Investigating the neural mechanisms of transcranial direct current stimulation effects on human cognition: current issues and potential solutions

Transcranial direct current stimulation (tDCS) has been studied extensively for its potential to enhance human cognitive functions in healthy individuals and to treat cognitive impairment in various clinical populations. However, little is known about how tDCS modulates the neural networks supporting cognition and the complex interplay with mediating factors that may explain the frequently observed variability of stimulation effects within and between studies. Moreover, research in this field has been characterized by substantial methodological variability, frequent lack of rigorous experimental control and small sample sizes, thereby limiting the generalizability of findings and translational potential of tDCS. The present manuscript aims to delineate how these important issues can be addressed within a neuroimaging context, to reveal the neural underpinnings, predictors and mediators of tDCS-induced behavioral modulation. We will focus on functional magnetic resonance imaging (fMRI), because it allows the investigation of tDCS effects with excellent spatial precision and sufficient temporal resolution across the entire brain. Moreover, high resolution structural imaging data can be acquired for precise localization of stimulation effects, verification of electrode positions on the scalp and realistic current modeling based on individual head and brain anatomy. However, the general principles outlined in this review will also be applicable to other imaging modalities. Following an introduction to the overall state-of-the-art in this field, we will discuss in more detail the underlying causes of variability in previous tDCS studies. Moreover, we will elaborate on design considerations for tDCS-fMRI studies, optimization of tDCS and imaging protocols and how to assure high-level experimental control. Two additional sections address the pressing need for more systematic investigation of tDCS effects across the healthy human lifespan and implications for tDCS studies in age-associated disease, and potential benefits of establishing large-scale, multidisciplinary consortia for more coordinated tDCS research in the future. We hope that this review will contribute to more coordinated, methodologically sound, transparent and reproducible research in this field. Ultimately, our aim is to facilitate a better understanding of the underlying mechanisms by which tDCS modulates human cognitive functions and more effective and individually tailored translational and clinical applications of this technique in the future

Neurostimulation improves reading and alters communication within reading networks in dyslexia

In the present study, we combined facilitatory transcranial magnetic stimulation (TMS) to the left TPC in adults with dyslexia with an overt word and pseudoword reading task during fMRI. Our aim was to improve phonological decoding in dyslexia and investigate TMS-induced changes in functional activation and effective connectivity within reading networks. We found TMS-induced improvements in pseudoword reading, reduced contributions of right-hemispheric regions during reading and substantial changes between the core reading nodes and an extended network involving the right cerebellum. Stronger coupling between temporo-occipital and frontal cortices was further directly linked to improvements in pseudoword reading. Collectively, we provide evidence for the crucial role of the left TPC for phonological decoding and show that TMS can successfully modulate reading networks to improve reading in dyslexia

Exploring the neuroanatomical and behavioural correlates of foreign language aptitude

