The Fundamental Mechanism of Transcranial Electrical Stimulation in Post-Stroke Rehabilitation

Abstract

Stroke is a leading cause of long-term disability, and many patients fail to achieve complete recovery following cerebral injury. Therefore, post-stroke rehabilitation is essential to restore impaired function. Transcranial electrical stimulation (tES), transcranial direct current stimulation (tDCS), and transcranial alternating current stimulation (tACS) have emerged as promising neuromodulation approaches to enhance post-stroke recovery. These treatments have therapeutic effects to restore impaired function by modulating cortical excitability and reorganizing brain tissue through electrical stimulation. However, the fundamental mechanisms underlying these therapeutic effects remain poorly understood. This review focused on the neurobiological mechanisms underlying tES that extend beyond cortical excitability and encompass long-term neuroplasticity, cerebral blood flow, neurometabolism, and neuroinflammatory modulation. Our summary provides a comprehensive understanding of tES processes and plays a vital role in the advancement of improved treatments. Additionally, our review promotes enhanced clinical outcomes through interactions with various stroke rehabilitation strategies.

Graphical abstract

Keywords

stroke / transcranial electrical stimulation / excitability / neuroplasticity / cerebral blood flow / neurometabolism / neuroinflammation

1. Introduction

Patients who experience stroke frequently fail to achieve full neurological recovery, resulting in long-term impairment that markedly diminishes quality of life and remains a leading cause of chronic disability [1, 2]. Despite advances in stroke management, no targeted therapy currently exists to repair damaged neural tissue and fully restore function, leaving most post-stroke patients dependent upon rehabilitation interventions [3, 4, 5]. Non-invasive brain stimulation encompasses electrical, magnetic, optical, and ultrasound modalities, with electrical stimulation being the most applicable [6, 7, 8].

Transcranial electrical stimulation (tES) for stroke rehabilitation is an emerging therapeutic approach that modulates cortical excitability and induces long-lasting effects, with minimal side effects [6, 9, 10]. tES includes two major modalities, direct current stimulation (tDCS) and transcranial alternating current stimulation (tACS), with the tDCS method predominantly utilized in clinical practice as a standard [11].

tDCS involves the administration of a constant weak direct current via two scalp electrodes to influence cortical excitability in a targeted cortical area [6, 12, 13, 14]. Early investigations of tDCS have focused on the modulation of cortical excitability influenced by polarity, producing contrasting effects on neuronal excitability [12, 13, 14]. Anodal tDCS increases cortical excitability, whereas cathodal tDCS decreases it [6, 12, 13].

tACS applies a mild sinusoidal alternating electric current at a designated frequency across the scalp to modulate brain activity and function as neural oscillation entrainment [15, 16, 17]. tACS is sometimes regarded as a variant of transcranial magnetic stimulation delivered at a predetermined frequency [17]. Cortical excitability, according to anodal and cathodal currents, can be clearly distinguished in tDCS; however, in tACS, the effects on cortical excitability are intricate because of the interplay between cortical excitation and inhibition [12, 13, 15, 18].

Despite many promising clinical trials of tES for stroke rehabilitation, either alone or in combination with other therapies, the number of study showing minimal or different effects remains a concern for its clinical application [7]. Moreover, the mechanisms underlying its beneficial effects remain to be elucidated. tDCS and tACS resemble electrical stimulation; however, because the electricity used differs between direct and alternating currents, the underlying mechanisms in the brain may manifest differently. tES may have therapeutic effects not only through cortical excitability induced by electrical stimulation but also via various biological mechanisms or processes. Elucidating these putative pathways is essential for the development of clinical interventions and therapeutic strategies. A literature search was conducted on PubMed using the terms “tES”, “tDCS”, and “tACS” alongside “stroke” to provide the transparency and reproducibility necessary for this mechanism-oriented review. Exclusion criteria were rigorously enforced for literature that solely presented clinical outcome measurements without significant mechanistic analysis or research that predominantly concentrated on non-electrical brain stimulation methods unless they elucidated a fundamental tES mechanism. Therefore, this study aimed to review the mechanisms of tDCS and tACS, with a focus on stroke rehabilitation.

2. Cortical Excitability and Neural Network Response

2.1 Re-Adjustment of Interhemispheric Imbalance

Following a unilateral stroke, the inhibitory interaction between the cerebral hemispheres is impaired through the transcallosal pathway [19, 20]. The lesioned ipsilateral hemisphere exhibits reduced excitability, whereas the undamaged contralateral hemisphere becomes hyperexcitable, disturbing the normal balance of interhemispheric inhibition via an excessive influence on the stroke-affected side. This phenomenon is referred to as interhemispheric imbalance, or the interhemispheric competition model [19, 21, 22]. The ipsilesional hemisphere remains inactive owing to continuous inhibition of the symmetrical healthy hemisphere, potentially worsening functional impairment for the patient [5, 19, 22].

Direct current electrical stimulation can modulate cortical excitability by modifying the membrane potential threshold through the involvement of N-methyl-d-aspartate (NMDA) receptors [12, 23, 24]. Clinically, anodal stimulation leads to neuronal depolarization, whereas cathodal stimulation leads to hyperpolarization [6, 12, 13]. Cortical excitability is crucial to restore brain function, with an equilibrium between excitatory glutamatergic and inhibitory gamma-aminobutyric acid-expressing (GABAergic) neurons [7, 23, 25]. Following stroke, neuronal inhibition increases in the peri-infarct region, which is mediated by gamma-aminobutyric acid (GABA) A receptors, resulting from impaired GABA transporter function [7, 26, 27]. This indicates that the interhemispheric imbalance is not solely a decrease in neuronal excitability but rather a neurochemical mechanism involving dysregulation of GABA tone. To correct this asymmetry, anodal tDCS is applied to the ipsilesional hemisphere to enhance cortical excitability, whereas cathodal tDCS is administered to the contralesional hemisphere to suppress excitability, with both approaches potentially combined through simultaneous bihemispheric stimulation [6, 21, 28, 29]. These configurations reduce the inhibitory influence of the intact hemisphere on the lesioned hemisphere.

To optimize the alleviation of interhemispheric imbalance, the integration of anodal tDCS and bihemispheric tDCS with constraint-induced movement therapy is utilized [30, 31, 32]. tDCS stimulation in each hemisphere varies according to polarity; nonetheless, a reduction in GABA levels has been noted [33], and the influence of tDCS on motor performance is negatively correlated with GABA concentration [34]. tACS in both hemispheres can phase-specifically modulate connections between brain regions within the corresponding frequency band; however, no studies have applied this to stroke [35].

The interhemispheric competition model serves as a valid mechanistic basis for the efficacy of tES treatment post-stroke by readjusting the balance of interhemispheric inhibition. However, the treatment of stroke with tES needs to be performed cautiously, considering the polarity, dose, and configuration of the electrodes based on the pathophysiological framework of stroke, as these factors yield varying clinical outcomes [10, 36, 37]. The application of anodal stimulation in the early stages of stroke considers the inflammatory response of the brain, which adversely affects clinical outcomes [10, 38], whereas cathodal tDCS facilitates superior clinical recovery in this stage [10, 39, 40]. Moreover, excessive stimulation of GABA signaling during stroke recovery diminishes neuronal excitability and impedes recovery [27, 41]. If tES is applied uniformly in this model without considering the pathological frameworks at different stages after stroke, it is likely to yield different clinical outcomes [7].

2.2 Structural Reserve as Integration of Interhemispheric Inhibition and Vicariation

A novel neural rehabilitation model, termed the bimodal balance-recovery model, was introduced to address the limitations of the oversimplified interhemispheric imbalance model, incorporating interhemispheric inhibition and structural reserve post-stroke [7]. When the structural reserve is high, the affected hemisphere can restore normal interhemispheric balance. However, when the structural reserve is low, the contribution of the contralesional hemisphere may be beneficial; this mechanism is referred to as vicariation or interhemispheric compensation [42]. Intact residual networks can compensate for the injured hemisphere through brain reorganization; hence, activation of the healthy hemisphere may represent an effective therapeutic strategy to promote functional recovery post-stroke [7, 43, 44]. The proposed model facilitates a patient-specific approach to brain stimulation tailored to the neural architecture of individual patients [7].

Clinical evidence from transcranial magnetic stimulation has questioned the ability of the interhemispheric competition model to modulate abnormal transcallosal inhibition, based on the assumption that all patients belong to a single cohort [45, 46, 47]. There were no signs of changes in the interhemispheric imbalance or contralesional hemisphere hyperexcitability [45, 47, 48, 49]. Interhemispheric inhibition and baseline impairments exhibit bimodality, allowing for a criterion impairment level for patient classification based on the bimodal effects of the contralateral brain [49].

In patients with moderate-to-severe impairment, enhancing contralesional excitability with anodal tDCS or bilateral stimulation can improve motor performance [50, 51]. Ipsilesional excitability initially diminishes and then increases, correlating with impairment and function; however, interhemispheric inhibition remains symmetrical [45]. In a rodent stroke model, the contralesional cortex exhibited anatomical adaptation in response to stroke size [52]. Patients with severe stroke primarily relied on the undamaged hemisphere for functional recovery, whereas those with minor stroke did not [53]. Application of anodal tDCS to the contralesional cortex shows markedly enhanced recovery relative to ipsilesional stimulation. Therefore, the intact hemisphere can facilitate the recovery of impaired function [54, 55]. These findings demonstrate that the role of the contralesional hemisphere in healing increases with lesion severity. To restore interhemispheric activity, tES therapeutic options are selected according to the degree of individual damage.

2.3 Neural Entrainment and Synchronizations

The brain inherently displays rhythmic activity at various frequencies—waves \delta (1–4 Hz), \theta (4–8 Hz), \alpha (8–12 Hz), \beta (12–30 Hz), and \gamma (30–80 Hz)—resulting from the firing timing of a group of neurons; these synchronizations are crucial for normal cerebral function [15, 56, 57]. Stroke leads to neuronal death or disruption of synaptic connections, resulting in diminished coordination of neuronal activity and disruption of endogenous oscillations [58, 59].

Externally applied electric fields modify the evoked network activity, and entrainment-based tACS can be synchronized to normalize endogenous oscillation patterns at a relevant frequency [15, 60, 61]. Periodic electrical stimulation of the tACS interacts with normal oscillations in the brain, synchronizing neuronal firing rhythms to the corresponding frequency [15, 16, 62]. tDCS, measured by electroencephalography, modulates oscillatory activity, which is a marker and facilitator of neuroplasticity [63, 64]. tDCS does not directly enforce the inherent frequency in the brain; rather, it indirectly modifies endogenous oscillations in the brain through long-term alterations in neuronal spike rates and synaptic activity [44, 65, 66].

Clinical studies support the potential of tES in altering neuronal networks via synchronization and coordination mechanisms [67, 68, 69]. In animal studies, tACS corrects atypical neuronal firing patterns by resynchronizing neurons to a healthy rhythm [70, 71]. The tDCS-induced interhemispheric brain coherence in the low gamma frequency range promotes functional recovery after stroke [72]. Therefore, the therapeutic benefits of administering tES are related to the recovery of healthy brain oscillations and the restoration of interregional coherence within cortical networks.

3. Enhancement of Neuroplasticity and Paired Associative Stimulation

3.1 Long-Term Neuroplasticity

Neuroplasticity is the unique capacity of the brain to create new synapses and reorganize neural networks in response to stimuli. Following a stroke, this mechanism is essential for the continuous regeneration of injured brain tissue. The basic theoretical premise of tES, which underlies its significant therapeutic efficacy, indicates the reorganization of brain circuits [17]. Long-term therapeutic benefits are produced via polarity-specific long-term potentiation (LTP), which is triggered by tES-induced intracellular calcium levels and forms connections with various downstream pathways [29, 54, 55, 73, 74].

