Priming transcranial direct current stimulation for improving hemiparetic upper limb in patients with subacute stroke: study protocol for a randomised controlled trial


Globally, stroke ranks as the third leading cause of death and disability combined,1 and almost 80% of patients suffer from upper limb motor impairment immediately after stroke onset.2 Poststroke motor impairment can be primarily attributed to direct damage to the corticospinal descending motor pathway, reduced regional activity and abnormal interhemispheric interactions caused by focal lesions. Neuroimaging studies have suggested that the recovery of motor functions after stroke is associated with adaptive neuroplasticity characterised by cortical reorganisation within or between brain cortices and rewiring of the corticospinal pathway.3 Therefore, to boost motor recovery in stroke rehabilitation, various neuroplasticity-enhancing techniques have been used to create a time window of temporarily enhanced neuroplasticity, permitting the brain to respond to subthreshold stimuli to accumulate motor-behavioural relearning.

Transcranial direct current stimulation (tDCS) is a non-invasive neuromodulation technique for selective and bidirectional conditioning of brain states.4–7 By applying weak and constant direct electrical currents to the cortex,8 tDCS can modify membrane potentials and alter the likelihood of neuronal activation.9 Studies have documented a relative decrease in corticospinal excitability in the ipsilesional hemisphere and an increase in corticospinal excitability in the contralesional hemisphere in patients with stroke.10 The revision of interhemispheric imbalance is thought to be the key principle of motor recovery following stroke,10 which has been supported by studies applying inhibitory transcranial magnetic stimulation (TMS) to the contralesional primary motor cortex (M1) and/or excitatory TMS to the ipsilesional M1.11 12 Similarly, three types of tDCS protocols have been extensively investigated in stroke populations: anodal stimulation targeting the ipsilesional M1 to enhance its excitability,8 13 14 cathodal stimulation targeting the contralesional M1 to reduce its maladaptive overactivity8 13 14 and dual tDCS (anode over the ipsilesional and cathode over the contralesional M1) targeting the reversal of imbalanced cortical activity.8 15 However, according to the most recent therapeutic guidelines for tDCS, a level A recommendation for these three protocols cannot be made for upper limb motor rehabilitation following stroke in view of several negative reports.16

These negative results can be partially attributed to the response variability of tDCS in human M1.17 Recently, researchers have designed a new facilitatory tDCS protocol, namely priming tDCS protocol,18 in which cathodal tDCS is carried out prior to anodal tDCS for a more robust and even stronger facilitatory effect. The underlying mechanism can be explained in accordance with the theory of metaplasticity. The likelihood of induced neuroplasticity is largely determined by the prior neuronal activity of the same neuron or neural circuits; low-level prior neuronal activity can enhance the likelihood of further long-term potentiation (LTP)-like effects, whereas high-level prior neuronal activity can enhance the likelihood of further long-term depression-like effects.19 Therefore, an inhibitory priming session can reduce the response variability of the brain to a subsequent facilitatory conditioning session by using therapeutically beneficial metaplasticity. Our team recently validated a priming protocol using theta burst magnetic stimulation (TBS) in patients with stroke. We found that preceding continuous TBS (an inhibitory protocol) enhanced subsequent facilitation induced by intermittent TBS (a facilitatory protocol), leading to superior upper limb motor improvement and neurophysiological outcomes in patients with stroke with higher functioning upper extremities.12 However, our study found that the application of metaplasticity-elicited priming TBS did not demonstrate superiority over a non-priming control in patients with lower functioning upper extremities.12 We concluded that this lack of superiority may be attributed to the preceding session involving continuous TBS, which, while increasing the likelihood of LTP induction, simultaneously significantly suppressed the overall cortical excitability of the ipsilesional M1. As a result, this suppression potentially counteracted the excitatory effect of the subsequent intermittent TBS, particularly in patients with lower functioning upper extremities. These patients typically have significantly reduced cortical excitability compared with individuals with higher functioning upper extremities.10 Therefore, the motivation for incorporating tDCS in the design of priming stimulation stems from the understanding that a subthreshold, low-intensity priming session using cathodal tDCS can elicit therapeutically beneficial metaplasticity without significantly impacting overall excitability.19 Therefore, this approach may maximise the induction of LTP in the subsequent session by combining anodal tDCS with motor training. In addition, tDCS is considered a favourable clinical option compared with TBS due to its cost-effectiveness and ease of operation.

