A cerebrovascular accident (CVA), often referred to as a stroke, arises from an interruption of blood flow or bleeding in a region of the brain, resulting in impaired brain function. According to the Global Burden of Disease (GBD) study, stroke is the primary cause of mortality among Chinese adults and the second leading cause of death worldwide (Zhou et al., 2019; GBD 2019 Stroke Collaborators, 2021). Reports indicate that 55–75% of stroke patients continue to experience upper extremity motor dysfunction within 3–6 months of onset (Kwakkel et al., 2003). Upper extremity motor dysfunction markedly influences the prognosis of patients, impacting their mobility, daily activities, and overall quality of life (Lai et al., 2002; Nichols-Larsen et al., 2005). For poststroke upper extremity rehabilitation, the task-oriented training (TOT) involves structured movement training based on daily activities to engage patients actively, thereby promoting motor function during the targeted task practice (Yoo and Park, 2015). Motor priming, a type of implicit learning wherein external stimulation prompts changes in the motor cortex and behavior, has been reported in motor skill learning recently (Jin et al., 2019). Previous research demonstrated that stimulation-based priming combined with TOT could facilitate brain reorganization and enhance upper extremity dexterity (Higgins et al., 2013; Alsubiheen et al., 2022). The most commonly used non-invasive brain stimulation techniques include transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) (Chhatbar et al., 2017; Bai et al., 2022; Zhang et al., 2022). However, the application of TMS and tDCS is limited by the need for high precision requirements and complex operations.

Vagus nerve stimulation (VNS) acts as a promising brain stimulation-based priming technique involving various forms of stimulation applied to the vagus nerve network. By regulating the balance of the autonomic nervous system and targeting neuroprotective and neuroplasticity pathways, VNS holds potential as a therapeutic tool in various neurological and psychiatric conditions (Ma et al., 2019). A multicenter, randomized, double-blind trial (VNS-REHAB) conducted by Dawson et al. (2021) confirmed the effectiveness of implanted VNS (iVNS) paired with upper extremity training in patients with ischemic stroke. However, the application process of iVNS may carry a high rate of potential complications due to its invasiveness (Ma et al., 2019). Recently, transcutaneous auricular VNS (taVNS), as a safe and easy-to-use neuromodulation technique, has been proposed, which utilizes transcutaneous stimulation of the cymba conchae and tragus innervated by fibers of the auricular branch of the vagus nerve (ABVN). Neuroimaging studies have demonstrated that taVNS can elicit brain activation effects similar to iVNS (Badran et al., 2018). Several studies (Capone et al., 2017; Redgrave et al., 2018; Wu et al., 2020; Chang et al., 2021; Li et al., 2022) have combined taVNS with regular rehabilitation training or robot-assisted arm training to improve upper extremity function in hemiplegic patients. To ensure consistency in taVNS research, an international consensus has been established for minimum reporting standards, which reported that a signal with a pulse width between 200 and 300 μs at 25 Hz, and a duty cycle of 30 s on, 30 s off has often been adopted in studies (Farmer et al., 2021). Nonetheless, the mechanism by which taVNS achieves task-specific benefits in upper extremity function remains unclear. In this study, we hypothesized that combining taVNS with TOT simultaneously would present a novel approach to enhance upper extremity function recovery in stroke patients. To assess the immediate and long-term effects of taVNS on hemodynamics and cortical excitability, we employed functional near-infrared spectroscopy (fNIRS) and single-pulse transcranial magnetic stimulation (TMS), respectively. The primary objective of this study was to investigate the effects and potential mechanisms of taVNS paired with TOT in facilitating the recovery of upper extremity function in patients with subacute stroke.

Forty-three patients were recruited, with 21 allocated to the VG and 21 to the SG. The flow chart can be found in Figure 1. Two participants (one from the VG and another from the SG) dropped out of the study due to unwillingness. In addition, eight participants in the VG and nine participants in the SG were excluded from the fNIRS data analysis. Detailed reasons for participants’ exclusion are shown in Figure 1.

In recent years, non-invasive VNS has garnered increasing attention due to its potential to improve upper extremity motor function in stroke patients, although its underlying neural mechanisms are not fully understood. In order to explore the feasibility of the current study, we conducted HRV and fNIRS examinations prior to the initiation of the treatment. The effectiveness of taVNS was assessed using HRV examination. Our current results demonstrated significant decreases in HR, LF/HF ratio, and a marked increase in SDNN in the VG, whereas no changes were observed in the SG. These results indicated that taVNS indeed modulated the vagus nerve network. Subsequently, using fNIRS, we observed greater hemodynamic responses in the bilateral PFC and the ipsilesional SMC during the grasp task with taVNS, suggesting an enhancement of cortical activation. In the fNIRS study conducted by Kunii et al. (2021), the effects of iVNS on cerebral blood flow (CBF) during a resting state and a verbal fluency task were investigated. They found that no changes in CBF were observed during the resting state, while the verbal fluency task led to a significant increase in CBF. Wang et al. (2023) found that a single session of taVNS could significantly bolster the activation within damaged cerebral territories in stroke patients without destabilizing cerebral lateralization. These findings provide evidence that taVNS can effectively activate the task-specific cortex during motor task performance, supporting the immediate effect of taVNS.