Moreover, successive tES stimulation causes initial cortical excitability, which is then transmitted to subcortical areas by activating NMDA receptors, intracellular calcium cascades, and other pathways [13, 14, 75]. Connection-based diffusion from the cerebral cortex to the subcortical areas following tES may be correlated with the therapeutic outcomes of functional recovery in stroke. Similar to neuroplasticity in the healthy brain, a time-limited window exists post-stroke to enhance functional recovery, which can be strengthened and reorganized through training and appropriate stimuli [5, 44].

tES promotes neuronal pathways that improve neuroplasticity and functional reorganization in injured brains [76]. Furthermore, combinations with other rehabilitation programs have a synergistic effect and improve therapies based on Hebbian plasticity [76, 77, 78, 79]. In animal studies, tDCS exerts neuroprotective effects by maintaining neuroplasticity by modifying genes linked to plasticity, neuroplastic biomarkers, and brain circuits [80, 81, 82]. Brain-derived neurotrophic factor (BDNF)-tropomyosin-related kinase receptor B (TrkB) signaling is involved in the therapeutic effects of tDCS on functional recovery and neuroplasticity [83, 84].

Neuroplasticity, a key feature of functional recovery after stroke, results from processes that facilitate the reorganization of brain networks. Thus, by providing stimulation in response to brain activity, tES-induced neuroplasticity may be similar to activity-dependent therapy [85, 86, 87]. tES augments the neuroplasticity of the brain, facilitating the recovery of functions in neural circuits impaired by stroke via the formation of new synapses. When combined with rehabilitative therapy, it promotes restoration of various functions, restoring and compensating for pre-injury functions through additional processes, including Hebbian plasticity, homeostatic metaplasticity, and vicariation.

3.2 Improvement With tES-Paired Associative Stimulation

Hebbian plasticity, proposed by Donald Hebb in 1949, is founded on the premise that “neurons that fire together, wire together” [88]. This theory offers a conceptual framework to elucidate how experience and learning facilitate the adaptation and enhancement of neural networks [89, 90]. Post-stroke, patients reacquire motor learning with repetitive practice similar to that of healthy individuals, indicating synaptic reinforcement at the cerebral level within the motor network based on Hebbian plasticity [91]. Synaptic connections are reinforced by the simultaneous activity of two neurons, which encompass coactivation, LTP, and long-term depression (LTD). Similar to LTP and LTD, repeated tES sessions cause long-lasting changes in synaptic efficacy via NMDA receptor-mediated calcium dynamics, resulting in the insertion or removal of synaptic receptors [92, 93, 94].

Clinical tDCS enhances Hebbian plasticity during task performance, particularly when administered in a task-specific manner that interacts with endogenous plasticity [95, 96]. Simultaneous stimulation of the motor cortex and peripheral nerves using paired associative tACS improves the corticospinal excitability of the motor neurons, eliciting an LTP-like response [97]. Providing temporally synchronized stimulation tailored to a patient’s movements or cerebral activity enhances the synaptic connections of the remaining neural pathways and intensifies learning effects. Therefore, the integration of other therapies, such as consistent exercise with tES, may facilitate the reorganization of the cerebral cortex and markedly improve functional recovery in patients with stroke.

Although Hebbian plasticity is a fundamental component of brain learning and memory, it can destabilize neural circuits if operating excessively or irregularly [98]. Homeostatic metaplasticity is a self-regulatory mechanism that modulates LTP or LTD in response to prior synaptic activity, thereby preventing excessive excitation or inhibition and maintaining brain activity within the physiological range [98, 99]. When numerous reinforcement processes occur, they exhibit a reduced sensitivity to further reinforcement.

The Bienenstock–Cooper–Munro (BCM)-like plasticity model, which dynamically adjusts the plasticity threshold according to the activation history, has been elucidated [100, 101, 102]. This mechanism can improve recovery outcomes after stroke by efficiently regulating synaptic responses based on the brain’s previous activity state and tES intervention. To prevent metaplastic inhibition, excitatory inputs must be modulated or delayed after the completion of a high-intensity task. In contrast, short-term tDCS before training can increase plasticity of the motor cortex, which magnifies synaptic changes during subsequent practice. Homeostatic metaplasticity offers a theoretical framework for the significance of the timing, dosage, and sequencing of tES and other training [102, 103]. Preconditioning the brain with cathodal tDCS enhances alterations in neuronal excitability caused by anodal tDCS or intermittent theta burst stimulation [103, 104]. Following cathodal tDCS, mesh glove stimulation can be used to modify motor cortex excitability in metaplastic modulation [105]. The concurrent application of anodal tDCS and neuromuscular electrical stimulation failed to yield the anticipated synergistic effect, which was potentially attributable to the influence of homeostatic plasticity [106]. However, tDCS does not significantly improve motor fatigue or excitability regulation in healthy adults, nor does it affect neuroexcitability, indicating a limited influence of metaplasticity on implicit motor learning [107, 108, 109].

The tES treatment of stroke promotes neuroplasticity in the injured brain by utilizing the principles of Hebbian plasticity and homeostatic metaplasticity. tES enables the simultaneous activation of neurons, reinforcement of activated neurons to restore functional connections, or reduction of excessive excitability to preserve the stability of the neural network. These mechanisms may alter the timing and intensity of tES to optimize neuroplasticity.

3.3 Augmentation of Growth Factors

tES facilitates motor recovery following stroke by stimulating the release of growth and neurotrophic factors that activate neuroplasticity, neuronal survival, and brain reorganization [29, 54, 83, 110]. tDCS activates calcium channels, which trigger the activation of downstream cascades that promote the production of growth factors [13, 29, 83, 111]. The brain contains numerous pathways composed of axonal fibers that connect various cerebral regions [112]. Thus, tES elevates intracellular calcium levels in the non-stimulated cortical hemisphere, indicating that calcium may trigger the release of neuroprotective factors at non-stimulated sites [29, 54, 73, 113].

tDCS increases neuroplasticity within the motor cortex through BDNF secretion and TrkB activation, which are crucial mechanisms to improve neurological function after stroke [29, 114]. The BDNF genotype in patients with stroke affects motor learning capability, as people with the atypical BDNF genotype show reduced efficacy in enhancing motor learning by tDCS [115, 116]. However, the therapeutic benefits of tDCS may differ according to catechol-O-methyltransferase genotype polymorphism, whereas those of BDNF may not [117]. A reduction in BDNF levels may have positive effects on recovery, highlighting the need for electrode positioning, polarity adjustments, and personalized intensity [70].

tDCS facilitates BDNF-TrkB signaling and functional recovery, with multifaceted biological mechanisms contributing to brain recovery and regeneration beyond simple electrical stimulation [29, 83, 84, 110]. tDCS provides a molecular basis for neuroplasticity by modifying neuroplasticity-related genes, such as BDNF, cyclic adenosine monophosphate (cAMP) response element-binding protein, synapsin I, and calcium/calmodulin-dependent protein kinase II [80, 81]. tDCS elevates BDNF levels and indicators of axonal regeneration in the peri-infarct cortex, along with neuroprotection and neuroplasticity facilitated by the BDNF/TrkB–phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt) pathways [118, 119]. Moreover, the continuous use of tDCS upregulates the expression of activity-dependent growth factors, such as BDNF, growth/differentiation factor 5 (GDF5), and platelet-derived growth factor subunit A (PDGFA) [54, 118]. These factors govern the proliferation and differentiation of oligodendrocyte progenitors, thereby facilitating functional recovery [54, 120, 121]. tDCS/tACS treatment regulates inflammatory cytokines in microglia as well as the activation of neurotrophic factors, such as BDNF and fibroblast growth factor 9 [70].

Neurotrophic factors stimulate the proliferation, differentiation, and survival of neural stem cells, facilitating neuronal formation and maturation [122, 123]. From a regenerative medicine perspective, tDCS activates adult neurogenesis regardless of its polarity; cathodal tDCS sometimes has an even greater effect [110, 124, 125]. Additionally, tDCS induces the differentiation of neural stem cells by inhibiting Notch1 signaling [126]. Furthermore, anodal tDCS treatment promotes cell proliferation in the ipsilesional striatum, subventricular zone, and corpus callosum [54].

One of the main mechanisms by which tES promotes neuroplasticity in stroke rehabilitation is the upregulation of the expression of growth factors, specifically BDNF. This highlights the essential functions of tES in neuronal survival, synapse formation and reformation, axonal growth, and restoration of connections between neurons, thus facilitating the reorganization and functional recovery of impaired brain circuits.

4. Reorganization by Enhancing Neurovascular Function and Neurometabolism

4.1 Regulation of Cerebral Blood Flow and Neurovascular Coupling

Diaschisis is a condition characterized by reduced brain activity in regions not directly affected by injury following stroke. It is reversible and gradually recovers over time and primarily occurs in regions that are structurally and functionally linked to the affected area owing to the loss of input and rapid neurochemical alterations [127, 128]. Vicariation, the functional reorganization of the brain in which diaschisis is irreversible and permanent, occurs when a permanently impaired part of the brain is functionally replaced by another area [129, 130]. The adult brain undergoes considerable structural and functional reorganization after injury, which is crucial for recovery [131, 132].

Diaschisis and vicariation during stroke recovery, and alterations such as disruption or compensation within functional networks, are closely related to cerebral blood flow (CBF) and neurometabolism, including the rates of oxygen and glucose consumption. The diaschitic region exhibits a reduction in CBF linked to the metabolic rate with no physical damage, whereas neuronal activity and blood flow serve as crucial indicators of functional reorganization of the brain [127, 129]. This entails the synchronized function of neurons, astrocytes, and vascular cells; however, neurovascular coupling (NVC) is impaired after a stroke [133]. The NVC regulates neuronal activity and CBF via coordinated signaling across many cells [134]. During the recovery phase, rehabilitation training or tES influences CBF, which, in turn, affects neuroplasticity and neuronal reorganization, leading to functional recovery.

Clinical trials have been conducted to investigate the interaction between nerves and blood vessels through tES because it may affect CBF velocity [51, 135]. tDCS modulates CBF and intracranial compliance, alleviating hemodynamic load on the affected hemisphere [136, 137]. In contrast, tACS alters cerebral hemodynamics, including enhanced blood flow velocity and diminished blood flow resistance [138]. The response of blood flow velocity to tDCS differs among individuals, which is attributable to variations in baseline neuronal excitability and cerebral blood velocity [135]. The integration of tDCS and computer-aided training effectively modulates CBF in patients with stroke [139]. Ischemia results in an imbalance in cerebral oxygen saturation, which is strongly correlated with EEG readings; however, this imbalance, resulting from cerebral vasomotor reactivity, is restored by tDCS [136, 140, 141, 142, 143]. In animal studies, tDCS reduced both the incidence and severity of cerebral vasospasms, with alterations in blood flow depending on polarity [144, 145, 146]. tACS affects CBF, leading to increased blood flow at specific frequencies [147, 148].

These results indicate that tDCS may elicit functional CBF responses by enhancing vasodilation, and tACS indirectly influences CBF via frequency-dependent modulation of neural rhythms. tES modulates CBF during stroke rehabilitation and is linked to neurometabolic activity that enhances the delivery of oxygen and nutrients to the affected areas or other regions. The augmentation of CBF may be associated with the restoration of NVC and the enhancement of neuroplasticity, which play a role in the remodeling of functional networks post-stroke.

4.2 Enhancement of Neurometabolism

Stroke markedly disrupts neurometabolism in the brain. Hence, cerebral reorganization or functional recovery refers to the restoration of the brain’s energy system [149, 150]. Restoration of ATP synthesis, mitochondrial activity, and metabolic conditions, together with NVC, is crucial to maintain the functionality of the recovered neural network [134, 151, 152]. Therefore, therapeutic strategies for stroke should employ a comprehensive approach that integrates functional induction with metabolic support, including the optimization of mitochondrial activities. The tES approach may enhance energy utilization and recovery in neuronal cells; however, data on its impact on mitochondrial activation and dynamics are limited.

