Closed-loop circuits make a close connection between the cerebellum and the cerebral cortex [1]. The cerebellum can modulate the cortex’s contralateral motor and non-motor areas via the cerebello-dentato-thalamo-cortical pathway to affect motor, cognitive, or other performance [2, 3]. Therefore, the cerebellum has thus been investigated over the years as a critical modulator of the activity of the cerebral cortex [4].

Different methods have explored the connections between the cerebral cortex and the cerebellum. In animal experiments, Kelly and Strick [5] injected transneuronal tracers into different regions of the cerebellum in macaque monkeys and discovered projections from the cerebellum to the M1 and prefrontal cortex (PFC). Imaging evidence from Krienen and Buckner [6] used functional magnetic resonance imaging (fMRI) to perform functional connectivity analysis between the cerebellum and seed points in the motor cortex, dorsolateral PFC, medial PFC, and anterior PFC. As a result, they constructed a provisional map of cerebellar topography based on functional correlations with the frontal cortex.

Non-invasive brain stimulation (NIBS) techniques have also been applied to explore this connection. Although cerebellum-brain inhibition (CBI) provides important evidence for cerebellar–cortical interactions [7, 8], it mainly reflects changes in M1 excitability and offers limited insight into task-related cortical dynamics across multiple brain regions, suggesting an inhibitory effect of the cerebellar hemisphere on the cerebral cortex. At the same time, in a study by Koch et al. [9], intermittent theta burst stimulation (iTBS) was applied to the uninjured cerebellar hemisphere to address persistent balance issues in patients with chronic supratentorial stroke. This intervention resulted in significant improvements in balance function and was accompanied by cortical remodeling in the posterior parietal cortex (PPC). These findings suggest that cerebellar NIBS may influence cortical plasticity by modulating cerebellar–cortical pathways. However, whether cerebellar NIBS can directly and reliably affect cerebral cortical activity remains unclear. This uncertainty largely stems from the limitations of CBI paradigms based on motor-evoked potentials (MEPs). MEPs primarily reflect the excitability of the corticospinal tract, which integrates contributions from multiple subcortical and cortical structures, including the cerebellum, thalamus, basal ganglia, and brainstem. Moreover, hemispheric cerebellar transcranial magnetic stimulation (TMS) may also influence adjacent brainstem regions through current spread. As a result, changes in MEP amplitude cannot be unequivocally attributed to modulation of M1 or broader cortical regions. Consequently, conventional TMS-based approaches provide only indirect and spatially limited evidence of cortical involvement, with observations largely restricted to M1. Nevertheless, some studies have begun using magnetic resonance imaging and electroencephalography (EEG) to reveal changes in the cerebral cortex following cerebellar stimulation [10, 11]. Due to equipment limitations, these observed changes have been based on the resting state. While resting-state analysis provides indirect evidence for balance function, real-time observation during balance tasks might offer more direct insights.

Functional near-infrared spectroscopy (fNIRS), like other brain imaging tools, can observe cortical changes at rest and can also conduct real-time monitoring during motor tasks. Red light frequencies between 700 and 900 nm were used by fNIRS, capable of detecting oxyhemoglobin (Oxy-Hb) and deoxyhemoglobin (Dxy-Hb) [12]. Compared with fMRI and EEG, fNIRS shows greater robustness to movement-related interference, which makes it particularly suitable for studying complex motor tasks [13]. Based on previous experiments [14], we conducted the resting-state data collection. By leveraging fNIRS’s resistance to motion artifacts, we also performed real-time data collection during balance and walking tasks, allowing us to observe changes more directly related to balance regulation. The regions of interest (ROIs) we selected are well-aligned with the balance. The premotor cortex (PMC) and supplementary motor area (SMA) play a crucial role in both anticipatory and compensatory postural adjustments [15]. The PFC has also been shown to be involved in complex balance tasks, while the sensorimotor cortex (SMC) is responsible for executing movements and contributing to balance regulation [16, 17]. Considering the cerebellar hemisphere’s deeper anatomical location, we used a double-cone coil with a greater depth range for stimulation [18, 19].

Therefore, the main objective of our experiment is to observe whether a single session of cerebellar iTBS can induce changes in the cerebral cortex. These changes are monitored not only at resting tasks but also in real-time during walking and unilateral support tasks, which are closely related to balance.

This study aimed to investigate whether cerebellar iTBS induces a cortical response, thereby validating the feasibility of a closed-loop cerebellar-cerebral cortex circuit using NIBS techniques. Significant differences in graph theory metrics were observed before and after the unilateral support task, while no significant differences were found during the walking and resting-state tasks. In contrast, the sham stimulation group exhibited no differences across any of the tasks.