However, to the best of our knowledge, it is still under exploration whether pairing a session of cathodal tDCS and a session of anodal tDCS will also capitalise on metaplasticity, thereby creating a brain state of temporarily increased neuroplasticity to facilitate the efficacy of motor learning in subsequent motor training for patients with stroke.

Therefore, the current study aims to investigate the effects of priming tDCS (real cathodal stimulation before real anodal stimulation) on improving upper limb motor functions and promoting neuroplastic changes in corticospinal excitability, regional cortical activity and functional connectivity (FC) in patients with stroke compared with a non-priming tDCS control (sham cathodal stimulation before real anodal stimulation). We further investigate the relationship between the motor functional improvement and changes in corticospinal excitability, regional cortical activity and FC, in order to elaborate the neural mechanism underlying the intervention.


The study protocol conforms to the Standard Protocol Items: Recommendations for Interventional Trials (SPIRIT) guidelines (see online supplemental SPIRIT checklist). The study schedule of participant recruitment, assessments and intervention has been summarised in figure 1.

Supplemental material

Figure 1
Figure 1

Schedule of participant recruitment, assessments and intervention. FMA, Fugl-Meyer Assessment; fNIRS, functional near-infrared spectroscopy; tDCS, transcranial direct current stimulation.

Study design

This study is designed as a single-centre, double-blind, sham-controlled, randomised controlled trial (RCT). Figure 2 shows an overview of the proposed trial. Research participants will be recruited among inpatients in a neurological rehabilitation ward at the Shanghai Yangzhi Rehabilitation Hospital. Eligible participants will be randomly allocated to the priming or non-priming groups at a ratio of 1:1. The included participants will receive 2 weeks of intervention, and assessments will be carried out before and after intervention, and at 2-week follow-up. The current study will start on 14 August 2023 and will end on 30 June 2025. Our data will be available by contacting the corresponding authors (JZ and ZB) on reasonable request.

Figure 2
Figure 2

Flow chart of the proposed randomised controlled trial and intervention (A) and demonstration of the intervention (B). tDCS, transcranial direct current stimulation.

Inclusion and exclusion criteria

Participants will be included if they meet all the following criteria: (1) first-ever unilateral ischaemic or haemorrhagic stroke; (2) purely subcortical lesion confirmed by MRI or CT; (3) subacute stroke, that is, 1–6 months since stroke onset; (4) aged 18–75 years; (5) moderate-to-severe upper limb motor dysfunction due to stroke, with a baseline Fugl-Meyer Assessment-upper extremity (FMA-UE) scores ranging from 20 to 50 and (6) ability to give written informed consent. Patients were excluded if they met any of the following criteria: (1) unstable medical condition, for example, severe heart or kidney diseases; (2) cognitive impairment (Mini-Mental State Examination <24); (3) with metal implants, such as heart stent, implant brain stimulator, pacemaker or artificial cochlear; (4) history of epileptic seizure, cerebral trauma or craniotomy; (5) currently taking psychotropic drugs and (6) neurological disorders other than stroke.

Sample size estimation

As the current study will investigate a completely novel tDCS protocol in patients with stroke, no previous studies can be referenced for sample size estimation. The sample size is calculated based on the minimal clinically important difference (MCID) of the FMA-UE in a population of patients with stroke, that is, 5.25 points, with an SD of 2.5.20 Given a power of 0.8, a two-tailed alpha error probability of 0.05 and a dropout rate of 10%, 67 participants are needed in each group, with a total sample size of 134.

Randomisation and allocation

Randomisation will be performed by the principal investigator not involved in recruitment, assessment, treatment or data analysis. The randomisation sequence will be generated using a computer program. To ensure allocation concealment, the sequence will be enclosed in an opaque sealed envelope, and eligible cases will be numbered by the principal investigator in order of recruitment. Participants’ demographic characteristics and baseline assessments will be collected prior to randomisation. A total of 134 eligible participants will be randomly assigned to the priming or non-priming groups at a ratio of 1:1.