The administration of tDCS to healthy individuals elevates cerebral energy consumption, leading to a temporary state of energy depletion, which causes changes in systemic glucose metabolism and related hormones [153]. Nonetheless, tDCS therapy in patients with stroke does not alter brain metabolite concentrations [154]. In preclinical animal study, tDCS decreases mitochondrial damage and preserves mitochondrial quality control signaling in neurons [155]. This indicates that tACS exhibits mitochondrial activity, which is linked to therapeutic advantages [151, 152, 156]. This suggests that tACS, similar to exercise therapy, affects neurotrophic factors, such as BDNF signaling, perhaps improving mitochondrial complex subunits and function through the peroxisome proliferator-activated receptor gamma co-activator (PGC-1\alpha)– fibronectin type III domain containing 5 (FNDC5)–BDNF pathway [157, 158].

Restoring and stabilizing the metabolic environment is essential for the efficacy of stroke rehabilitation and is necessary to optimize these effects from the early stages of stroke until recovery [149, 150]. Employing tES to restore and regulate brain metabolism may facilitate functional recovery by enhancing neuroplasticity and reorganization. This function is likely associated with enhanced blood flow metabolism, and further investigation is required.

5. Enhancing Neuronal Survival via Anti-Neuroinflammation

Following ischemic stroke, a severe reduction in CBF leads to an infarct zone characterized by a damaged ischemic core, resulting in irreversible necrosis along the surrounding penumbra [159]. Clinical research supports the prevention of neuronal death by positioning the penumbra/peri-infarct region, which is defined by a partially preserved blood supply and temporary neuronal viability, and is a pivotal concept in stroke neuroprotection [160, 161, 162]. Major research targets in stroke, including excitotoxicity, oxidative stress, apoptosis, and inflammation, are all related to neuronal survival in the penumbra [163, 164]. Cathodal tDCS therapy in patients with stroke exhibits a potentially beneficial effect by reducing the extent of ischemic and infarct regions through the inhibition of excitotoxic effects and penumbral salvage [39, 40].

Preclinical animal studies on changes in cerebral infarction through the regulation of inflammation and neural death are more diverse than clinical studies. tDCS enhances activated microglia in both anodal and cathodal configurations, eliciting an innate immune response [165]; however, cathodal tDCS facilitates superior recovery by modulating neuroinflammatory responses and apoptosis [10, 166, 167]. The polarity of tDCS, especially that of cathodal tDCS, reduces cell death through the alleviation of inflammatory markers and oxidative stress and the regulation of NMDA receptors [168, 169]. tDCS promotes neuronal survival via the Cezanne-SIRT6-DNA damage regulation pathway or via DNA fragmentation [170, 171]. The anti-inflammatory response and enhancement of nerve plasticity and angiogenesis associated with tDCS are facilitated by the cyclic guanosine monophosphate-adenosine monophosphate (cGMP-AMP) synthase stimulator of the interferon gene pathway [172]. Cathodal tDCS suppresses ferroptosis through the dual mechanisms of mitophagy inhibition and nuclear factor erythroid 2-related factor 2 (Nrf2) activation [167].

Electrical stimulation at 20 Hz diminishes neuroinflammation and apoptosis-related markers via the BDNF/TrkB and PI3K/Akt/mTOR pathways [173], whereas 20 Hz tACS modulates 11 genes typically associated with apoptosis and inflammatory responses, along with tDCS [70]. Research on the effects of tDCS on detrimental factors in stroke has demonstrated its neuroprotective effects via the regulation of propionic acid [174], 3-oxo-lithocholic acid [175], isoleucine [176], and surplus locus protein 4 [177]. The effects of tDCS vary according to the polarity, and its application during the ischemia and reperfusion stages can markedly improve neuroprotection against cerebral injury [169, 171].

tDCS treatment exerts therapeutic effects by inhibiting neuronal damage and reducing the levels of inflammatory factors via the NMDA-mediated sterol regulatory element-binding protein 1 (SREBP1) pathway [55]. tDCS/tACS treatments also commonly regulate stroke brain immune-related targets in response to damage-associated molecular patterns (DAMPs), including DAMPs (S100 calcium-binding protein A9 (s100a9)) and its receptor (toll-like receptor 4 (Tlr4) and myeloid differentiation primary response 88 (Myd88)), phagocytosis receptor (triggering receptor expressed on myeloid cells 2 (Trem2)), signaling cascade (signal transducer and activator of transcription 1 (Stat1) and nuclear factor kappa-light-chain-enhancer of activated B cells (NFkB)), and cytokines (interleukin (IL)-6 (IL-6), IL-1\beta, and tumor necrosis factor alpha (TNF-\alpha)) [70].

Cathodal tDCS demonstrates superior neuroprotection in relevant stroke models. This may be due to the modulation of neuroinflammatory pathways that provide neuroprotective effects, particularly during the acute and subacute periods after ischemia. Following a stroke, tES modulates inflammation-related mechanisms to mitigate excessive responses and safeguard the surrounding tissues. Furthermore, its anti-inflammatory effects enhance neuronal survival by regulating various apoptotic and survival-related factors. The anti-neuroinflammatory properties of tES are important in preventing brain injury and promoting functional recovery during rehabilitation.

6. Indirect Involvement of the Peripheral Nerve System

tES focuses on neuroplasticity and network reorganization by modulating cortical excitability and related processes; however, the possibility of indirect involvement of the peripheral nervous system (PNS) has been raised. In humans and rodents, tES accounts for approximately 75% of the current applied to the scalp, owing to the soft tissue and the skull, making it insufficient to entrain neural activity [178, 179]. To stimulate the cortex, either a higher intensity current than conventional tES is employed [178], or different high-frequency currents are delivered to the brain to create temporal interference at different frequencies, selectively stimulating the deep brain [180]. Because of the transcranial and transcutaneous stimulation, the effect of tACS on the motor system can be elicited solely by transcutaneous stimulation [179].

Involvement of the PNS may be an important consideration in the design of tES therapy. The effects of tES can be modulated or enhanced by the PNS, and synergistic outcomes can be achieved by combining central and PNS stimulations [181, 182]. Similar to cortical excitability in the tES, calcium channels involving NMDA receptors are important signaling components that represent the therapeutic advantages of PNS [70, 183]. Recent studies have indicated the possible indirect effects of tDCS via the trigeminal nerve [184, 185] and have highlighted the role of the vagus nerve in the outcomes of tDCS [186].

Clinical and physiological parameters improve when tDCS and PNS are applied simultaneously, proving to be more effective than when used in isolation [181, 187, 188]. Nonetheless, it is equally accurate that combined stimulation does not invariably exhibit additional effects or consistently reveals enhanced efficacy [108, 189]. Electrical stimulation techniques, including tDCS, tACS, and electroacupuncture, positively influence motor function in a stroke model via a shared mechanism that modulates neuronal cell death, regardless of whether the stimulation targets the cerebral cortex or the peripheral somatosensory input [70]. Therefore, these therapeutic interventions can induce both bottom-up and top-down bidirectional neural activation, leading to synergistic advantages [182].

7. Collaborative Relationships Across Mechanisms

We investigated the therapeutic effects of tES in stroke rehabilitation, including cortical excitability, neuroplasticity, neurovascular and neurometabolic stability, inflammatory regulation, and the role of peripheral nerves. These categories of molecular mechanisms indicating therapeutic effects interact interdependently, going beyond simple cortical excitability. This dynamic interaction forms a complex loop that enhances its therapeutic benefits (Fig. 1A). tDCS normalizes cortical connections disrupted by stroke through neural rewiring, leading to the recovery of motor function [190]. Enhancements in motor and cognitive functions correlate with elevated expression of growth factors, including GDF5 and PDGFA, along with neurogenesis in the subventricular zone [54]. Motor function recovery includes activation of the BDNF/TrkB pathway, synaptic remodeling, restoration of brain connections, and potential enhancement of angiogenesis [83]. For recovery of motor and cognitive functions, NMDA-dependent SREBP1 signaling and related inflammatory factors, such as cyclooxygenase-2 (COX-2) and Akt, are involved [55]. Therefore, the modulation of neuronal excitability elicited by tES facilitates neuroplasticity and the secretion of neurotrophic factors, such as BDNF, thereby maintaining long-term functional recovery. They also diminish neuroplasticity deficits along with anti-neuroinflammation and reorganization through neurovascular and neurometabolic stability.

8. Conclusions

There are still gaps in the knowledge regarding the stimulation protocols, parameters, long-term effects, and basic mechanisms of tES in relation to stroke rehabilitation. This highlights the inapplicability of animal models in humans and their disconnection from therapeutic practices. However, there are questions regarding whether tES can activate the cortex by passing through the skull. Rodent studies have used higher intensities than human studies, even when considering head size, complicating the direct application to clinical scenarios [191, 192, 193, 194]. Clinically, field intensities higher than those employed in traditional tES guarantee consistent cortical excitability in the human brain [178, 193].

tES treatment requires enhanced precision in current administration and spatial focality in the cerebral cortex. The traditional large rectangular pad induces nonfocal current diffusion [195, 196] thus, small ring-based high-definition (HD) electrodes are employed to overcome this [197, 198]. The modified HD electrode with integrated needles enhances the focus relative to traditional HD electrodes; however, it exhibits limitations in accurate stimulation [55]. Moreover, newly developed electrodes and excessive stimulus intensity intended to enhance cortical activity and local stimulation may induce uncomfortable sensations such as edge effects; therefore, further clinical and animal studies are needed to address this problem [199, 200].

The goal of post-stroke treatment to date has been to reduce the initial impact and complications and to maximize functional ability through comprehensive physical therapy, employing a strategy aimed at activating a limited window of neuroplasticity during the early stages [5, 7, 44]. However, depending on the stage after stroke, functional recovery is achieved through stage-specific modulation of inflammation, neuroplasticity, NVC, network reorganization, and structural remodeling [44, 83, 201]. The design approach for tES needs to be varied based on the stage of the stroke because it may produce different outcomes depending on parameters, such as polarity, frequency, intensity, duration, electrode configuration, stimulation target, and number of sessions (Fig. 1B). tES is non-invasive and is expected to promote functional recovery in patients with stroke through various mechanisms with repeated applications. However, careful prescription design of tES is required, considering its parameters and the post-stroke stage. Along with the standardization of each parameter, analysis of the long-term effects of repeated applications and personalized stimulation combined with other treatments will lead to improved tES strategies.

References

[1] Iadecola C, Anrather J. Stroke research at a crossroad: asking the brain for directions. Nature Neuroscience. 2011; 14: 1363–1368. https://doi.org/10.1038/nn.2953.

[2] Feigin VL, Roth GA, Naghavi M, Parmar P, Krishnamurthi R, Chugh S, et al. Global burden of stroke and risk factors in 188 countries, during 1990-2013: a systematic analysis for the Global Burden of Disease Study 2013. The Lancet. Neurology. 2016; 15: 913–924. https://doi.org/10.1016/S1474-4422(16)30073-4.

[3] Campbell BCV, Khatri P. Stroke. Lancet. 2020; 396: 129–142. https://doi.org/10.1016/S0140-6736(20)31179-X.

[4] Langhorne P, Bernhardt J, Kwakkel G. Stroke rehabilitation. Lancet. 2011; 377: 1693–1702. https://doi.org/10.1016/S0140-6736(11)60325-5.