During the walking task, we found significant interaction factors in LPFC and LPMC/LSMA. However, further simple effect analysis of these two brain regions revealed no significant differences. This pattern likely reflects diverging trends between groups, which is sufficient to produce a significant interaction term but insufficient to yield statistically significant simple effects under conservative multiple-comparison correction. From the results, we observed that the real stimulation group showed an increasing trend in $\mathrm{\Delta }$HBO levels after the intervention, whereas the sham stimulation group exhibited a slight decrease in activation compared to baseline. We speculated that the iTBS could elicit the activation of the contralateral cortex to some extent. It should be noted, however, that this speculation remains tentative and requires further validation. Tan et al. [16] found that cerebellar iTBS can enhance cortical excitability. This study applied iTBS to the cerebellar vermis and observed its immediate effects on cortical excitability during balance tasks in healthy individuals, revealing that a single session of cerebellar vermis iTBS could increase cortical excitability in the bilateral SMA during balance tasks in healthy adults. The discrepancy between our findings and those reported by Tan et al.’s study [16] may be partly attributed to differences in cerebellar stimulation sites. Tan et al. [16] targeted the cerebellar vermis, which has strong bilateral connections with the SMA [28]. In contrast, the present study stimulated the cerebellar hemisphere, which predominantly influences the contralateral cortex via the dentato–thalamo–cortical pathway. Consequently, hemispheric stimulation may produce more distributed or subtle cortical network modulation rather than focal increases in SMA excitability. These differences highlight the importance of stimulation site selection when interpreting the cortical effects of cerebellar iTBS. Similarly, another study [29] used TMS combined with EEG to assess cortical changes induced by cerebellar iTBS, showing that TMS-evoked potentials significantly increased after a single iTBS intervention. This metric is associated with intracortical inhibitory networks, suggesting a relative increase in cortical excitability. Other studies [30, 31] have indicated that when excitatory NIBS protocols (such as high-frequency repeated transcranial magnetic stimulation [rTMS] or iTBS) are applied to the cerebellar hemispheres, they may activate superficial stellate and basket cells, leading to inhibition of Purkinje cells. The reduced inhibitory synaptic connections between Purkinje cells and the deep cerebellar nuclei result in increased excitatory synaptic connections from the deep cerebellar nuclei to the motor cortex via the ventral thalamus [5], thereby enhancing cortical excitability. In the left sensorimotor cortex (LSMC), no tendency was observed in this study, which may be related to the walking task design itself. Walking tasks primarily involve lower limb movements, and the sensory and motor cortices for the lower limbs are anatomically located in the ventromedial regions of the precentral and postcentral gyri [32]. This area is relatively deep and may lie beyond the detection range of fNIRS [33]. In the study by Koenraadt et al. [34], it was also found that $\mathrm{\Delta }$HBO in S1 and M1 during walking showed little difference compared to the baseline state. Quiet standing constitutes an active postural control state rather than a passive rest condition, due to the sustained anti-gravity muscle activity and ongoing balance adjustments required. Thus, the design of the baseline state requires further consideration—a point overlooked in current research. Furthermore, walking is a highly automated task primarily reliant on subcortical automatic mechanisms, suggesting that the motor cortex’s contribution may not differ significantly between standing and walking states.

An interesting phenomenon observed during walking, regardless of whether the stimulation was real or sham, was that participants exhibited smaller changes in cortical blood flow compared to the resting state. It is well known that the centers controlling walking are primarily governed at lower levels of the neuroaxis, including brainstem regions, spinal pattern-generating circuits, and cerebellar circuits. These structures generate patterns of intermuscular and interlimb coordination, promoting the automation of walking and minimizing reliance on cortical control [33]. Disruption of these structures can lead to significant consumption of cortical resources, as demonstrated in stroke and Parkinson’s disease patients [35]. In the study by Koenraadt et al. [34], it was found that participants exhibited decreased activation in the SMA and PFC during normal walking compared to resting states. However, during precision stepping, activation in the SMA and PFC increased compared to resting states. When walking requires more control, the activation of motor control-related cortices, such as the PFC and SMA, also correspondingly increases. In our results, the significant interaction factor in activation was particularly evident in the LPFC and LPMC/LSMA, further suggesting that the changes in activation during normal walking compared to resting states may be closely related to walking automation. Additionally, even though we emphasized maintaining natural conditions during data collection, it remains uncertain whether the collected resting state truly represents rest. The observer effect also plays a significant role in this context. This factor may also lead to participants exhibiting higher activation during resting states.

In the balance task, we designed a unilateral support task to engage participants in postural control. We observed an increase in $\sigma$ and Lp, and a reduction in Eglob. A healthy brain network typically exhibits small-world organization, characterized by an optimal balance between local specialization and global integration [36]. In the present study, changes in key graph-theoretical metrics—including Lp, Eglob, and $\sigma$—were used to evaluate alterations in network organization during the unilateral support task. Specifically, increased Lp together with reduced Eglob indicates decreased efficiency of global information transfer, while changes in $\sigma$ reflect deviations from an optimal small-world configuration [26, 37]. Current research [38, 39, 40] has shown that this efficient brain network balance can be disrupted in some neurological diseases. Our results indicate that after a single session of iTBS, the brain network seems to deviate from optimal small-world organization and reduced global integration [26]. In healthy individuals, the brain typically maintains a state of balanced stability. Although NIBS techniques have demonstrated positive effects on functional improvement in stroke patients, potentially based on the interhemispheric inhibition theory [41], which involves inducing cortical reorganization towards a pattern observed in healthy brains, our NIBS approach may disrupt the originally balanced brain network in healthy individuals. Katagiri et al. [42] found that a single session of anodal and cathodal transcranial direct current stimulation (tDCS) on the cerebellum suppressed the acquisition of postural control skills in healthy individuals compared to sham stimulation. Thus, for healthy individuals, it indicated that a single session of iTBS may disrupt the existing balance of the brain network.

