High-definition tDCS of the temporo-parietal cortex enhances access to newly learned words

Learning associations between words and their referents is crucial for language learning in the developing and adult brain and for language re-learning after neurological injury. Non-invasive transcranial direct current stimulation (tDCS) to the posterior temporo-parietal cortex has been suggested to enhance this process. However, previous studies employed standard tDCS set-ups that induce diffuse current flow in the brain, preventing the attribution of stimulation effects to the target region. This study employed high-definition tDCS (HD-tDCS) that allowed the current flow to be constrained to the temporo-parietal cortex, to clarify its role in novel word learning. In a sham-controlled, double-blind, between-subjects design, 50 healthy adults learned associations between legal non-words and unfamiliar object pictures. Participants were stratified by baseline learning ability on a short version of the learning paradigm and pairwise randomized to active (20 mins; N = 25) or sham (40 seconds; N = 25) HD-tDCS. Accuracy was comparable during the baseline and experimental phases in both HD-tDCS conditions. However, active HD-tDCS resulted in faster retrieval of correct word-picture pairs. Our findings corroborate the critical role of the temporo-parietal cortex in novel word learning, which has implications for current theories of language acquisition

Task load modulates tDCS effects on brain network for phonological processing

Motor participation in phonological processing can be modulated by task nature across the speech perception to speech production range. The pars opercularis of the left inferior frontal gyrus (LIFG) would be increasingly active across this range, because of changing motor demands. Here, we investigated with simultaneous tDCS and fMRI whether the task load modulation of tDCS effects translates into predictable patterns of functional connectivity. Findings were analysed under the "multi-node framework", according to which task load and the network structure underlying cognitive functions are modulators of tDCS effects. In a within-subject study, participants (N = 20) performed categorical perception, lexical decision and word naming tasks [which differentially recruit the target of stimulation (LIFG)], which were repeatedly administered in three tDCS sessions (anodal, cathodal and sham). The LIFG, left superior temporal gyrus and their right homologues formed the target network subserving phonological processing. C-tDCS inhibition and A-tDCS excitation should increase with task load. Correspondingly, the larger the task load, the larger the relevance of the target for the task and smaller the room for compensation of C-tDCS inhibition by less relevant nodes. Functional connectivity analyses were performed with partial correlations, and network compensation globally inferred by comparing the relative number of significant connections each condition induced relative to sham. Overall, simultaneous tDCS and fMRI was adequate to show that motor participation in phonological processing is modulated by task nature. Network responses induced by C-tDCS across phonological processing tasks matched predictions. A-tDCS effects were attributed to optimisation of network efficiency.</p

Repetition-imitation and aphasia: Neural Basis and treatment

Study 2: a single-case study was developed. Repetition performance was widely assessed in a patient with crossed conduction aphasia and striatal/capsular vascular lesions encompassing the right arcuate fasciculus (AF) and inferior frontal-occipital fasciculus (IFOF), the temporal stem and the white matter underneath the supramarginal gyrus. He showed lexicality effects repeating better words than non-words, but manipulation of other lexical-semantic variables exerted less influence on repetition performance. Imageability and frequency effects, production of meaning-based paraphrases during sentence repetition, or better performance on repeating novel sentences than overlearned clichés were hardly ever observed in this patient. Diffusion tensor imaging disclosed damage to the right long direct segment of the AF and IFOF with relative sparing of the anterior indirect and posterior segments of the AF, together with fully developed left perisylvian white matter pathways. These findings suggest that striatal/capsular lesions extending into the right AF and IFOF in some individuals with right hemisphere language dominance are associated with atypical repetition patterns which might reflect reduced interactions between phonological and lexical-semantic processes. Study 3: A within-patient design, with multiple assessments, was developed with the same patient as in Study 2A. The initial dose of Donepezil (5 mg/day) was titulated up to 10 mg/day and administered alone (without aphasia therapy) during 3 months (Endpoint 1). Then, the drug was combined with a verbal repetition-imitation therapy (Look-Listen and Repeat - LLR) (1 hour/day) during 2 months (Endpoint 2). Language evaluations, diffusion tensor imaging (DTI) and voxel-based morphometry (VBM) were performed at baseline and at both endpoints. Donepezil alone and combined with LLR induced marked improvement in measures of speech production (Correct Information Units/minute during picture description, and repetition of word lists, idiomatic-novel phrases and sentences). Greater benefits were observed after combined therapy and the obtained gains in speech production remained well-above baseline scores even four months after combined therapy interruption. Longitudinal DTI showed structural plasticity in the right frontal aslant tract (FAT) with both interventions and VBM additionally revealed increased grey matter density in cortical areas connected through the FAT. Donepezil alone and combined with LLR improved speech production deficits by inducing structural plastic changes in white matter tracts and grey matter areas spared by the lesion in the injured hemisphere. Conclusions: Cholinergic modulation and intensive verbal repetition-imitation therapy improve speech production deficits in crossed conduction aphasia by inducing right hemisphere structural plasticity. Conclusions: MSRT become a simultaneous training of attention, auditory and visual input processing with the activation of different semantic fields, STM, executive processing, and language production. Treatment with donepezil has a positive effect in communication skills of CA patient. We observe a summative effect between donepezil and MSRT. The improvement associates neuroplasticity changes in neuroimaging.In the past two decades, single-case studies evaluated the effect of massed repetition training to improve speech production and short-term memory deficits in conduction aphasia (CA). Improvements were reported in treated language and memory domains with modest generalisation of gains to spontaneous speech or auditory comprehension. Although these results are encouraging, sentence repetition training has not been compared with distributed speech-language therapy, and no studies have examined the role of pharmacological interventions to enhance gains promoted by these behavioural interventions in CA. To explore repetition and imitation in the context of neural basis and treatment of aphasia, we developed three studies (STUDY 1, 2 & 3). Study 1: The effects of massed sentence repetition therapy (MSRT) were compared to those of distributed speech-language therapy (DSLT) in measures of verbal output, short-term memory and repetition in patients with chronic post-stroke CA receiving treatment with the cholinesterase inhibitor donepezil (DP). Both interventions improved performance in speech production tasks, but better improvements were found with DP-MSRT than with DP-DSLT. Larger treatment effects were found for DP-MSRT in comparison with baselines and DP-DSLT in repetition of word pairs and triplets, and novel and experimental sentences with generalisation of gains to aphasia severity, connected speech and non-treated control sentences