[5] Hordacre B, McCambridge AB, Ridding MC, Bradnam LV. Can Transcranial Direct Current Stimulation Enhance Poststroke Motor Recovery? Development of a Theoretical Patient-Tailored Model. Neurology. 2021; 97: 170–180. https://doi.org/10.1212/WNL.0000000000012187.

[6] Hummel F, Celnik P, Giraux P, Floel A, Wu WH, Gerloff C, et al. Effects of non-invasive cortical stimulation on skilled motor function in chronic stroke. Brain. 2005; 128: 490–499. https://doi.org/10.1093/brain/awh369.

[7] Di Pino G, Pellegrino G, Assenza G, Capone F, Ferreri F, Formica D, et al. Modulation of brain plasticity in stroke: a novel model for neurorehabilitation. Nature Reviews. Neurology. 2014; 10: 597–608. https://doi.org/10.1038/nrneurol.2014.162.

[8] Feng J, Li Z. Progress in Noninvasive Low-Intensity Focused Ultrasound Neuromodulation. Stroke. 2024; 55: 2547–2557. https://doi.org/10.1161/STROKEAHA.124.046679.

[9] Hummel FC, Cohen LG. Non-invasive brain stimulation: a new strategy to improve neurorehabilitation after stroke? The Lancet. Neurology. 2006; 5: 708–712. https://doi.org/10.1016/S1474-4422(06)70525-7.

[10] Peruzzotti-Jametti L, Cambiaghi M, Bacigaluppi M, Gallizioli M, Gaude E, Mari S, et al. Safety and efficacy of transcranial direct current stimulation in acute experimental ischemic stroke. Stroke. 2013; 44: 3166–3174. https://doi.org/10.1161/STROKEAHA.113.001687.

[11] Elsner B, Kugler J, Pohl M, Mehrholz J. Transcranial direct current stimulation (tDCS) for improving activities of daily living, and physical and cognitive functioning, in people after stroke. The Cochrane Database of Systematic Reviews. 2020; 11: CD009645. https://doi.org/10.1002/14651858.CD009645.pub4.

[12] Nitsche MA, Paulus W. Excitability changes induced in the human motor cortex by weak transcranial direct current stimulation. The Journal of Physiology. 2000; 527: 633–639. https://doi.org/10.1111/j.1469-7793.2000.t01-1-00633.x.

[13] Nitsche MA, Fricke K, Henschke U, Schlitterlau A, Liebetanz D, Lang N, et al. Pharmacological modulation of cortical excitability shifts induced by transcranial direct current stimulation in humans. The Journal of Physiology. 2003; 553: 293–301. https://doi.org/10.1113/jphysiol.2003.049916.

[14] Liebetanz D, Nitsche MA, Tergau F, Paulus W. Pharmacological approach to the mechanisms of transcranial DC-stimulation-induced after-effects of human motor cortex excitability. Brain. 2002; 125: 2238–2247. https://doi.org/10.1093/brain/awf238.

[15] Fröhlich F, McCormick DA. Endogenous electric fields may guide neocortical network activity. Neuron. 2010; 67: 129–143. https://doi.org/10.1016/j.neuron.2010.06.005.

[16] Helfrich RF, Schneider TR, Rach S, Trautmann-Lengsfeld SA, Engel AK, Herrmann CS. Entrainment of brain oscillations by transcranial alternating current stimulation. Current Biology. 2014; 24: 333–339. https://doi.org/10.1016/j.cub.2013.12.041.

[17] Alekseichuk I, Turi Z, Amador de Lara G, Antal A, Paulus W. Spatial Working Memory in Humans Depends on Theta and High Gamma Synchronization in the Prefrontal Cortex. Current Biology. 2016; 26: 1513–1521. https://doi.org/10.1016/j.cub.2016.04.035.

[18] Khatoun A, Asamoah B, Mc Laughlin M. Simultaneously Excitatory and Inhibitory Effects of Transcranial Alternating Current Stimulation Revealed Using Selective Pulse-Train Stimulation in the Rat Motor Cortex. The Journal of Neuroscience. 2017; 37: 9389–9402. https://doi.org/10.1523/JNEUROSCI.1390-17.2017.

[19] Murase N, Duque J, Mazzocchio R, Cohen LG. Influence of interhemispheric interactions on motor function in chronic stroke. Annals of Neurology. 2004; 55: 400–409. https://doi.org/10.1002/ana.10848.

[20] Boddington LJ, Reynolds JNJ. Targeting interhemispheric inhibition with neuromodulation to enhance stroke rehabilitation. Brain Stimulation. 2017; 10: 214–222. https://doi.org/10.1016/j.brs.2017.01.006.

[21] Lindenberg R, Renga V, Zhu LL, Nair D, Schlaug G. Bihemispheric brain stimulation facilitates motor recovery in chronic stroke patients. Neurology. 2010; 75: 2176–2184. https://doi.org/10.1212/WNL.0b013e318202013a.

[22] Xu J, Branscheidt M, Schambra H, Steiner L, Widmer M, Diedrichsen J, et al. Rethinking interhemispheric imbalance as a target for stroke neurorehabilitation. Annals of Neurology. 2019; 85: 502–513. https://doi.org/10.1002/ana.25452.

[23] Werhahn KJ, Mortensen J, Kaelin-Lang A, Boroojerdi B, Cohen LG. Cortical excitability changes induced by deafferentation of the contralateral hemisphere. Brain. 2002; 125: 1402–1413. https://doi.org/10.1093/brain/awf140.

[24] Stagg CJ, Nitsche MA. Physiological basis of transcranial direct current stimulation. The Neuroscientist. 2011; 17: 37–53. https://doi.org/10.1177/1073858410386614.

[25] Eisenstein T, Furman-Haran E, Tal A. Early excitatory-inhibitory cortical modifications following skill learning are associated with motor memory consolidation and plasticity overnight. Nature Communications. 2024; 15: 906. https://doi.org/10.1038/s41467-024-44979-9.

[26] Andrushko JW, Zirk DT. Functional relevance of stroke-related losses in GABA_B -mediated interhemispheric inhibition for alternative modes of stroke recovery. The Journal of Physiology. 2018; 596: 3463–3464. https://doi.org/10.1113/JP276555.

[27] Clarkson AN, Huang BS, Macisaac SE, Mody I, Carmichael ST. Reducing excessive GABA-mediated tonic inhibition promotes functional recovery after stroke. Nature. 2010; 468: 305–309. https://doi.org/10.1038/nature09511.

[28] Hummel FC, Voller B, Celnik P, Floel A, Giraux P, Gerloff C, et al. Effects of brain polarization on reaction times and pinch force in chronic stroke. BMC Neuroscience. 2006; 7: 73. https://doi.org/10.1186/1471-2202-7-73.

[29] Fritsch B, Reis J, Martinowich K, Schambra HM, Ji Y, Cohen LG, et al. Direct current stimulation promotes BDNF-dependent synaptic plasticity: potential implications for motor learning. Neuron. 2010; 66: 198–204. https://doi.org/10.1016/j.neuron.2010.03.035.

[30] Figlewski K, Blicher JU, Mortensen J, Severinsen KE, Nielsen JF, Andersen H. Transcranial Direct Current Stimulation Potentiates Improvements in Functional Ability in Patients With Chronic Stroke Receiving Constraint-Induced Movement Therapy. Stroke. 2017; 48: 229–232. https://doi.org/10.1161/STROKEAHA.116.014988.

[31] Abdullahi A, Truijen S, Saeys W. Neurobiology of Recovery of Motor Function after Stroke: The Central Nervous System Biomarker Effects of Constraint-Induced Movement Therapy. Neural Plasticity. 2020; 2020: 9484298. https://doi.org/10.1155/2020/9484298.

[32] Garrido M M, Álvarez E E, Acevedo P F, Moyano V Á Castillo N N, Cavada Ch G. Early transcranial direct current stimulation with modified constraint-induced movement therapy for motor and functional upper limb recovery in hospitalized patients with stroke: A randomized, multicentre, double-blind, clinical trial. Brain Stimulation. 2023; 16: 40–47. https://doi.org/10.1016/j.brs.2022.12.008.

[33] Bachtiar V, Johnstone A, Berrington A, Lemke C, Johansen-Berg H, Emir U, et al. Modulating Regional Motor Cortical Excitability with Noninvasive Brain Stimulation Results in Neurochemical Changes in Bilateral Motor Cortices. The Journal of Neuroscience. 2018; 38: 7327–7336. https://doi.org/10.1523/JNEUROSCI.2853-17.2018.

[34] McCambridge AB, Stinear JW, Byblow WD. Revisiting interhemispheric imbalance in chronic stroke: A tDCS study. Clinical Neurophysiology. 2018; 129: 42–50. https://doi.org/10.1016/j.clinph.2017.10.016.

[35] Schwab BC, Misselhorn J, Engel AK. Modulation of large-scale cortical coupling by transcranial alternating current stimulation. Brain Stimulation. 2019; 12: 1187–1196. https://doi.org/10.1016/j.brs.2019.04.013.

[36] Fregni F, Pascual-Leone A. Technology insight: noninvasive brain stimulation in neurology-perspectives on the therapeutic potential of rTMS and tDCS. Nature Clinical Practice. Neurology. 2007; 3: 383–393. https://doi.org/10.1038/ncpneuro0530.

[37] Shinde AB, Lerud KD, Munsch F, Alsop DC, Schlaug G. Effects of tDCS dose and electrode montage on regional cerebral blood flow and motor behavior. NeuroImage. 2021; 237: 118144. https://doi.org/10.1016/j.neuroimage.2021.118144.

[38] Yoon KJ, Oh BM, Kim DY. Functional improvement and neuroplastic effects of anodal transcranial direct current stimulation (tDCS) delivered 1 day vs. 1 week after cerebral ischemia in rats. Brain Research. 2012; 1452: 61–72. https://doi.org/10.1016/j.brainres.2012.02.062.

[39] Pruvost-Robieux E, Benzakoun J, Turc G, Marchi A, Mancusi RL, Lamy C, et al. Cathodal Transcranial Direct Current Stimulation in Acute Ischemic Stroke: Pilot Randomized Controlled Trial. Stroke. 2021; 52: 1951–1960. https://doi.org/10.1161/STROKEAHA.120.032056.

[40] Bahr-Hosseini M, Nael K, Unal G, Iacoboni M, Liebeskind DS, Bikson M, et al. High-definition Cathodal Direct Current Stimulation for Treatment of Acute Ischemic Stroke: A Randomized Clinical Trial. JAMA Network Open. 2023; 6: e2319231. https://doi.org/10.1001/jamanetworkopen.2023.19231.

[41] Carmichael ST. Brain excitability in stroke: the yin and yang of stroke progression. Archives of Neurology. 2012; 69: 161–167. https://doi.org/10.1001/archneurol.2011.1175.

[42] Finger S. Chapter 51: recovery of function: redundancy and vicariation theories. Handbook of Clinical Neurology. 2010; 95: 833–841. https://doi.org/10.1016/S0072-9752(08)02151-9.

[43] Jaillard A, Martin CD, Garambois K, Lebas JF, Hommel M. Vicarious function within the human primary motor cortex? A longitudinal fMRI stroke study. Brain. 2005; 128: 1122–1138. https://doi.org/10.1093/brain/awh456.

[44] Murphy TH, Corbett D. Plasticity during stroke recovery: from synapse to behaviour. Nature Reviews. Neuroscience. 2009; 10: 861–872. https://doi.org/10.1038/nrn2735.

[45] Stinear CM, Petoe MA, Byblow WD. Primary Motor Cortex Excitability During Recovery After Stroke: Implications for Neuromodulation. Brain Stimulation. 2015; 8: 1183–1190. https://doi.org/10.1016/j.brs.2015.06.015.

[46] Bertolucci F, Chisari C, Fregni F. The potential dual role of transcallosal inhibition in post-stroke motor recovery. Restorative Neurology and Neuroscience. 2018; 36: 83–97. https://doi.org/10.3233/RNN-170778.

