience high communicative pressure and psychological strain. Subsequently, they often develop social withdrawal to hide their stuttering. Therefore, a reduced quality of life is measured in some PWS (Carter, Breen, Yaruss, & Beilby, 2017; Kohmäscher, 2017; Natke & Alpermann, 2010). Stuttering therapies enhance speech fluency and support patients in their handling with adverse emotions and attitudes towards their stuttering (Neumann et al., 2016). Even though the effectiveness of different types of intense stuttering therapies has been evaluated on a behavioural level, there is a research gap regarding the long-term effects of intense stuttering therapies on brain structure and function. In fact, to our knowledge, changes of white matter integrity following the participation in an intense stuttering therapy have not been investigated yet. Therefore, the present thesis investigates the effects of therapy-induced long-term white matter plasticity changes as well as brain activation changes in adolescent and adult PWS. For this purpose, we recruited stuttering patients taking part in the “Kasseler Stottertherapie” (Euler, Gudenberg, A. W. v., Jung, & Neumann, 2009). This is an evidence-based fluency shaping therapy approach accomplished in a group setting and with a high intensity (Euler et al., 2009; Euler, Anders, Merkel, & von Gudenberg, A Wolff, 2016; Euler & Wolff v. Gudenberg, 2000; Neumann et al., 2016). In addition, a case report of a cessation of stuttering after a left cerebellar haemorrhage is presented in this thesis. In the first study of this dissertation, we used diffusion tensor imaging (DTI) to evaluate long-term therapy-induced changes of white matter integrity in stuttering patients. For this purpose, we added two control groups and compared the longitudinal structural changes of the intervention group with the structural changes of stuttering control participants not taking part in any therapy and healthy control participants. By using Tract-Based Spatial Statistics (Smith et al., 2006; Smith et al., 2007), we investigated changes in fibre integrity within whole-brain and region of interest analyses. Our results show that the effects of therapy in the intervention group were versatile: Referring to the behavioural level, a significant decline of stuttering severity as well as of the impact of stuttering on the quality of life were detected and attributed to the stuttering treatment. Regarding white matter integrity changes, we observed a significant increase of fractional anisotropy (FA) in the left superior longitudinal fasciculus. In contrast to the intervention group, a significant decrease of white matter integrity was found in stuttering and healthy control participants. This white matter decline could have been triggered through the process of ageing. The second purpose of this study was to replicate previous findings of a reduction of white matter integrity in PWS compared to healthy controls (Cykowski, Fox, Ingham, Ingham, & Robin, 2010; Neef, Anwander, & Friederici, 2015; Sommer, Koch, Paulus, Weiller, & Buchel, 2002). We were able to confirm this reduced white matter integrity in right hemispheric brain regions including parts of the inferior longitudinal fasciculus close to the callosal body, cingulum, inferior-fronto-occipital fasciculus and the corticospinal tract. With our study, we provided first evidence that an intense stuttering therapy has the potential to change white matter plasticity in stuttering patients. Future studies are necessary to replicate this result and to relate this outcome to the aetiology of stuttering. In the second study of this thesis, we evaluated long-term changes in brain activation induced by an intense stuttering therapy and its maintenance phase. We compared brain activation changes in the treatment group with the changes measured in both control groups (healthy participants and stuttering participants not currently taking part in any stuttering therapy). The research aim was to investigate therapy-induced activation changes and to discuss them with regard to the therapeutic principles of action. The following results were obtained: In comparison to healthy and stuttering control participants, stuttering patients showed an increase of activity in motor (e.g. left and right rolandic operculum) and in cognition and emotion processing areas (e.g. left amygdala, right supramarginal gyrus). The effect of therapy was also traceable on the behavioural level. Only stuttering patients of the intervention group showed a significant decline of stuttering severity and a significantly decreased impact of stuttering on the quality of life. Our results underline the importance of also considering non-motor brain regions meaningful for therapeutic achievements as well as for the aetiology of stuttering. The third part of this thesis consists of a case report about the cessaction of stuttering after a cerebellar haemorrhage. The 52 years old female patient stuttered since childhood and had taken part in a stuttering-related magnet resonance imaging (MRI) research study at the University Medical Center Göttingen. After taking part in the study, she developed a left acoustic neuroma which was subsequently surgically removed. Postoperatively, the patient presented