Retrieval from memory: Vulnerable or inviolable?

We show that retrieval from semantic memory is vulnerable even to the mere presence of speech. Irrelevant speech impairs semantic fluency—namely, lexical retrieval cued by a semantic category name—but only if it is meaningful (forward speech compared to reversed speech or words compared to nonwords). Moreover, speech related semantically to the retrieval category is more disruptive than unrelated speech. That phonemic fluency—in which participants are cued with the first letter of words they are to report—was not disrupted by the mere presence of meaningful speech, only by speech in a related phonemic category, suggests that distraction is not mediated by executive processing load. The pattern of sensitivity to different properties of sound as a function of the type of retrieval cue is in line with an interference-by-process approach to auditory distraction

The cerebellar role in Executive Functions:new insights from behavioral and structural neuroimaging data

“Executive functions” (EFs) are a set of cognitive processes that allow to select and monitor behaviours to achieve specific goals. Although it has been proposed that the cerebellum is involved in EFs by means of specific anatomical connections with the lateral prefrontal cortex, its specific role in these processes needs to be clarified. Aim of the present study was to investigate the EFs in subject with cerebellar pathology to characterize their profile of executive impairment. Twenty-three patients with cerebellar atrophy (CA), 18 patients with focal cerebellar damage (FCD), and 43 matched healthy controls (CT) were enrolled in the study and underwent an extensive evaluation of the EFs. A one-way Anova and Tukey’s post hoc test were performed. Moreover a principal components analysis with 3 factors (Planning, Set shifting and Cognitive Inhibition) was executed to identify possible shared process among impaired EFs tasks. Finally, in order to investigate the link between executive impairment and the pattern of cerebellar structural alterations, T1 weighted scans were also collected for voxel-based morphometry analysis and cerebellar lesion characterization. The neuropsychological assessment evidenced that CA was significantly impaired in planning tasks while FCD was significantly impaired in set shifting tasks. By using the neuroimaging analysis, the damaged cerebellar regions have been identified in CA and FCD. The structural alteration patterns have been related to the executive impairment patterns. The hypothesis that, in presence of a cerebellar pathology, different profiles of EFs alteration depend on cerebellar damage localization will be discussed

Treating lexical retrieval using letter fluency and tDCS in primary progressive aphasia:a single-case study

Background: In early stages, individuals with primary progressive aphasia (PPA) report language symptoms while scoring within norm in formal language tests. Early intervention is important due to the progressive nature of the disease. Method: We report a single-case study of an individual with logopenic variant PPA (lvPPA). We tested whether letter fluency, used as a therapy task, can improve lexical retrieval when combined with tDCS to either the left inferior-frontal gyrus (IFG) or the left inferior parietal lobe (IPL), administered in two separate therapy phases separated by a wash-out period of 3 months. Outcomes and results: We observed increases in number of words retrieved during a letter fluency task in trained and untrained letters, when letter fluency therapy (LeFT) was administered with anodal tDCS. When LeFT was combined with left IFG stimulation, words produced in a letter fluency task were lower frequency and higher age of acquisition after treatment, compared to before treatment and there was also an increase in accuracy and response times in an untrained picture-naming task. Conclusions: The results indicate that letter fluency therapy combined anodal tDCS is effective in improving lexical retrieval, particularly when left IFG stimulation was used. Effects generalize beyond the trained task, albeit slowing down of responses in picture naming. This task may provide a useful clinical intervention strategy for patients with mild anomia, who are not challenged enough by traditional naming therapies