[47] Di Pino G, Di Lazzaro V. The balance recovery bimodal model in stroke patients between evidence and speculation: Do recent studies support it? Clinical Neurophysiology. 2020; 131: 2488–2490. https://doi.org/10.1016/j.clinph.2020.07.004.

[48] McDonnell MN, Stinear CM. TMS measures of motor cortex function after stroke: A meta-analysis. Brain Stimulation. 2017; 10: 721–734. https://doi.org/10.1016/j.brs.2017.03.008.

[49] Lin YL, Potter-Baker KA, Cunningham DA, Li M, Sankarasubramanian V, Lee J, et al. Stratifying chronic stroke patients based on the influence of contralesional motor cortices: An inter-hemispheric inhibition study. Clinical Neurophysiology. 2020; 131: 2516–2525. https://doi.org/10.1016/j.clinph.2020.06.016.

[50] Duan G, Yuan Y, Hua H, Zhang X, Liu B, Li Y, et al. Effect of transcranial direct current stimulation on swallowing improvement and cortical activity in hemispheric stroke patients: a randomized, controlled trial. Scientific Reports. 2025; 15: 19586. https://doi.org/10.1038/s41598-025-04939-9.

[51] Iyer PC, Madhavan S. Non-invasive brain stimulation in the modulation of cerebral blood flow after stroke: A systematic review of Transcranial Doppler studies. Clinical Neurophysiology. 2018; 129: 2544–2551. https://doi.org/10.1016/j.clinph.2018.09.019.

[52] Kim SY, Jones TA. Lesion size-dependent synaptic and astrocytic responses in cortex contralateral to infarcts in middle-aged rats. Synapse. 2010; 64: 659–671. https://doi.org/10.1002/syn.20777.

[53] Biernaskie J, Szymanska A, Windle V, Corbett D. Bi-hemispheric contribution to functional motor recovery of the affected forelimb following focal ischemic brain injury in rats. The European Journal of Neuroscience. 2005; 21: 989–999. https://doi.org/10.1111/j.1460-9568.2005.03899.x.

[54] Ahn SM, Jung DH, Lee HJ, Pak ME, Jung YJ, Shin YI, et al. Contralesional Application of Transcranial Direct Current Stimulation on Functional Improvement in Ischemic Stroke Mice. Stroke. 2020; 51: 2208–2218. https://doi.org/10.1161/STROKEAHA.120.029221.

[55] Jung DH, Lee JH, Lee HJ, Park JW, Jung YJ, Shin HK, et al. Therapeutic effects of a novel electrode for transcranial direct current stimulation in ischemic stroke mice. Theranostics. 2024; 14: 1325–1343. https://doi.org/10.7150/thno.90779.

[56] Buzsáki G, Draguhn A. Neuronal oscillations in cortical networks. Science. 2004; 304: 1926–1929. https://doi.org/10.1126/science.1099745.

[57] Csicsvari J, Jamieson B, Wise KD, Buzsáki G. Mechanisms of gamma oscillations in the hippocampus of the behaving rat. Neuron. 2003; 37: 311–322. https://doi.org/10.1016/s0896-6273(02)01169-8.

[58] Finnigan SP, Rose SE, Walsh M, Griffin M, Janke AL, McMahon KL, et al. Correlation of quantitative EEG in acute ischemic stroke with 30-day NIHSS score: comparison with diffusion and perfusion MRI. Stroke. 2004; 35: 899–903. https://doi.org/10.1161/01.STR.0000122622.73916.d2.

[59] Biskamp J, Isla Cainzos S, Higgen FL, Gerloff C, Magnus T. Normalization of Aperiodic Electrocorticography Components Indicates Fine Motor Recovery After Sensory Cortical Stroke in Mice. Stroke. 2022; 53: 2945–2953. https://doi.org/10.1161/STROKEAHA.122.039335.

[60] Pogosyan A, Gaynor LD, Eusebio A, Brown P. Boosting cortical activity at Beta-band frequencies slows movement in humans. Current Biology: CB. 2009; 19: 1637–1641. https://doi.org/10.1016/j.cub.2009.07.074.

[61] Polanía R, Nitsche MA, Korman C, Batsikadze G, Paulus W. The importance of timing in segregated theta phase-coupling for cognitive performance. Current Biology. 2012; 22: 1314–1318. https://doi.org/10.1016/j.cub.2012.05.021.

[62] Witkowski M, Garcia-Cossio E, Chander BS, Braun C, Birbaumer N, Robinson SE, et al. Mapping entrained brain oscillations during transcranial alternating current stimulation (tACS). NeuroImage. 2016; 140: 89–98. https://doi.org/10.1016/j.neuroimage.2015.10.024.

[63] Jacobson L, Ezra A, Berger U, Lavidor M. Modulating oscillatory brain activity correlates of behavioral inhibition using transcranial direct current stimulation. Clinical Neurophysiology. 2012; 123: 979–984. https://doi.org/10.1016/j.clinph.2011.09.016.

[64] Wang C, Yang X, Guo D, Huo W, Yu N, Zhang Y. Transcranial direct current stimulation-induced changes in motor cortical connectivity are associated with motor gains following ischemic stroke. Scientific Reports. 2024; 14: 15645. https://doi.org/10.1038/s41598-024-66464-5.

[65] Carmichael ST. Cellular and molecular mechanisms of neural repair after stroke: making waves. Annals of Neurology. 2006; 59: 735–742. https://doi.org/10.1002/ana.20845.

[66] Liu M, Xu G, Yu H, Wang C, Sun C, Guo L. Effects of Transcranial Direct Current Stimulation on EEG Power and Brain Functional Network in Stroke Patients. IEEE Transactions on Neural Systems and Rehabilitation Engineering. 2023; 31: 335–345. https://doi.org/10.1109/TNSRE.2022.3223116.

[67] Kitatani R, Koganemaru S, Maeda A, Mikami Y, Matsuhashi M, Mima T, et al. Gait-synchronized oscillatory brain stimulation modulates common neural drives to ankle muscles in patients after stroke: A pilot study. Neuroscience Research. 2020; 156: 256–264. https://doi.org/10.1016/j.neures.2019.11.001.

[68] Yuan K, Chen C, Lou WT, Khan A, Ti ECH, Lau CCY, et al. Differential Effects of 10 and 20 Hz Brain Stimulation in Chronic Stroke: A tACS-fMRI Study. IEEE Transactions on Neural Systems and Rehabilitation Engineering. 2022; 30: 455–464. https://doi.org/10.1109/TNSRE.2022.3153353.

[69] Omae E, Shima A, Tanaka K, Yamada M, Cao Y, Nakamura T, et al. Case report: An N-of-1 study using amplitude modulated transcranial alternating current stimulation between Broca’s area and the right homotopic area to improve post-stroke aphasia with increased inter-regional synchrony. Frontiers in Human Neuroscience. 2024; 18: 1297683. https://doi.org/10.3389/fnhum.2024.1297683.

[70] Lee H, Lee J, Jung D, Oh H, Shin H, Choi B. Neuroprotection of Transcranial Cortical and Peripheral Somatosensory Electrical Stimulation by Modulating a Common Neuronal Death Pathway in Mice with Ischemic Stroke. International Journal of Molecular Sciences. 2024; 25: 7546. https://doi.org/10.3390/ijms25147546.

[71] Johnson L, Alekseichuk I, Krieg J, Doyle A, Yu Y, Vitek J, et al. Dose-dependent effects of transcranial alternating current stimulation on spike timing in awake nonhuman primates. Science Advances. 2020; 6: eaaz2747. https://doi.org/10.1126/sciadv.aaz2747.

[72] Schjetnan AG, Gidyk DC, Metz GA, Luczak A. Anodal transcranial direct current stimulation with monopolar pulses improves limb use after stroke by enhancing inter-hemispheric coherence. Acta Neurobiologiae Experimentalis. 2019; 79: 290–301.

[73] Monai H, Ohkura M, Tanaka M, Oe Y, Konno A, Hirai H, et al. Calcium imaging reveals glial involvement in transcranial direct current stimulation-induced plasticity in mouse brain. Nature Communications. 2016; 7: 11100. https://doi.org/10.1038/ncomms11100.

[74] Wischnewski M, Engelhardt M, Salehinejad MA, Schutter DJLG, Kuo MF, Nitsche MA. NMDA Receptor-Mediated Motor Cortex Plasticity After 20 Hz Transcranial Alternating Current Stimulation. Cerebral Cortex. 2019; 29: 2924–2931. https://doi.org/10.1093/cercor/bhy160.

[75] Weber MJ, Messing SB, Rao H, Detre JA, Thompson-Schill SL. Prefrontal transcranial direct current stimulation alters activation and connectivity in cortical and subcortical reward systems: a tDCS-fMRI study. Human Brain Mapping. 2014; 35: 3673–3686. https://doi.org/10.1002/hbm.22429.

[76] Bao SC, Khan A, Song R, Kai-Yu Tong R. Rewiring the Lesioned Brain: Electrical Stimulation for Post-Stroke Motor Restoration. Journal of Stroke. 2020; 22: 47–63. https://doi.org/10.5853/jos.2019.03027.

[77] Hong X, Lu ZK, Teh I, Nasrallah FA, Teo WP, Ang KK, et al. Brain plasticity following MI-BCI training combined with tDCS in a randomized trial in chronic subcortical stroke subjects: a preliminary study. Scientific Reports. 2017; 7: 9222. https://doi.org/10.1038/s41598-017-08928-5.

[78] Amaya-Pascasio L, García-Pinteño J, Sánchez-Kuhn A, Uceda Sánchez C, Fernández-Martín P, León Domene JJ, et al. Neuromodulation of Executive Dysfunction in Patients with Acute Stroke Using Transcranial Direct Current Stimulation: Study Protocol for a Triple-Blind, Randomized Sham-Controlled Trial. Cerebrovascular Diseases. 2024; 53: 335–345. https://doi.org/10.1159/000531860.

[79] Zhang X, Meesen R, Swinnen SP, Feys H, Woolley DG, Cheng HJ, et al. Combining muscle-computer interface guided training with bihemispheric tDCS improves upper limb function in patients with chronic stroke. Journal of Neurophysiology. 2024; 131: 1286–1298. https://doi.org/10.1152/jn.00316.2023.

[80] Kim MS, Koo H, Han SW, Paulus W, Nitsche MA, Kim YH, et al. Repeated anodal transcranial direct current stimulation induces neural plasticity-associated gene expression in the rat cortex and hippocampus. Restorative Neurology and Neuroscience. 2017; 35: 137–146. https://doi.org/10.3233/RNN-160689.

[81] Barbati SA, Cocco S, Longo V, Spinelli M, Gironi K, Mattera A, et al. Enhancing Plasticity Mechanisms in the Mouse Motor Cortex by Anodal Transcranial Direct-Current Stimulation: The Contribution of Nitric Oxide Signaling. Cerebral Cortex. 2020; 30: 2972–2985. https://doi.org/10.1093/cercor/bhz288.

[82] Dai C, Wang J, Li J, Wang J, Zhang L, Yin C, et al. Repetitive anodal transcranial direct current stimulation improves neurological recovery by preserving the neuroplasticity in an asphyxial rat model of cardiac arrest. Brain Stimulation. 2021; 14: 407–416. https://doi.org/10.1016/j.brs.2021.02.008.

[83] Longo V, Barbati SA, Re A, Paciello F, Bolla M, Rinaudo M, et al. Transcranial Direct Current Stimulation Enhances Neuroplasticity and Accelerates Motor Recovery in a Stroke Mouse Model. Stroke. 2022; 53: 1746–1758. https://doi.org/10.1161/STROKEAHA.121.034200.