For the resting-state results, we did not observe any positive outcomes at the levels of FC or graph theory metrics. One possible reason for this could be the relatively low spatial resolution of fNIRS, which may have limited our ability to detect changes in FC within the observed brain regions. Wang et al. [11] investigated changes in swallowing-related brain regions using resting-state fMRI after a single session of iTBS on the cerebellar hemisphere. They found changes in the middle temporal gyrus, precuneus, and superior frontal gyrus that could not be detected by fNIRS. Additionally, it is not only our study that failed to find significant results in the resting state. Thomson et al. [43] also reported similar findings. They suggested that the hemodynamic response induced by sub-threshold repetitive TMS in a single trial is generally very mild and may be masked when averaging epochs, which presents a challenge for fNIRS-based measurements. Regarding FC during the single-leg support task, no significant differences were observed. Currently, no studies are exploring whether cerebellar iTBS can induce changes in functional connectivity between brain regions. However, we speculate that there may be functional changes between the cerebellum and the cerebral cortex, which warrant further investigation using fMRI in future research.

Our study still has certain limitations. First, there are inherent limitations of near-infrared spectroscopy, which cannot detect subcortical structures that also participate in the cerebellar-cortical circuitry. Second, this study only intervened with healthy individuals, and no research has confirmed that cerebellar NIBS improves balance function in healthy people. We assessed the balance function (Berg Balance Scale) of all participants before and after the intervention and found both full scores without any differences, so we did not mention this result. We speculated that the scale we selected has low sensitivity among healthy individuals. Future experiments should use more precise balance measures for further observation. Third, leg dominance was not formally assessed, and all participants performed the unilateral support task using the right leg. Although this approach ensured task standardization, future studies should consider evaluating lower-limb dominance and counterbalancing the supporting leg to examine potential laterality effects. Fourth, we did not perform within-task time-course analyses (e.g., early vs. late epochs) because subdividing the recording may reduce the robustness of connectivity and graph metrics and would require concurrent behavioral/physiological fatigue measures for reliable interpretation. Furthermore, in recent years, fNIRS systems equipped with short-separation channels have been increasingly adopted, as they provide important advantages for improving signal specificity and reducing motion-related and systemic physiological artifacts. The incorporation of such hardware designs facilitates more accurate separation of cortical hemodynamic responses from superficial and non-neuronal signals. Future studies may therefore consider employing fNIRS systems with short-separation channels to further enhance data quality during dynamic motor tasks.

This study provides evidence that cerebellar iTBS can induce cortical responses, supporting the hypothesis of a cerebellar-cerebral cortex circuit involved in balance regulation. The changes in brain network properties during the unilateral support task indicate that iTBS may disrupt the efficient balance of brain networks in healthy individuals. However, no significant effects were observed during the walking and resting tasks, which may be due to the limitations of fNIRS in detecting subtle functional connectivity changes. Overall, these findings contribute to our understanding of the neural mechanisms underlying cerebellar NIBS and its potential therapeutic applications in improving balance. Further research is required to explore the clinical relevance of these findings.

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.

CG and AG contributed equally to this work. CG, AG, TW, and SZ conceived and designed the study. CG, AG, QW, JS, CK, and YS performed the research, including participant recruitment, assessment, and data collection. CG and AG analyzed and interpreted the data and drafted the manuscript. QW, JS, CK, and YS contributed to data curation and provided critical feedback on the manuscript. TW and SZ supervised the study, provided important intellectual input, and critically revised the manuscript for important intellectual content. TW and SZ also took primary responsibility for the overall coordination of the study and communication during the research and publication process. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

This study was approved by the Changzhou De’an Hospital Human Ethics Committee (CZDALL-2023-013) and registered with the China Clinical Trial Registration Center (ChiCTR2300077916) on November 23, 2023. The research was conducted in accordance with the principles embodied in the Declaration of Helsinki and in accordance with local statutory requirements. Prior to participation, all participants provided written informed consent and received financial compensation.

We are sincerely grateful to all the researchers (excluding the co-authors) and participants for their valuable contributions to this study.

This work was supported by the National Natural Science Foundation of China (NSFC) (Grant No. 82302882) and the Key Medical Discipline Project of Jiangsu Province (No. XK201110).

The authors declare no conflict of interest.

Supplementary material associated with this article can be found, in the online version, at https://doi.org/10.31083/JIN47653.