Diese Dissertation befasst sich mit dem Konzept der Sprachbegabung, das ein angeborenes Potenzial, Fremdsprachen zu erlernen, designiert. Das Hauptziel lag darin, die neuroanatomischen Grundlagen von Sprachbegabung zu erforschen, und herauszufinden, ob strukturelle Variation im Hörkortex mit Sprachbegabung korreliert. Ein weiteres Ziel lag in der Erforschung der Korrelate von Sprachbegabung auf der Verhaltensebene, wobei Musikalität und Arbeitsgedächtniskapazität im Fokus standen. Die Ergebnisse der zwei durchgeführten Studien mit Kindern und Erwachsenen zeigten, dass mehrere Gyri im Hörkortex der rechten Hemisphäre mit höherer Musikalität (bei Erwachsenen) sowie höherer Sprachbegabung (in beiden Gruppen) einhergingen. Das Auftreten einzelner Gyri im rechten Hörkortex hingegen korrelierte mit niedriger Sprachbegabung. Außerdem waren nicht nur Sprachbegabung und Arbeitsgedächtniskapazität sehr eng verknüpft, sondern die Ergebnisse der Erwachsenen zeigten auch einen schwachen Zusammenhang mit muskalischer Erfahrung. In der Studie mit Kindern und Jugendlichen wurde des Weiteren entdeckt, dass arithmetische Kompetenz sehr stark mit Sprachbegabung und Arbeitsgedächtnis assoziiert war. Zudem spiegelten sich die Schulnoten der Kinder sehr gut in ihrer Leistung bei den Testungen wieder und korrelierten positiv mit der Selbsteinschätzung ihres Fremdsprachlernpotenzials, obwohl Sprachbegabung nicht mit Intelligenz und sozio-ökonomischem Status einherging. Zusammenfassend scheint der rechte Hörkortex ein wichtiges Areal für Sprachbegabung zu sein, da strukturelle Variation zu besserer funktioneller Konnektivität wie auch zu besseren strukturellen Vernetzungen zu anderen Regionen führen könnte. Außerdem hängt Sprachbegabung nicht nur eng mit Arbeitsgedächtniskapazität zusammen, sondern korreliert auch mit musikalischen Fertigkeiten bei Erwachsenen sowie schulischen und arithmetischen Leistungen bei Jugendlichen.The aim of the present thesis was twofold. The first aim was to find out whether auditory cortex morphology is a potential neuroanatomical marker of foreign language aptitude. The second aim was to investigate the relationships between language aptitude, musicality and working memory in both adults and children. In the younger population, the relationships between intelligence, socio-economic status, arithmetic skills and language aptitude were further investigated. Behavioural and neuroanatomical analyses were performed in two studies with different age groups, namely children and teenagers on the one hand, and adults on the other hand. On the neural level, both studies revealed a strong relationship between language aptitude and right-hemispheric auditory cortex morphology. Adults and children with high language aptitude scores possessed more complete posterior duplications in their right auditory cortex. Conversely, subjects with low aptitude more frequently possessed single gyri in their right auditory cortex. On the behavioural level, significant correlations between working memory and language aptitude were found, while a weak link between musical experience and language aptitude could only be found in the adults. In the child population, scores on the arithmetic competence test and school performance correlated with language aptitude, which was further found to be independent of general intelligence and socio-economic status.To conclude, the findings suggest an intricate link between right auditory cortex morphology and language aptitude, so that a higher number of gyri and higher grey matter volumes in right auditory cortex seem to present neuroanatomical markers of high language aptitude. Additionally, language aptitude was not only significantly associated with working memory capacity, but it was linked to musicality (in adults) and arithmetic skills, as well as to school achievement, in the younger population.eingereicht von Mag. phil. Sabrina Turker BAZusammenfassungen in Deutsch und EnglischKarl-Franzens-Universität Graz, Dissertation, 2019OeBB(VLID)437540

Cortical, subcortical and cerebellar contributions to language processing: a meta-analytic review of 403 neuroimaging experiments

We here provide the full dataset for our most recent meta-analysis on language processing, exploring subdomain-specific processes for semantics, syntax, phonology and prosody in 403 experiments. Data were analyzed with GingerALE (version 3.0.2; https://brainmap.org/ale/) and the data analysis sheet, as well as a detailed list of all studies is provided

Adaptive short-term plasticity in the typical reading network

Data and code of TMS-fMRI study published as Turker, Kuhnke et al. 2023 NeuroImag

Disrupted network interactions serve as a neural marker of dyslexia

Abstract Dyslexia, a frequent learning disorder, is characterized by severe impairments in reading and writing and hypoactivation in reading regions in the left hemisphere. Despite decades of research, it remains unclear to date if observed behavioural deficits are caused by aberrant network interactions during reading and whether differences in functional activation and connectivity are directly related to reading performance. Here we provide a comprehensive characterization of reading-related brain connectivity in adults with and without dyslexia. We find disrupted functional coupling between hypoactive reading regions, especially between the left temporo-parietal and occipito-temporal cortices, and an extensive functional disruption of the right cerebellum in adults with dyslexia. Network analyses suggest that individuals with dyslexia process written stimuli via a dorsal decoding route and show stronger reading-related interaction with the right cerebellum. Moreover, increased connectivity within networks is linked to worse reading performance in dyslexia. Collectively, our results provide strong evidence for aberrant task-related connectivity as a neural marker for dyslexia that directly impacts behavioural performance. The observed differences in activation and connectivity suggest that one effective way to alleviate reading problems in dyslexia is through modulating interactions within the reading network with neurostimulation methods

Cortical, subcortical, and cerebellar contributions to language processing: A meta-analytic review of 403 neuroimaging experiments.