Assessor-blinded and participant-blinded methods will be used in this study. An independent assessor blinded to the group allocation will conduct the outcome measurement. To blind the participants, a sham tDCS protocol will be used for the non-priming intervention.


Participants in the priming group will receive real cathodal tDCS (cathode electrode over the ipsilesional M1 and anodal electrode over the orbit of the contralesional side) treatment for 20 min, which is the priming stimulation of our tDCS protocol. Subsequently, the polarity of the stimulation will be reversed, with the anodal electrode over the ipsilesional M1 and cathode electrode over the orbit of the contralesional side for 20 min to enhance the excitability of the ipsilesional M1. During the excitatory stimulation period, the participants will concurrently perform robot-assisted upper limb training. This is because tDCS preferentially modulates the neural network already involved, which is called activity-dependent modulation.21 Furthermore, our previous work and others have shown the time-dependent interaction effects of tDCS with other training modalities.22 23 In the non-priming group, the intervention procedure will be similar to that in the priming tDCS group. However, the first real cathodal tDCS will be replaced by a sham cathodal tDCS (cathode over the ipsilesional M1 and anodal electrode over the orbit of the unaffected side) treatment. Each intervention session will last 40 min, with five sessions per week for 2 weeks. The total number of stimulation sessions is in line with our previous priming TBS work12 and most of the previous studies using tDCS in patients with stroke.4 The study procedure is illustrated in figure 2.

Transcranial direct current stimulation

The stimulation will be delivered using a Soterix Medical 1×1 stimulator through two 35 cm×35 cm saline-soaked surface sponge electrodes. A headband will be used to attach the electrodes. The current intensity of the stimulation is 2 mA, with 30 s of ramping up at the start and 30 s of ramping down at the end of the stimulation. In the non-priming group, sham cathode tDCS will be applied, including a 30 s ramp-up and 30 s ramp-down only, and the current will be switched to 0 mA without the participants’ awareness during the other time.

We intend to administer a 40 min session of tDCS, which is slightly longer than the typically used 20–30 min duration.4 Nonetheless, it is important to note that 40 min tDCS remains a safe approach for poststroke patients.24 Additionally, the reversal of the montage further mitigates the risk of cathodal iontophoresis burn, which may be caused by prolonged cathodal tDCS.

Participants will be withdrawn from the study if any of the following situations occur: (1) epileptic seizure, (2) recurrent stroke, (3) intolerance to completing the study or (4) other treatments due to worsening disease.

Upper extremity motor training assisted by robotic device

In line with our previous meta-analysis25 and the large-scale RCT by Rodgers et al,26 robot-assisted training has been shown to be non-inferior to conventional rehabilitation and more effective than usual care in poststroke upper extremity rehabilitation. Robot-assisted training is selected as a standard upper extremity training in the current study because offers better control and customisation to the patient’s functional level. All patients will receive anode stimulation to the ipsilesional M1 area while undergoing 20 min of robot-assisted upper limb training (Armguider, ZD Medtech, Shanghai, China), which can provide antigravity support for the shoulder, elbow, wrist and hand. In the first training session, the range of movement will be individually set up, and a training mode will be chosen depending on the residual motor function of the affected upper extremity. For example, an assisted movement mode is selected for patients who cannot actively complete tasks with a full range of motion, and the machine can provide partial assistance when patients are unable to complete them. The active movement mode will be selected for patients who can voluntarily perform tasks with a full range of motion without any assistance from the machine. To provide real-time visual feedback during task-oriented training, the motor functional tasks are shown on a monitor approximately 1 m away from the patients. Real-time auditory feedback is also provided to indicate the performance quality (figure 2).

