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Background and purpose: Parkinson's disease (PD) reduces independence and quality of life through deterioration of upper limb motor function. Transcranial direct current stimulation (tDCS) may offer an alternative, adjunctive therapy for PD. However, the efficacy of tDCS for upper limb motor rehabilitation in PD is unknown. In this systematic review, evidence is compiled regarding the effects of tDCS on upper limb motor function in PD. Methods: Studies of tDCS applied to PD patients that assessed upper limb motor function, conducted between January 2000 and November 2018, were screened for inclusion via a systematic search of Medline, Cochrane, PsycINFO, EMBASE, CINAHL, and Web of Science. Results: Ten out of 606 studies were included and their findings synthesized into five categories regarding the effects of tDCS on: (1) Unified Parkinson's Disease Rating Scale motor section (UPDRS III), (2) upper limb motor tasks, (3) manual dexterity, (4) reaction time, and (5) neurophysiology. Conclusions: When applied to the primary motor cortex, tDCS may improve UPDRS III and the speed and force of movement. Considerable variation was found in tDCS parameters and further study is needed to clarify the long-term effects of tDCS on both simple and complex motor tasks and to compile relevant neurophysiological evidence. [ABSTRACT FROM AUTHOR]

Transcranial direct current stimulation (tDCS) is a non-invasive technique able to modulate cortical excitability in a polarity-dependent way. At present, only few studies investigated the effects of tDCS on the modulation of functional connectivity between remote cortical areas. The aim of this study was to investigate—through graph theory analysis—how bipolar tDCS modulate cortical networks high-density EEG recordings were acquired before and after bipolar cathodal, anodal and sham tDCS involving the primary motor and pre-motor cortices of the dominant hemispherein 14 healthy subjects. Results showed that, after bipolar anodal tDCS stimulation, brain networks presented a less evident “small world” organization with a global tendency to be more random in its functional connections with respect to prestimulus condition in both hemispheres. Results suggest that tDCS globally modulates the cortical connectivity of the brain, modifying the underlying functional organization of the stimulated networks, which might be related to changes in synaptic efficiency of the motor network and related brain areas. This study demonstrated that graph analysis approach to EEG recordings is able to intercept changes in cortical functions mediated by bipolar anodal tDCS mainly involving the dominant M1 and related motor areas. Concluding, tDCS could be an useful technique to help understanding brain rhythms and their topographic functional organization and specificity. [ABSTRACT FROM AUTHOR]

Background: Transcranial direct current stimulation (tDCS) is a technique for brain modulation that has potential to be used in motor neurorehabilitation. Considering that the cerebellum and motor cortex exert influence on the motor network, their stimulation could enhance motor functions, such as motor imagery, and be utilized for brain-computer interfaces (BCIs) during motor neurorehabilitation.Methods: A new tDCS montage that influences cerebellum and either right-hand or feet motor area is proposed and validated with a simulation of electric field. The effect of current density (0, 0.02, 0.04 or 0.06 mA/cm2) on electroencephalographic (EEG) classification into rest or right-hand/feet motor imagery was evaluated on 5 healthy volunteers for different stimulation modalities: 1) 10-minutes anodal tDCS before EEG acquisition over right-hand or 2) feet motor cortical area, and 3) 4-seconds anodal tDCS during EEG acquisition either on right-hand or feet cortical areas before each time right-hand or feet motor imagery is performed. For each subject and tDCS modality, analysis of variance and Tukey-Kramer multiple comparisons tests (p <0.001) are used to detect significant differences between classification accuracies that are obtained with different current densities. For tDCS modalities that improved accuracy, t-tests (p <0.05) are used to compare μ and β band power when a specific current density is provided against the case of supplying no stimulation.Results: The proposed montage improved the classification of right-hand motor imagery for 4 out of 5 subjects when the highest current was applied for 10 minutes over the right-hand motor area. Although EEG band power changes could not be related directly to classification improvement, tDCS appears to affect variably different motor areas on μ and/or β band.Conclusions: The proposed montage seems capable of enhancing right-hand motor imagery detection when the right-hand motor area is stimulated. Future research should be focused on applying higher currents over the feet motor cortex, which is deeper in the brain compared to the hand motor cortex, since it may allow observation of effects due to tDCS. Also, strategies for improving analysis of EEG respect to accuracy changes should be implemented. [ABSTRACT FROM AUTHOR]