The TOT approach aims to teach stroke patients specific task strategies and improve their ability to adapt to the environment through functional tasks related to daily life. Previous fMRI studies (Jang et al., 2003; McCombe Waller et al., 2014) have reported that TOT could improve upper extremity function in post-stroke patients. These studies found that activity in the related motor cortex increased with the recovery of upper extremity motor function. In our study, we observed improvements in the motor capacity of the upper extremity (FMA-UE), arm-hand capacity (ARAT), and activities of daily living (MBI) in both groups. However, patients in the taVNS group demonstrated greater improvements in upper extremity function, which is consistent with previous research (Hoonhorst et al., 2015; Capone et al., 2017; Redgrave et al., 2018; Chang et al., 2021; Li et al., 2022). Notably, the improvement observed in the VG was clinically significant, surpassing the minimal clinically important difference (MCID) of 9–10 points for the FMA-UE in individuals with subacute stroke (Singer and Garcia-Vega, 2017). Khodaparast et al. (2016) observed that the rate of forelimb strength recovery (86%) in ischemic stroke rats was significantly higher after receiving VNS with rehabilitation training compared to the simple rehabilitation group (47%) and the delayed VNS group (42%). Morrison et al. (2019, 2020) reported that intracortical microstimulation with VNS can improve motor cortical plasticity in mice. In this study, significant improvements were observed in upper extremity motor function, indicating that taVNS paired with task-specific activities may promote neuroplastic changes.

Neuroanatomy research has provided insights into the mechanisms underlying the activation of the locus ceruleus-noradrenaline (LC-NE) release system by taVNS. Norepinephrine (NE) is an excitatory neurotransmitter, while the locus coeruleus (LC) is the primary source of the norepinephrine-producing neurons in the brain. Studies indicated that both short-term and long-term VNS can increase neuronal firing rates of the LC, leading to increased NE concentrations in the amygdala, hippocampus, and prefrontal cortex. The VNS-induced NE could maintain long-term activity (Groves et al., 2005; Hulsey et al., 2017). NE can enhance cortical excitability and plasticity, which are associated with daytime vigilance, attention, and motor learning (Duffau, 2006; Ciampa et al., 2022). The fMRI studies by Kraus et al. (2007) and Dietrich et al. (2008) have detected blood oxygenation level-dependent (BOLD) signal activations in the bilateral sensorimotor cortex and prefrontal cortex during taVNS. Furthermore, Frangos and Komisaruk (2017) observed sustained activation of the bilateral precentral gyrus for nearly 10 min (9 min, in fact) following taVNS. Combining these findings with the results of fNIRS in our trial, it is plausible to speculate that taVNS can induce bilateral hemisphere activation during the treatment, which may be attributable to the related neurotransmitter release.

In this study, we utilized single-pulse TMS to assess long-term changes in motor cortex excitability by measuring MEPs in both hemispheres. The study found a relatively low elicitation rate of ipsilesional MEPs in patients at baseline. However, the elicitation rate of ipsilesional MEPs increased significantly from 15% before treatment to 45% after treatment in the VG, which is consistent with previous studies (Escudero et al., 1998; Powell et al., 2019; Yarossi et al., 2019). Moreover, the average latency of ipsilesional MEPs in the VG decreased from 63.53 ms to 55.36 ms, while the amplitude increased from 87.60 μV to 140.18 μV, suggesting a trend toward increased cortical excitability in the ipsilesional M1. Cakar et al. (2016) reported a positive correlation between shorter MEP latency, increased amplitude, and improved motor function in stroke patients. Additionally, we observed a reduction in the latency of contralesional MEPs, indicating the modulation of the contralesional M1. Increased intracortical excitability of contralesional M1 has been observed in subacute and chronic stroke patients (Bütefisch, 2003; Bütefisch et al., 2008), with suggestions that the increased excitability is associated with better recovery of upper extremity function (Bütefisch, 2015). Bi-hemispheric transcranial direct current stimulation (tDCS) studies using fMRI have supported this notion, proposing a cooperative role of contralesional M1 rather than competition (Waters et al., 2017). Furthermore, a randomized controlled trial conducted by Hsu et al. (2023) demonstrated that improvements in FMA-UE scores in subacute stroke patients correlated with changes in functional connectivity within the bilateral intra-hemisphere networks. In our study, we found a significant correlation between the decrease in latency of contralesional MEPs and improvements in FMA-UE scores, which is consistent with previous literature (Wang et al., 2020). Based on these findings, the long-term combined intervention may modulate bilateral motor cortical excitability and facilitate training-specific motor performance.

This study has several limitations that need to be addressed in future research. First, the small sample size and the high dropout rate of the fNIRS examination in this study might result in insufficient statistical power, potentially increasing the chance of a type I error. A larger sample size is needed to confirm these findings and achieve more reliable results. Second, it is worth noting that, due to the limited number of positive events (elicited ipsilesional MEPs), the significance and generalizability of this result should be carefully considered. Finally, follow-up assessments were not conducted to investigate the sustained effects of taVNS in this study, which should be considered in future studies.

In conclusion, our study provides evidence that the combined intervention of taVNS with TOT is effective in enhancing upper extremity motor function in patients with post-stroke hemiplegia. This improvement can be attributed to the modulation of cortical excitability in both hemispheres, which facilitates the remodeling of motor function.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving humans were approved by the Ethics Committee of Sir Run Run Hospital, Nanjing Medical University (No. 2020-SR-001). The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study.

M-HW: Conceptualization, Data curation, Formal analysis, Software, Visualization, Writing – original draft. Y-XW Conceptualization, Writing – review & editing. MX: Investigation, Methodology, Validation, Writing – review & editing. L-YC: Investigation, Methodology, Writing – review & editing. M-FH: Investigation, Methodology, Writing – review & editing. FL: Conceptualization, Project administration, Supervision, Writing – review & editing. Z-LJ: Conceptualization, Funding acquisition, Project administration, Resources, Supervision, Writing – review & editing.