Clinical assessments

The primary outcome is the FMA-UE, which includes 33 items related to the proximal or distal joints of the upper limb movement. Each item is scored as 0, 1 or 2, and the final score is the sum of the 33 items, ranging from 0 to 66. The FMA-UE mainly assesses reflex activity, motor control and muscle strength of the upper limb affected by stroke. The FMA-UE demonstrates good test-retest reliability, excellent internal consistency and good convergent validity with other tools.27 The MCID of FMA-UE is 5.25,20 28 29 which means that an improvement in FMA-UE >5.25 is considered clinically meaningful. Secondary outcomes include the Wolf Motor Function Test (WMFT) and Modified Barthel Index (MBI). The WMFT quantifies the upper limb motor function by timing and scoring the performance of 15 functional tasks, assessing single-joint or multiple-joint motor performance. Studies have shown that the WMFT is a reliable and valid tool for motor impairment and is sensitive to changes over time in patients with early stroke.30 The WMFT should be performed on an appropriately sized desk. To ensure that the participants understand the test, each task should be demonstrated twice before starting the test. The MBI is a reliable and valid tool that can be used to assess independence in daily activities in patients with stroke.31


Neuroplasticity will be investigated using motor-evoked potentials (MEP) and functional near-infrared spectroscopy (fNIRS). We will reveal the associated neuroplastic effect at three levels: (1) the cortical regional activity measured by the signal intensity of oxyhaemoglobin using resting-state fNIRS; (2) the strength of the intracortical and intercortical motor connections measured by FC analyses using resting-state fNIRS and (3) the excitability of the corticospinal descending motor pathway, measured by single-pulse TMS-induced MEP.

TMS-induced MEP

The ipsilesional corticospinal tract is immediately damaged in patients with stroke with motor deficits, and its excitability is decreased compared with the contralesional side and that of healthy individuals. Since tDCS aims to enhance decreased excitability by directly modulating the membrane potentials of neurons, we postulate that our priming tDCS protocol is more effective than non-priming tDCS for corticospinal excitability. Corticospinal excitability is assessed using a figure-of-eight cooling coil (external diameter of each wing: 75 mm) connected to a magnetic stimulator (NS3000, YIRUIDE, Wuhan, China). The participants are seated in a TMS-specific adjustable chair with head and back support, and kept awake with their eyes open. Biphasic TMS pulses are delivered to the motor hotspots of the first dorsal interosseous muscle. The motor hotspot is defined as the position where the largest and most reliable MEPs can be obtained from the first dorsal interosseous muscle. The MEPs are recorded from the contralateral muscle using disposable Ag-AgCl surface electrodes positioned in a belly tendon montage, and a ground electrode is placed on the ulnar styloid process. Resting motor threshold (RMT) is defined as the minimum intensity (% of maximal stimulator output) that can elicit peak-to-peak MEP amplitudes >50 µV in at least 5 out of 10 trials.32 To measure corticospinal excitability indexed by MEPs, 20 single-pulse trials with intertrial intervals of 4–5 s, at an intensity of 120% RMT are recorded while the participants are in a resting state. Specifically, the peak-to-peak amplitudes of MEPs are extracted; a larger MEP amplitude indicates higher corticospinal excitability. Raw MEP signals will be digitised at 5 kHz (Dantec Keypoint Focus; Natus, Paris, France) and stored on a laptop for offline analysis. If MEPs are not detectable with a stimulation intensity of 100% of the maximal machine output, the patient will be labelled MEP-negative.

fNIRS data collection

Functional magnetic resonance imaging (fMRI) studies have shown that the fluctuations of resting-state blood oxygen level-dependent (BOLD) signals are similar to those of task-based signals.33 During data acquisition, resting-state scanning does not require participants to perform any behavioural tasks, and they are instructed to rest only with their eyes open or closed. Because of the lack of specific requirements, resting-state scanning has become popular in recent studies on patients with stroke who have difficulty with motor performance. Previous fMRI studies have shown that the FC between the bilateral M1 is reduced immediately after stroke onset,34 is gradually normalised and is associated with motor recovery.35 36 Compared with fMRI, fNIRS has the advantage of good patient compliance, and poststroke reorganisation of resting-state FC (rsFC) has been investigated in patients with stroke using fNIRS scanning. Recent studies have shown disrupted and stronger intrahemispheric FC in the ipsilesional and contralesional hemispheres, respectively,37 38 and that the abnormal FC pattern can be reversed by motor training.38 Therefore, the current study will employ fNIRS to investigate whether the priming tDCS protocol is more effective than non-priming tDCS in normalising FC patterns in patients with stroke.