Stroke is a leading cause of worldwide disability, and up to 75% of survivors suffer from some degree of arm paresis. Recently, rehabilitation of stroke patients has focused on recovering motor skills by taking advantage of use-dependent neuroplasticity, where high-repetition of goal-oriented movement is at times combined with non-invasive brain stimulation, such as transcranial direct current stimulation (tDCS). Merging the two approaches is thought to provide outlasting clinical gains, by enhancing synaptic plasticity and motor relearning in the motor cortex primary area. However, this general approach has shown mixed results across the stroke population. In particular, stroke location has been found to correlate with the likelihood of success, which suggests that different patients might require different protocols. Understanding how motor rehabilitation and stimulation interact with ongoing neural dynamics is crucial to optimize rehabilitation strategies, but it requires theoretical and computational models to consider the multiple levels at which this complex phenomenon operate. In this work, we argue that biophysical models of cortical dynamics are uniquely suited to address this problem. Specifically, biophysical models can predict treatment efficacy by introducing explicit variables and dynamics for damaged connections, changes in neural excitability, neurotransmitters, neuromodulators, plasticity mechanisms, and repetitive movement, which together can represent brain state, effect of incoming stimulus, and movement-induced activity. In this work, we hypothesize that effects of tDCS depend on ongoing neural activity and that tDCS effects on plasticity may be also related to enhancing inhibitory processes. We propose a model design for each step of this complex system, and highlight strengths and limitations of the different modeling choices within our approach. Our theoretical framework proposes a change in paradigm, where biophysical models can contribute to the future design of novel protocols, in which combined tDCS and motor rehabilitation strategies are tailored to the ongoing dynamics that they interact with, by considering the known biophysical factors recruited by such protocols and their interaction. [ABSTRACT FROM AUTHOR]

Learning to control our movements is accompanied by neuroplasticity of motor areas of the brain. The mechanisms of neuroplasticity are diverse and produce what is referred to as the motor engram, i.e., the neural trace of the motor memory. Transcranial direct current stimulation (tDCS) alters the neural and behavioral correlates of motor learning, but its precise influence on the motor engram is unknown. In this review, we summarize the effects of tDCS on neural activity and suggest a few key principles: (1) Firing rates are increased by anodal polarization and decreased by cathodal polarization, (2) anodal polarization strengthens newly formed associations, and (3) polarization modulates the memory of new/preferred firing patterns. With these principles in mind, we review the effects of tDCS on motor control, motor learning, and clinical applications. The increased spontaneous and evoked firing rates may account for the modulation of dexterity in non-learning tasks by tDCS. The facilitation of new association may account for the effect of tDCS on learning in sequence tasks while the ability of tDCS to strengthen memories of new firing patterns may underlie the effect of tDCS on consolidation of skills. We then describe the mechanisms of neuroplasticity of motor cortical areas and how they might be influenced by tDCS. We end with current challenges for the fields of brain stimulation and motor learning. [ABSTRACT FROM AUTHOR]

We systematically investigated the effects of cathodal and anodal Transcranial Direct Current Stimulation (CtDCS, AtDCS) on the electric activity of primary motor cortex during a motor task. High-density electroencephalography was used to define the spatial diffusion of tDCS after effects. Ten healthy subjects performed a finger tapping task with the right hand before and after three separate sessions of 20 minutes of Sham, AtDCS or CtDCS over left primary motor cortex (M1). During movement, we found an increment of low alpha band Event-Related Desynchronization (ERD) in bilateral central, frontal areas and in the left inferior parietal region, as well as an increment of beta ERD in fronto-central and parieto-occipital regions, after AtDCs compared to Sham and CtDCS. In the rest pre-movement period, after Sham as well as AtDCS, we documented an increment of low alpha band power over the course of pre- and post-stimulation recording sessions, localized in the sensorimotor and parieto-occipital regions. On the contrary, after CtDCS no increment of low alpha power was found. Finally beta band coherence among signals from left sensorimotor cortex and activity of bilateral parietal, occipital and right frontal regions was higher after AtDCS compared with Sham condition. Similarly, theta coherence with parietal and frontal regions was enhanced after AtDCS. We hypothesize that the local modulation of membrane polarization, as well as long-lasting synaptic modification induced by tDCS over M1, could result in changes of both local band power and functional architecture of the motor network. Hum Brain Mapp 35:2220-2232, 2014. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique that has been shown to alter cortical excitability and activity via application of weak direct currents. Beyond intracortical effects, functional imaging as well as behavioral studies are suggesting additional tDCS-driven alterations of subcortical areas, however, direct evidence for such effects is scarce. We aimed to investigate the impact of tDCS on cortico-subcortical functional networks by seed functional connectivity analysis of different striatal and thalamic regions to prove tDCS-induced alterations of the cortico-striato-thalamic circuit. fMRI resting state data sets were acquired immediately before and after 10 min of bipolar tDCS during rest, with the anode/cathode placed over the left primary motor cortex (M1) and the cathode/anode over the contralateral frontopolar cortex. To control for possible placebo effects, an additional sham stimulation session was carried out. Functional coupling between the left thalamus and the ipsilateral primary motor cortex (M1) significantly increased following anodal stimulation over M1. Additionally, functional connectivity between the left caudate nucleus and parietal association cortices was significantly strengthened. In contrast, cathodal tDCS over M1 decreased functional coupling between left M1 and contralateral putamen. In summary, in this study, we show for the first time that tDCS modulates functional connectivity of cortico-striatal and thalamo-cortical circuits. Here we highlight that anodal tDCS over M1 is capable of modulating elements of the cortico-striato-thalamo-cortical functional motor circuit. Hum Brain Mapp 33:2499-2508, 2012. © 2011 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]