with a cerebellar haemorrhage and, as a consequence, various neurological symptoms and impairments. After the rehabilitation period, the patient reported a cessation of her stuttering as a consequence of the cerebellar haemorrhage. We became conscious of her clinical course and invited her to a revisited measurement. The aim of this second measurement and the case report were to elucidate neurophysiological processes which are responsible for the cessation of stuttering. For the revisited measurement, we used the same behavioural measurements and functional and diffusion MRI protocols as in the previous study measurement. To compare the (functional) MRI data of the single patient with a reference group, we added a control group with healthy participants and another control group with stuttering participants to our analyses. The study outcomes were manifold: The conducted lesion analysis indicated a large cerebellar lesion, including approximately 1/5 of the left cerebellum. The tract-based spatial statistics analysis showed a primary white matter decrease caused by the haemorrhage in the lesioned parts of the cerebellum. A secondary white matter impairment was detected in the corpus callosum, right inferior fronto-occipital fasciculus, left anterior thalamic radiation, left cingulum and right posterior corona radiata. The whole-brain functional MRI (fMRI) analysis revealed a modality-related difference in brain activity from the first to the second measurement. During covert speaking, parietal and temporal areas showed an increase of activation. This increase of activation was not traceable during covert humming. Also in our region of interest (ROI) analysis in the left and right BA 44, the single case patient showed a hyperactivation during covert speaking at the second measurement when comparing her with the control groups. This hyperactivation was again modality-related and therefore only measureable during covert speaking. Our results were discussed in respect to the cerebello-thalamo-cortical pathway. The cerebellar disinhibition caused by the lesion might have led to an overactivity in thalamus and motor cortex, represented by the hyperactivation during covert speaking. Therefore, the cerebellar disinhibition and its triggered overactivation along the cerebello-thalamo-cortical pathway might have facilitated the cessation of stuttering. Taken together, the object of the current thesis was to evaluate the long-term longitudinal effects of an intense stuttering therapy or a brain lesion on brain structure and function in PWS. The results provide first evidence that the reduction of white matter integrity (often seen as the deficit in neural processing in PWS; see Packman, 2012) can be altered through an intense stuttering therapy. Furthermore, our research demonstrates that an intense stuttering therapy has the potential to enhance an increase of brain activation in areas that were hypoactivated before therapy. This enhancement even takes place in non-motor regions. And last, this thesis underlines the importance of the cerebello-thalamo-cortical pathway for the aetiology of stuttering.

Using tDCS to improve speech processes in typical speakers and people who stutter

Stuttering is a speech disorder for which treatment options are limited. Brain stimulation methods such as tDCS used as an adjunct to treatments enhance positive effects of intervention. This thesis addressed whether tDCS applied to the left inferior frontal gyrus would improve speech processes in typical speakers (TS) and people who stutter (PWS). Study 1.1 with TS showed that tDCS resulted in enhanced performance (reduction in speech reaction times) for three, but not one, syllable words in a picture naming task. Such interaction between stimuli complexity and tDCS was explored in Study 1.2. A picturenaming task was used with three syllable stimuli. Primes either facilitated the speech plan (low complexity) or required speech-plan reformulation. When anodal tDCS was applied, incongruent trials alone were significantly quicker than sham trials, replicating the effect of difficulty. Study 3 with TS applied tDCS whilst participants repeated tongue twisters. Anodal tDCS resulted in significantly faster tongue twister completion times compared to sham or cathodal stimulation. The studies with TS indicated that tDCS improves speech processes, particularly when task complexity is high. Study 4, applied tDCS to PWS alongside a challenging intervention known to reduce stuttering. There were reductions in stuttering during conversation that trended towards significance (sample size was small). Finally, we examined inferior frontal gyrus neural activity in PWS and TS whilst conversing socially or to a recording. The left inferior frontal gyrus showed significant and unique responses during face-to-face conversation compared to audio conversation. Findings indicated that the left inferior frontal gyrus is differentially involved when PWS communicate in different styles. This thesis demonstrated that tDCS is a promising adjunct for improving speech production processes in TS and PWS to use with challenging tasks and interventions. Further research is required to understand mechanisms of effect and to further refine effects for this promising approach