[84] Chen Y, Mao L, Zhou Q, Bai D, Kong Y. Role of BDNF-TrkB signaling in the improvement of motor function and neuroplasticity after ischemic stroke in rats by transcranial direct current stimulation. Brain Research Bulletin. 2025; 220: 111164. https://doi.org/10.1016/j.brainresbull.2024.111164.

[85] Nudo RJ, Milliken GW, Jenkins WM, Merzenich MM. Use-dependent alterations of movement representations in primary motor cortex of adult squirrel monkeys. The Journal of Neuroscience. 1996; 16: 785–807. https://doi.org/10.1523/JNEUROSCI.16-02-00785.1996.

[86] Nudo RJ. Plasticity. NeuroRx. 2006; 3: 420–427. https://doi.org/10.1016/j.nurx.2006.07.006.

[87] Taub E, Uswatte G, Elbert T. New treatments in neurorehabilitation founded on basic research. Nature Reviews. Neuroscience. 2002; 3: 228–236. https://doi.org/10.1038/nrn754.

[88] Hebb DO. Organization of Behavior: A Neuropsychological Theory. Wiley: New York, NY. 1949.

[89] Ziemann U, Ilić TV, Pauli C, Meintzschel F, Ruge D. Learning modifies subsequent induction of long-term potentiation-like and long-term depression-like plasticity in human motor cortex. The Journal of Neuroscience. 2004; 24: 1666–1672. https://doi.org/10.1523/JNEUROSCI.5016-03.2004.

[90] Buetefisch C, Heger R, Schicks W, Seitz R, Netz J. Hebbian-type stimulation during robot-assisted training in patients with stroke. Neurorehabilitation and Neural Repair. 2011; 25: 645–655. https://doi.org/10.1177/1545968311402507.

[91] Stefan K, Kunesch E, Cohen LG, Benecke R, Classen J. Induction of plasticity in the human motor cortex by paired associative stimulation. Brain. 2000; 123: 572–584. https://doi.org/10.1093/brain/123.3.572.

[92] Ranieri F, Podda MV, Riccardi E, Frisullo G, Dileone M, Profice P, et al. Modulation of LTP at rat hippocampal CA3-CA1 synapses by direct current stimulation. Journal of Neurophysiology. 2012; 107: 1868–1880. https://doi.org/10.1152/jn.00319.2011.

[93] Kronberg G, Bridi M, Abel T, Bikson M, Parra LC. Direct Current Stimulation Modulates LTP and LTD: Activity Dependence and Dendritic Effects. Brain Stimulation. 2017; 10: 51–58. https://doi.org/10.1016/j.brs.2016.10.001.

[94] Frase L, Mertens L, Krahl A, Bhatia K, Feige B, Heinrich SP, et al. Transcranial direct current stimulation induces long-term potentiation-like plasticity in the human visual cortex. Translational Psychiatry. 2021; 11: 17. https://doi.org/10.1038/s41398-020-01134-4.

[95] Kronberg G, Rahman A, Sharma M, Bikson M, Parra LC. Direct current stimulation boosts hebbian plasticity in vitro. Brain Stimulation. 2020; 13: 287–301. https://doi.org/10.1016/j.brs.2019.10.014.

[96] Weightman M, Brittain JS, Hall A, Miall RC, Jenkinson N. Timing is everything: Event-related transcranial direct current stimulation improves motor adaptation. Brain Stimulation. 2022; 15: 750–757. https://doi.org/10.1016/j.brs.2022.05.003.

[97] McNickle E, Carson RG. Paired associative transcranial alternating current stimulation increases the excitability of corticospinal projections in humans. The Journal of Physiology. 2015; 593: 1649–1666. https://doi.org/10.1113/jphysiol.2014.280453.

[98] Abraham WC, Bear MF. Metaplasticity: the plasticity of synaptic plasticity. Trends in Neurosciences. 1996; 19: 126–130. https://doi.org/10.1016/s0166-2236(96)80018-x.

[99] Zenke F, Gerstner W, Ganguli S. The temporal paradox of Hebbian learning and homeostatic plasticity. Current Opinion in Neurobiology. 2017; 43: 166–176. https://doi.org/10.1016/j.conb.2017.03.015.

[100] Bienenstock EL, Cooper LN, Munro PW. Theory for the development of neuron selectivity: orientation specificity and binocular interaction in visual cortex. The Journal of Neuroscience. 1982; 2: 32–48. https://doi.org/10.1523/JNEUROSCI.02-01-00032.1982.

[101] Narayanan R, Johnston D. The h current is a candidate mechanism for regulating the sliding modification threshold in a BCM-like synaptic learning rule. Journal of Neurophysiology. 2010; 104: 1020–1033. https://doi.org/10.1152/jn.01129.2009.

[102] Müller-Dahlhaus F, Ziemann U. Metaplasticity in human cortex. The Neuroscientist. 2015; 21: 185–202. https://doi.org/10.1177/1073858414526645.

[103] Pourmajidian M, Lauber B, Sidhu SK. Preconditioning cathodal transcranial direct current stimulation facilitates the neuroplastic effect of subsequent anodal transcranial direct current stimulation applied during cycling in young adults. Neuroscience Letters. 2020; 714: 134597. https://doi.org/10.1016/j.neulet.2019.134597.

[104] Bian L, Zhang L, Huang G, Song D, Zheng K, Xu X, et al. Effects of Priming Intermittent Theta Burst Stimulation With High-Definition tDCS on Upper Limb Function in Hemiparetic Patients With Stroke: A Randomized Controlled Study. Neurorehabilitation and Neural Repair. 2024; 38: 268–278. https://doi.org/10.1177/15459683241233259.

[105] Trudgen A, Cirillo J, Byblow WD. Somatosensory and transcranial direct current stimulation effects on manual dexterity and motor cortex function: A metaplasticity study. Brain Stimulation. 2019; 12: 938–947. https://doi.org/10.1016/j.brs.2019.02.010.

[106] Schabrun SM, Chipchase LS, Zipf N, Thickbroom GW, Hodges PW. Interaction between simultaneously applied neuromodulatory interventions in humans. Brain Stimulation. 2013; 6: 624–630. https://doi.org/10.1016/j.brs.2012.09.009.

[107] Kuo MF, Unger M, Liebetanz D, Lang N, Tergau F, Paulus W, et al. Limited impact of homeostatic plasticity on motor learning in humans. Neuropsychologia. 2008; 46: 2122–2128. https://doi.org/10.1016/j.neuropsychologia.2008.02.023.

[108] Tsuiki S, Sasaki R, Miyaguchi S, Kojima S, Saito K, Inukai Y, et al. The effect of combined transcranial direct current stimulation and peripheral nerve electrical stimulation on corticospinal excitability. PLoS ONE. 2019; 14: e0214592. https://doi.org/10.1371/journal.pone.0214592.

[109] Boda MR, Otieno LA, Smith AE, Goldsworthy MR, Sidhu SK. Metaplastic neuromodulation via transcranial direct current stimulation has no effect on corticospinal excitability and neuromuscular fatigue. Experimental Brain Research. 2024; 242: 1999–2012. https://doi.org/10.1007/s00221-024-06874-z.

[110] Braun R, Klein R, Walter HL, Ohren M, Freudenmacher L, Getachew K, et al. Transcranial direct current stimulation accelerates recovery of function, induces neurogenesis and recruits oligodendrocyte precursors in a rat model of stroke. Experimental Neurology. 2016; 279: 127–136. https://doi.org/10.1016/j.expneurol.2016.02.018.

[111] Colgan LA, Hu M, Misler JA, Parra-Bueno P, Moran CM, Leitges M, et al. PKCα integrates spatiotemporally distinct Ca²⁺ and autocrine BDNF signaling to facilitate synaptic plasticity. Nature Neuroscience. 2018; 21: 1027–1037. https://doi.org/10.1038/s41593-018-0184-3.

[112] Iordanova B, Vazquez A, Kozai TD, Fukuda M, Kim SG. Optogenetic investigation of the variable neurovascular coupling along the interhemispheric circuits. Journal of Cerebral Blood Flow and Metabolism. 2018; 38: 627–640. https://doi.org/10.1177/0271678X18755225.

[113] Lee HJ, Jung DH, Jung YJ, Shin HK, Choi BT. Transcranial alternating current stimulation rescues motor deficits in a mouse model of Parkinson’s disease via the production of glial cell line-derived neurotrophic factor. Brain Stimulation. 2022; 15: 645–653. https://doi.org/10.1016/j.brs.2022.04.002.

[114] Marangolo P, Fiori V, Gelfo F, Shofany J, Razzano C, Caltagirone C, et al. Bihemispheric tDCS enhances language recovery but does not alter BDNF levels in chronic aphasic patients. Restorative Neurology and Neuroscience. 2014; 32: 367–379. https://doi.org/10.3233/RNN-130323.

[115] van der Vliet R, Ribbers GM, Vandermeeren Y, Frens MA, Selles RW. BDNF Val66Met but not transcranial direct current stimulation affects motor learning after stroke. Brain Stimulation. 2017; 10: 882–892. https://doi.org/10.1016/j.brs.2017.07.004.

[116] Fridriksson J, Elm J, Stark BC, Basilakos A, Rorden C, Sen S, et al. BDNF genotype and tDCS interaction in aphasia treatment. Brain Stimulation. 2018; 11: 1276–1281. https://doi.org/10.1016/j.brs.2018.08.009.

[117] Ai Y, Liu Y, Yin M, Zhang L, Luo J, Zhang S, et al. Interactions between tDCS treatment and COMT Val158Met in poststroke cognitive impairment. Clinical Neurophysiology. 2024; 158: 43–55. https://doi.org/10.1016/j.clinph.2023.12.011.

[118] Jung DH, Ahn SM, Pak ME, Lee HJ, Jung YJ, Kim KB, et al. Therapeutic effects of anodal transcranial direct current stimulation in a rat model of ADHD. eLife. 2020; 9: e56359. https://doi.org/10.7554/eLife.56359.

[119] Zhou Q, Chen Y, Tang H, Zhang L, Ma Y, Bai D, et al. Transcranial direct current stimulation alleviated ischemic stroke induced injury involving the BDNF-TrkB signaling axis in rats. Heliyon. 2023; 9: e14946. https://doi.org/10.1016/j.heliyon.2023.e14946.

[120] Yeh HJ, Ruit KG, Wang YX, Parks WC, Snider WD, Deuel TF. PDGF A-chain gene is expressed by mammalian neurons during development and in maturity. Cell. 1991; 64: 209–216. https://doi.org/10.1016/0092-8674(91)90222-k.

[121] Murtie JC, Zhou YX, Le TQ, Vana AC, Armstrong RC. PDGF and FGF2 pathways regulate distinct oligodendrocyte lineage responses in experimental demyelination with spontaneous remyelination. Neurobiology of Disease. 2005; 19: 171–182. https://doi.org/10.1016/j.nbd.2004.12.006.

[122] Schäbitz WR, Steigleder T, Cooper-Kuhn CM, Schwab S, Sommer C, Schneider A, et al. Intravenous brain-derived neurotrophic factor enhances poststroke sensorimotor recovery and stimulates neurogenesis. Stroke. 2007; 38: 2165–2172. https://doi.org/10.1161/STROKEAHA.106.477331.

[123] Choi SH, Bylykbashi E, Chatila ZK, Lee SW, Pulli B, Clemenson GD, et al. Combined adult neurogenesis and BDNF mimic exercise effects on cognition in an Alzheimer’s mouse model. Science. 2018; 361: eaan8821. https://doi.org/10.1126/science.aan8821.

[124] Pikhovych A, Stolberg NP, Jessica Flitsch L, Walter HL, Graf R, Fink GR, et al. Transcranial Direct Current Stimulation Modulates Neurogenesis and Microglia Activation in the Mouse Brain. Stem Cells International. 2016; 2016: 2715196. https://doi.org/10.1155/2016/2715196.