Language is a key human faculty for communication and interaction that provides invaluable insight into the human mind. Previous work has dissected different linguistic operations, but the large-scale brain networks involved in language processing are still not fully uncovered. Particularly, little is known about the subdomain-specific engagement of brain areas during semantic, syntactic, phonological, and prosodic processing and the role of subcortical and cerebellar areas. Here, we present the largest coordinate-based meta-analysis of language processing including 403 experiments. Overall, language processing primarily engaged bilateral fronto-temporal cortices, with the highest activation likelihood in the left posterior inferior frontal gyrus (IFG). Whereas we could not detect any syntax-specific regions, semantics specifically engaged left posterior temporal areas (left fusiform and occipitotemporal cortex) and the left frontal pole. Phonology showed highest subdomain-specificity in bilateral auditory and left postcentral regions, whereas prosody engaged specifically the right amygdala and the right IFG. Across all subdomains and modalities, we found strong bilateral subcortical and cerebellar contributions. Especially the right cerebellum was engaged during various processes, including speech production, visual, and phonological tasks. Collectively, our results emphasize consistent recruitment and high functional modularity for general language processing in bilateral domain-specific (temporo-frontal) and domain-general (medial frontal/anterior cingulate cortex) regions but also a high specialization of different subareas for different linguistic subdomains. Our findings refine current neurobiological models of language by adding novel insight into the general sensitivity of the language network and subdomain-specific functions of different brain areas and highlighting the role of subcortical and cerebellar regions for different language operations

Neurostimulation improves reading and alters communication within reading networks in dyslexia

The left temporo-parietal cortex (TPC) is critical for phonological decoding during reading and appears hypoactive in dyslexia. Therefore, a promising approach to alleviate phonological deficits in dyslexia is to modulate left TPC functioning. However, it is unclear how neurostimulation alters activity and network interactions in dyslexia. To address this gap, we combined facilitatory transcranial magnetic stimulation (TMS) to the left TPC in adults with dyslexia with an overt word and pseudoword reading task during fMRI. We found TMS-induced improvements in pseudoword reading, reduced contributions of right-hemispheric regions during reading and substantial changes between the core reading nodes and an extended network involving the right cerebellum. Stronger coupling between temporo-occipital and frontal cortices was further directly linked to improvements in pseudoword reading. Collectively, we provide evidence for the crucial role of the left TPC for phonological decoding and show that TMS can successfully modulate reading networks to improve reading in dyslexia

“When Music Speaks”: Auditory Cortex Morphology as a Neuroanatomical Marker of Language Aptitude and Musicality

Recent research has shown that the morphology of certain brain regions may indeed correlate with a number of cognitive skills such as musicality or language ability. The main aim of the present study was to explore the extent to which foreign language aptitude, in particular phonetic coding ability, is influenced by the morphology of Heschl’s gyrus (HG; auditory cortex), working memory capacity, and musical ability. In this study, the auditory cortices of German-speaking individuals (N = 30; 13 males/17 females; aged 20–40 years) with high and low scores in a number of language aptitude tests were compared. The subjects’ language aptitude was measured by three different tests, namely a Hindi speech imitation task (phonetic coding ability), an English pronunciation assessment, and the Modern Language Aptitude Test (MLAT). Furthermore, working memory capacity and musical ability were assessed to reveal their relationship with foreign language aptitude. On the behavioral level, significant correlations were found between phonetic coding ability, English pronunciation skills, musical experience, and language aptitude as measured by the MLAT. Parts of all three tests measuring language aptitude correlated positively and significantly with each other, supporting their validity for measuring components of language aptitude. Remarkably, the number of instruments played by subjects showed significant correlations with all language aptitude measures and musicality, whereas, the number of foreign languages did not show any correlations. With regard to the neuroanatomy of auditory cortex, adults with very high scores in the Hindi testing and the musicality test (AMMA) demonstrated a clear predominance of complete posterior HG duplications in the right hemisphere. This may reignite the discussion of the importance of the right hemisphere for language processing, especially when linked or common resources are involved, such as the inter-dependency between phonetic and musical aptitude