In this study, the concentration changes of oxyhaemoglobin (ΔHbO) will be acquired by a continuous ware optical system (ETC-4100, Hitachi Medical, Japan). This system generates near-infrared light with two wavelengths, 690 and 830 nm, and the signals are sampled at 10 Hz, which is the same as in our previous work.39 40 Two 3×3 probe sets will be used to cover the hand-representative areas of the contralesional and ipsilesional M1, constituting 12 channels over each hemisphere (figure 3). The distance between the source and detector will be fixed at 3 cm. The positions of the probes will be determined according to the international 10–10 system of electrode placement, and a three-dimensional digitizer (PATRIOT, Polhemus) will be used to confirm whether the channels are precisely located on M1. Data will be acquired in a relatively dark room. The participants will sit on a chair with arm and head supports, keeping their eyes stationary, eyes open and gazing at a black cross with a white background almost 2 m away. fNIRS data acquisition will be performed in a relatively dark room in a relaxed environment. Our study will acquire 10 min of resting-state fNIRS data before and after the intervention and at follow-up.41

Figure 3
Figure 3

Channel location of the functional near-infrared spectroscopy measurement.

fNIRS data processing

The resting-state fNIRS data will be preprocessed using the HomER2 toolbox in MATLAB 2014a (MathWorks),42 which is similar to the method in our previous work.39 40 First, the raw signals are converted to changes in the optical density. Next, motion artefacts are corrected using a spline interpolation algorithm. Third, a bandpass filter between 0.01 and 0.2 Hz is applied to the signals to remove physiological noises and drifts. Fourth, the processed optical density is converted to ΔHbO based on the modified Beer-Lambert law. rsFC is obtained by calculating the Pearson’s correlation between channels.43 All channel-by-channel Pearson’s correlation coefficients form a matrix according to the channel arrangement, and the Fisher’s Z transformation is applied for subsequent statistical analysis.44

Safety profile investigation

A side effect survey will be conducted at the end of each tDCS session. In the event of any moderate-to-severe side effects occurring in the participants, such as headaches, insomnia or acute mood changes, the experimental session will be immediately suspended.

Statistical analysis

Statistical analyses will be performed using SPSS V.22.0. The demographic and baseline clinical data will be compared using the t-test or Mann-Whitney U test and χ2 test, where appropriate. A two-way repeated-measures analysis of variance (ANOVA) (between-subjects factor: group; within-subjects factor: time and group-by-time interaction effect) will be used to compare the differences between the priming and non-priming groups regarding the primary and secondary outcomes. Mauchly’s spherical test will be performed before analysis. If the data do not conform to the spherical test, we will use the results of the multivariate test or the corrected results using one-way ANOVA. The level of significance will be set at p<0.05 (two-tailed). When any significant group-by-time interaction effect is found, pairwise comparisons will be performed. If there is no interaction effect, the main effect will be analysed. Otherwise, the analysis will focus on the simple main effects. In addition, we will use Pearson’s correlation coefficient to analyse the correlation between motor outcomes and neuroplastic indices.

Patient and public involvement

The patients and/or the public were not involved in the design, conduct, reporting, or dissemination of this research.

Ethics and dissemination

This study was registered in the Chinese Clinical Trial Registry on 12 August 2023 (; reference number: ChiCTR2300074681). This study has been approved by the Research Ethics Committee of the Shanghai Yangzhi Rehabilitation Center (reference number: Yangzhi2023-022) and will be conducted in accordance with the Declaration of Helsinki of 1964, as revised in 2013. Written informed consent will be obtained from each participant before the start of the study (see online supplemental file 1: consent form and information sheet). We will inform the Research Ethics Committee of any modifications made to the study protocol. This study will exclusively involve participants who provide written informed consent, ensuring confidentiality throughout the entire process. All the original data will be securely maintained with strict privacy. Written data will be stored in a secure location during the study, and on completion, the principal investigator will input the data into a computer, preserving a backup on a hard drive that can be safely stored. The input data will be verified by a different research investigator. Personal data will be permanently saved in the hospital’s data management system for clinical purposes. An interim analysis will be conducted once 50% of the patients have been included and completed the follow-up assessment. We expect that the findings of this study will be presented at international conferences and published in peer-reviewed journals.

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