[125] Walter HL, Pikhovych A, Endepols H, Rotthues S, Bärmann J, Backes H, et al. Transcranial-Direct-Current-Stimulation Accelerates Motor Recovery After Cortical Infarction in Mice: The Interplay of Structural Cellular Responses and Functional Recovery. Neurorehabilitation and Neural Repair. 2022; 36: 701–714. https://doi.org/10.1177/15459683221124116.

[126] Zhang K, Guo L, Zhang J, Rui G, An G, Zhou Y, et al. tDCS Accelerates the Rehabilitation of MCAO-Induced Motor Function Deficits via Neurogenesis Modulated by the Notch1 Signaling Pathway. Neurorehabilitation and Neural Repair. 2020; 34: 640–651. https://doi.org/10.1177/1545968320925474.

[127] Feeney DM, Baron JC. Diaschisis. Stroke. 1986; 17: 817–830. https://doi.org/10.1161/01.str.17.5.817.

[128] Finger S, Koehler PJ, Jagella C. The Monakow concept of diaschisis: origins and perspectives. Archives of Neurology. 2004; 61: 283–288. https://doi.org/10.1001/archneur.61.2.283.

[129] Weiller C, Ramsay SC, Wise RJ, Friston KJ, Frackowiak RS. Individual patterns of functional reorganization in the human cerebral cortex after capsular infarction. Annals of Neurology. 1993; 33: 181–189. https://doi.org/10.1002/ana.410330208.

[130] Cramer SC, Nelles G, Benson RR, Kaplan JD, Parker RA, Kwong KK, et al. A functional MRI study of subjects recovered from hemiparetic stroke. Stroke. 1997; 28: 2518–2527. https://doi.org/10.1161/01.str.28.12.2518.

[131] Ward NS, Brown MM, Thompson AJ, Frackowiak RSJ. Neural correlates of motor recovery after stroke: a longitudinal fMRI study. Brain. 2003; 126: 2476–2496. https://doi.org/10.1093/brain/awg245.

[132] Dancause N, Nudo RJ. Shaping plasticity to enhance recovery after injury. Progress in Brain Research. 2011; 192: 273–295. https://doi.org/10.1016/B978-0-444-53355-5.00015-4.

[133] Girouard H, Iadecola C. Neurovascular coupling in the normal brain and in hypertension, stroke, and Alzheimer disease. Journal of Applied Physiology. 2006; 100: 328–335. https://doi.org/10.1152/japplphysiol.00966.2005.

[134] Iadecola C. The Neurovascular Unit Coming of Age: A Journey through Neurovascular Coupling in Health and Disease. Neuron. 2017; 96: 17–42. https://doi.org/10.1016/j.neuron.2017.07.030.

[135] Iyer PC, Rosenberg A, Baynard T, Madhavan S. Influence of neurovascular mechanisms on response to tDCS: an exploratory study. Experimental Brain Research. 2019; 237: 2829–2840. https://doi.org/10.1007/s00221-019-05626-8.

[136] Kim H, Kim J, Lee G, Lee J, Kim YH. Task-Related Hemodynamic Changes Induced by High-Definition Transcranial Direct Current Stimulation in Chronic Stroke Patients: An Uncontrolled Pilot fNIRS Study. Brain Sciences. 2022; 12: 453. https://doi.org/10.3390/brainsci12040453.

[137] Nascimento MR, Bernardes TS, Santos KCS, Silva GF, Bazan R, Sande de Souza LAP, et al. Effects of anodal transcranial direct current stimulation on intracranial compliance in the subacute phase of stroke. Clinical Neurology and Neurosurgery. 2024; 247: 108597. https://doi.org/10.1016/j.clineuro.2024.108597.

[138] Wu JF, Wang HJ, Wu Y, Li F, Bai YL, Zhang PY, et al. Efficacy of transcranial alternating current stimulation over bilateral mastoids (tACS_b⁢m) on enhancing recovery of subacute post-stroke patients. Topics in Stroke Rehabilitation. 2016; 23: 420–429. https://doi.org/10.1080/10749357.2016.1175218.

[139] Chen Y, Zhao Z, Huang J, Wang T, Qu Y. Computer-aided cognitive training combined with tDCS can improve post-stroke cognitive impairment and cerebral vasomotor function: a randomized controlled trial. BMC Neurology. 2024; 24: 132. https://doi.org/10.1186/s12883-024-03613-3.

[140] Vernieri F, Assenza G, Maggio P, Tibuzzi F, Zappasodi F, Altamura C, et al. Cortical neuromodulation modifies cerebral vasomotor reactivity. Stroke. 2010; 41: 2087–2090. https://doi.org/10.1161/STROKEAHA.110.583088.

[141] Guhathakurta D, Dutta A. Computational Pipeline for NIRS-EEG Joint Imaging of tDCS-Evoked Cerebral Responses-An Application in Ischemic Stroke. Frontiers in Neuroscience. 2016; 10: 261. https://doi.org/10.3389/fnins.2016.00261.

[142] Takai H, Tsubaki A, Sugawara K, Miyaguchi S, Oyanagi K, Matsumoto T, et al. Effect of Transcranial Direct Current Stimulation over the Primary Motor Cortex on Cerebral Blood Flow: A Time Course Study Using Near-infrared Spectroscopy. Advances in Experimental Medicine and Biology. 2016; 876: 335–341. https://doi.org/10.1007/978-1-4939-3023-4_42.

[143] Kim H, Lee G, Lee J, Kim YH. Alterations in learning-related cortical activation and functional connectivity by high-definition transcranial direct current stimulation after stroke: an fNIRS study. Frontiers in Neuroscience. 2023; 17: 1189420. https://doi.org/10.3389/fnins.2023.1189420.

[144] Hu S, Zheng T, Dong Y, Du J, Liu L. Effect of Anodal Direct-Current Stimulation on Cortical Hemodynamic Responses With Laser-Speckle Contrast Imaging. Frontiers in Neuroscience. 2018; 12: 503. https://doi.org/10.3389/fnins.2018.00503.

[145] Wachter D, Wrede A, Schulz-Schaeffer W, Taghizadeh-Waghefi A, Nitsche MA, Kutschenko A, et al. Transcranial direct current stimulation induces polarity-specific changes of cortical blood perfusion in the rat. Experimental Neurology. 2011; 227: 322–327. https://doi.org/10.1016/j.expneurol.2010.12.005.

[146] Malinova V, Bleuel K, Stadelmann C, Iliev B, Tsogkas I, Psychogios MN, et al. The impact of transcranial direct current stimulation on cerebral vasospasm in a rat model of subarachnoid hemorrhage. Journal of Cerebral Blood Flow and Metabolism. 2021; 41: 2000–2009. https://doi.org/10.1177/0271678X21990130.

[147] Turner DA, Degan S, Galeffi F, Schmidt S, Peterchev AV. Rapid, Dose-Dependent Enhancement of Cerebral Blood Flow by transcranial AC Stimulation in Mouse. Brain Stimulation. 2021; 14: 80–87. https://doi.org/10.1016/j.brs.2020.11.012.

[148] Degan S, Feng Y, Hoffmann U, Turner DA. Placement of Extracranial Stimulating Electrodes and Measurement of Cerebral Blood Flow and Intracranial Electrical Fields in Anesthetized Mice. Journal of Visualized Experiments. 2023; 10.3791/65195. https://doi.org/10.3791/65195.

[149] Moskowitz MA, Lo EH, Iadecola C. The science of stroke: mechanisms in search of treatments. Neuron. 2010; 67: 181–198. https://doi.org/10.1016/j.neuron.2010.07.002.

[150] Jung JY, Lee HS, Kang DG, Kim NS, Cha MH, Bang OS, et al. 1H-NMR-based metabolomics study of cerebral infarction. Stroke. 2011; 42: 1282–1288. https://doi.org/10.1161/STROKEAHA.110.598789.

[151] Lin MT, Beal MF. Mitochondrial dysfunction and oxidative stress in neurodegenerative diseases. Nature. 2006; 443: 787–795. https://doi.org/10.1038/nature05292.

[152] Mattson MP, Gleichmann M, Cheng A. Mitochondria in neuroplasticity and neurological disorders. Neuron. 2008; 60: 748–766. https://doi.org/10.1016/j.neuron.2008.10.010.

[153] Binkofski F, Loebig M, Jauch-Chara K, Bergmann S, Melchert UH, Scholand-Engler HG, et al. Brain energy consumption induced by electrical stimulation promotes systemic glucose uptake. Biological Psychiatry. 2011; 70: 690–695. https://doi.org/10.1016/j.biopsych.2011.05.009.

[154] Azarnia S, Ezzati K, Saberi A, Naghdi S, Abdollahi I, Jaberzadeh S. The Effect of Uni-Hemispheric Dual-Site Anodal tDCS on Brain Metabolic Changes in Stroke Patients: A Randomized Clinical Trial. Brain Sciences. 2023; 13: 1100. https://doi.org/10.3390/brainsci13071100.

[155] Wang H, Ma W, Hu W, Li X, Shen N, Li Z, et al. Cathodal bilateral transcranial direct-current stimulation regulates selenium to confer neuroprotection after rat cerebral ischaemia-reperfusion injury. The Journal of Physiology. 2024; 602: 1175–1197. https://doi.org/10.1113/JP285806.

[156] Koopman WJH, Willems PHGM, Smeitink JAM. Monogenic mitochondrial disorders. The New England Journal of Medicine. 2012; 366: 1132–1141. https://doi.org/10.1056/NEJMra1012478.

[157] Cheng A, Wan R, Yang JL, Kamimura N, Son TG, Ouyang X, et al. Involvement of PGC-1α in the formation and maintenance of neuronal dendritic spines. Nature Communications. 2012; 3: 1250. https://doi.org/10.1038/ncomms2238.

[158] Wrann CD, White JP, Salogiannnis J, Laznik-Bogoslavski D, Wu J, Ma D, et al. Exercise induces hippocampal BDNF through a PGC-1α/FNDC5 pathway. Cell Metabolism. 2013; 18: 649–659. https://doi.org/10.1016/j.cmet.2013.09.008.

[159] Astrup J, Siesjö BK, Symon L. Thresholds in cerebral ischemia - the ischemic penumbra. Stroke. 1981; 12: 723–725. https://doi.org/10.1161/01.str.12.6.723.

[160] Donnan GA, Baron JC, Ma H, Davis SM. Penumbral selection of patients for trials of acute stroke therapy. The Lancet. Neurology. 2009; 8: 261–269. https://doi.org/10.1016/S1474-4422(09)70041-9.

[161] Esposito E, Li W, T Mandeville E, Park JH, Şencan I, Guo S, et al. Potential circadian effects on translational failure for neuroprotection. Nature. 2020; 582: 395–398. https://doi.org/10.1038/s41586-020-2348-z.

[162] Tuo QZ, Zhang ST, Lei P. Mechanisms of neuronal cell death in ischemic stroke and their therapeutic implications. Medicinal Research Reviews. 2022; 42: 259–305. https://doi.org/10.1002/med.21817.

[163] Dirnagl U, Iadecola C, Moskowitz MA. Pathobiology of ischaemic stroke: an integrated view. Trends in Neurosciences. 1999; 22: 391–397. https://doi.org/10.1016/s0166-2236(99)01401-0.

[164] Qin C, Yang S, Chu YH, Zhang H, Pang XW, Chen L, et al. Signaling pathways involved in ischemic stroke: molecular mechanisms and therapeutic interventions. Signal Transduction and Targeted Therapy. 2022; 7: 215. https://doi.org/10.1038/s41392-022-01064-1.

[165] Rueger MA, Keuters MH, Walberer M, Braun R, Klein R, Sparing R, et al. Multi-session transcranial direct current stimulation (tDCS) elicits inflammatory and regenerative processes in the rat brain. PLoS ONE. 2012; 7: e43776. https://doi.org/10.1371/journal.pone.0043776.

[166] Zhang KY, Rui G, Zhang JP, Guo L, An GZ, Lin JJ, et al. Cathodal tDCS exerts neuroprotective effect in rat brain after acute ischemic stroke. BMC Neuroscience. 2020; 21: 21. https://doi.org/10.1186/s12868-020-00570-8.

[167] Li XD, Ning YX, Pei YF, Niu JY, Luo J, Zhang YN, et al. Cathodal Transcranial Direct Current Stimulation Attenuates Cerebral Ischemia-Reperfusion Injury by Coordinating Mitophagy Inhibition and Nrf2 Activation Against Ferroptosis. Antioxidants & Redox Signaling. 2025; 43: 693–708. https://doi.org/10.1089/ars.2025.0914.

[168] Kaviannejad R, Karimian SM, Riahi E, Ashabi G. A Single Immediate Use of the Cathodal Transcranial Direct Current Stimulation Induces Neuroprotection of Hippocampal Region Against Global Cerebral Ischemia. Journal of Stroke and Cerebrovascular Diseases. 2022; 31: 106241. https://doi.org/10.1016/j.jstrokecerebrovasdis.2021.106241.

[169] Kaviannejad R, Karimian SM, Riahi E, Ashabi G. Using dual polarities of transcranial direct current stimulation in global cerebral ischemia and its following reperfusion period attenuates neuronal injury. Metabolic Brain Disease. 2022; 37: 1503–1516. https://doi.org/10.1007/s11011-022-00985-8.

[170] Cheng J, Fan YQ, Jiang HX, Chen SF, Chen J, Liao XY, et al. Transcranial direct-current stimulation protects against cerebral ischemia-reperfusion injury through regulating Cezanne-dependent signaling. Experimental Neurology. 2021; 345: 113818. https://doi.org/10.1016/j.expneurol.2021.113818.

[171] Kaviannejad R, Karimian SM, Riahi E, Ashabi G. The neuroprotective effects of transcranial direct current stimulation on global cerebral ischemia and reperfusion via modulating apoptotic pathways. Brain Research Bulletin. 2022; 186: 70–78. https://doi.org/10.1016/j.brainresbull.2022.05.013.

[172] Huang J, Bao C, Yang C, Qu Y. Dual-tDCS Ameliorates Cerebral Injury and Promotes Motor Function Recovery via cGAS-STING Signaling Pathway in a Rat Model of Ischemic Stroke. Molecular Neurobiology. 2025; 62: 4484–4498. https://doi.org/10.1007/s12035-024-04574-x.

[173] Wang LC, Wei WY, Ho PC, Wu PY, Chu YP, Tsai KJ. Somatosensory Cortical Electrical Stimulation After Reperfusion Attenuates Ischemia/Reperfusion Injury of Rat Brain. Frontiers in Aging Neuroscience. 2021; 13: 741168. https://doi.org/10.3389/fnagi.2021.741168.

[174] Kong X, Hu W, Cui Y, Gao J, Yao X, Ren J, et al. Transcranial Direct-Current Stimulation Regulates MCT1-PPA-PTEN-LONP1 Signaling to Confer Neuroprotection After Rat Cerebral Ischemia-Reperfusion Injury. Molecular Neurobiology. 2022; 59: 7423–7438. https://doi.org/10.1007/s12035-022-03051-7.

[175] Kong X, Yao X, Ren J, Gao J, Cui Y, Sun J, et al. tDCS Regulates ASBT-3-OxoLCA-PLOD2-PTEN Signaling Pathway to Confer Neuroprotection Following Rat Cerebral Ischemia-Reperfusion Injury. Molecular Neurobiology. 2023; 60: 6715–6730. https://doi.org/10.1007/s12035-023-03504-7.

[176] Yao X, Kong X, Ren J, Cui Y, Chen S, Cheng J, et al. Transcranial direct-current stimulation confers neuroprotection by regulating isoleucine-dependent signalling after rat cerebral ischemia-reperfusion injury. The European Journal of Neuroscience. 2023; 58: 3330–3346. https://doi.org/10.1111/ejn.16091.

[177] Hu W, Kong X, Cui Y, Wang H, Gao J, Wang X, et al. Surfeit Locus Protein 4 as a Novel Target for Therapeutic Intervention in Cerebral Ischemia-Reperfusion Injury. Molecular Neurobiology. 2024; 61: 2033–2048. https://doi.org/10.1007/s12035-023-03687-z.

[178] Vöröslakos M, Takeuchi Y, Brinyiczki K, Zombori T, Oliva A, Fernández-Ruiz A, et al. Direct effects of transcranial electric stimulation on brain circuits in rats and humans. Nature Communications. 2018; 9: 483. https://doi.org/10.1038/s41467-018-02928-3.

[179] Asamoah B, Khatoun A, Mc Laughlin M. tACS motor system effects can be caused by transcutaneous stimulation of peripheral nerves. Nature Communications. 2019; 10: 266. https://doi.org/10.1038/s41467-018-08183-w.

[180] Grossman N, Bono D, Dedic N, Kodandaramaiah SB, Rudenko A, Suk HJ, et al. Noninvasive Deep Brain Stimulation via Temporally Interfering Electric Fields. Cell. 2017; 169: 1029–1041.e16. https://doi.org/10.1016/j.cell.2017.05.024.

[181] Celnik P, Paik NJ, Vandermeeren Y, Dimyan M, Cohen LG. Effects of combined peripheral nerve stimulation and brain polarization on performance of a motor sequence task after chronic stroke. Stroke. 2009; 40: 1764–1771. https://doi.org/10.1161/STROKEAHA.108.540500.

[182] Qi F, Nitsche MA, Ren X, Wang D, Wang L. Top-down and bottom-up stimulation techniques combined with action observation treatment in stroke rehabilitation: a perspective. Frontiers in Neurology. 2023; 14: 1156987. https://doi.org/10.3389/fneur.2023.1156987.

[183] Shin HK, Lee SW, Choi BT. Modulation of neurogenesis via neurotrophic factors in acupuncture treatments for neurological diseases. Biochemical Pharmacology. 2017; 141: 132–142. https://doi.org/10.1016/j.bcp.2017.04.029.

[184] Majdi A, Asamoah B, Mc Laughlin M. Understanding novel neuromodulation pathways in tDCS: brain stem recordings in rats during trigeminal nerve direct current stimulation. Translational Psychiatry. 2024; 14: 456. https://doi.org/10.1038/s41398-024-03158-6.

[185] Chen L, Deng Z, Asamoah B, Laughlin MM. Trigeminal nerve direct current stimulation causes sustained increase in neural activity in the rat hippocampus. Brain Stimulation. 2024; 17: 648–659. https://doi.org/10.1016/j.brs.2024.05.005.

[186] Lin DJ, Finklestein SP, Cramer SC. New Directions in Treatments Targeting Stroke Recovery. Stroke. 2018; 49: 3107–3114. https://doi.org/10.1161/STROKEAHA.118.021359.

[187] Sattler V, Acket B, Raposo N, Albucher JF, Thalamas C, Loubinoux I, et al. Anodal tDCS Combined With Radial Nerve Stimulation Promotes Hand Motor Recovery in the Acute Phase After Ischemic Stroke. Neurorehabilitation and Neural Repair. 2015; 29: 743–754. https://doi.org/10.1177/1545968314565465.

[188] Yagüe S, Veciana M, Martínez-Yélamos A, Pedro J, Cardona P, Quesada H, et al. Effects of Bihemispheric Transcranial Direct Current Stimulation Combined With Repetitive Peripheral Nerve Stimulation in Acute Stroke Patients. Journal of Clinical Neurophysiology. 2023; 40: 63–70. https://doi.org/10.1097/WNP.0000000000000840.

[189] Menezes IS, Cohen LG, Mello EA, Machado AG, Peckham PH, Anjos SM, et al. Combined Brain and Peripheral Nerve Stimulation in Chronic Stroke Patients With Moderate to Severe Motor Impairment. Neuromodulation. 2018; 21: 176–183. https://doi.org/10.1111/ner.12717.

[190] Blaschke SJ, Vlachakis S, Pallast N, Walter HL, Volz LJ, Wiedermann D, et al. Transcranial Direct Current Stimulation Reverses Stroke-Induced Network Alterations in Mice. Stroke. 2023; 54: 2145–2155. https://doi.org/10.1161/STROKEAHA.123.042808.

[191] Liebetanz D, Koch R, Mayenfels S, König F, Paulus W, Nitsche MA. Safety limits of cathodal transcranial direct current stimulation in rats. Clinical Neurophysiology. 2009; 120: 1161–1167. https://doi.org/10.1016/j.clinph.2009.01.022.

[192] Bikson M, Grossman P, Thomas C, Zannou AL, Jiang J, Adnan T, et al. Safety of Transcranial Direct Current Stimulation: Evidence Based Update 2016. Brain Stimulation. 2016; 9: 641–661. https://doi.org/10.1016/j.brs.2016.06.004.

[193] Liu A, Vöröslakos M, Kronberg G, Henin S, Krause MR, Huang Y, et al. Immediate neurophysiological effects of transcranial electrical stimulation. Nature Communications. 2018; 9: 5092. https://doi.org/10.1038/s41467-018-07233-7.

[194] Alekseichuk I, Mantell K, Shirinpour S, Opitz A. Comparative modeling of transcranial magnetic and electric stimulation in mouse, monkey, and human. NeuroImage. 2019; 194: 136–148. https://doi.org/10.1016/j.neuroimage.2019.03.044.

[195] Brunoni AR, Nitsche MA, Bolognini N, Bikson M, Wagner T, Merabet L, et al. Clinical research with transcranial direct current stimulation (tDCS): challenges and future directions. Brain Stimulation. 2012; 5: 175–195. https://doi.org/10.1016/j.brs.2011.03.002.

[196] Javadi AH, Walsh V. Transcranial direct current stimulation (tDCS) of the left dorsolateral prefrontal cortex modulates declarative memory. Brain Stimulation. 2012; 5: 231–241. https://doi.org/10.1016/j.brs.2011.06.007.

[197] Datta A, Bansal V, Diaz J, Patel J, Reato D, Bikson M. Gyri-precise head model of transcranial direct current stimulation: improved spatial focality using a ring electrode versus conventional rectangular pad. Brain Stimulation. 2009; 2: 201–7, 207.e1. https://doi.org/10.1016/j.brs.2009.03.005.

[198] Bikson M, Datta A, Rahman A, Scaturro J. Electrode montages for tDCS and weak transcranial electrical stimulation: role of “return” electrode’s position and size. Clinical Neurophysiology. 2010; 121: 1976–1978. https://doi.org/10.1016/j.clinph.2010.05.020.

[199] Minhas P, Datta A, Bikson M. Cutaneous perception during tDCS: role of electrode shape and sponge salinity. Clinical Neurophysiology. 2011; 122: 637–638. https://doi.org/10.1016/j.clinph.2010.09.023.

[200] Kronberg G, Bikson M. Electrode assembly design for transcranial Direct Current Stimulation: a FEM modeling study. Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Annual International Conference. 2012; 2012: 891–895. https://doi.org/10.1109/EMBC.2012.6346075.

[201] Zaleska MM, Mercado MLT, Chavez J, Feuerstein GZ, Pangalos MN, Wood A. The development of stroke therapeutics: promising mechanisms and translational challenges. Neuropharmacology. 2009; 56: 329–341. https://doi.org/10.1016/j.neuropharm.2008.10.006.

Funding

Pusan National University