Neuromodulation techniques, which can change central excitability and induce neuroplasticity, have been successfully used for rehabilitation after central neural system injury or function disorder (1, 2). Transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) have been widely studied in the past (3, 4). Transcranial Ultrasound stimulation (TUS), which can pass through the skull and influence central nervous function mechanically, has become a research focus for research in recent years (5). As a novel neuromodulation technique, TUS has the advantages of being non-invasive and providing high spatial resolution and greater penetration depth. Preliminary studies suggest that TUS can locally regulate sensory induction and cortical function (6).

Ultrasound can be classified into high, medium, or low intensities according to the power applied. High-intensity ultrasound (>200 W/cm²) mainly inhibits neuronal activity through nerve ablation, while medium-intensity ultrasound (100–200 W/cm²) is normally used to disrupt the blood-brain barrier non-invasively. Low-intensity ultrasound (<100 W/cm²), on the other hand, does not involve a significant accumulation of thermal energy, which may cause DNA fragmentation, coagulative necrosis, and cell death (7, 8), relying instead on a direct mechanical effect (9).

Previous studies mostly focused on animal experiments-very few studies discussed the effect of TUS on the human cortex (10, 11). The animal studies reported both excitation and inhibition effects of low-intensity TUS on the central neural system (12, 13). One human study reported intervention in the right prefrontal gyrus in healthy individuals and found an inhibitory effect on the cortex but a positive effect on mood (14). The therapeutic effect of TUS on the cortex may be related to parameters, such as focused or non-focused, intensity, frequency, and stimulation location (15, 16). Focused TUS and non-focused TUS can lead to a differential stimulation effect according to the volume of the brain tissue impacted (17). Frequency and intensity may cause different physiological effects (8). In humans, TUS delivered to the M1 region at 0.50 MHz, 6.16 W/cm² inhibited neuronal excitability (11). Diagnostic imaging ultrasound (unfocused TUS) stimulation with a frequency of 2.32 MHz and intensity of 34.96 W/cm² increased neuronal excitability (10). Using multiple Focused TUS transducers to stimulate the primary sensory cortex elicited various tactile sensations in the absence of any external sensory stimuli (18). Sanguinetti reported a focused TUS study with a randomized, placebo-controlled, double-blind design, in which participants received 30 s of 500 kHz focused TUS or a placebo control. They found that focused TUS can be used to modulate mood and emotional regulation networks in the prefrontal cortex when targeting the right inferior frontal gyrus in healthy human volunteers (14).

In this study, we used the tapping action and motor evoked potentials (MEPs) (19) to investigate the effect of low-intensity TUS on hand motor function and cortical excitability in healthy adults.

Transcranial ultrasound stimulation (TUS) is a novel neuromodulation technique that has not yet been widely adopted in clinical practice (8, 13). In this study, TUS was used to stimulate the motor area of the brain in healthy adults, and it could enhance hand responsiveness, shorten the transmission time of MEPs, and enhance motor cortex excitability, indicating that low-frequency and low-intensity TUS has a positive effect on central excitability and hand function.

Some previous studies reported that TUS can regulate neuronal activity (8). The short-term neuronal activity involves inhibition or activation, while in the long term it may involve the reorganization of neural circuits (neuroplasticity), which could potentially improve brain function (7). A previous study has shown that transcranial-focused ultrasound increased cortical blood flow in the activated area of the motor cortex (27). Most researchers consider that the non-thermal effects of ultrasound play a role in the regulation of neuronal activity, including the opening of mechanically-gated ion channels and changes in membrane permeability caused by cavitation effects (13, 28). Due to the plasticity of the human cortex, low-intensity TUS can increase the excitability of the target brain circuit within a short time, suggesting that endogenous motor cortex activity can be enhanced by regulating excitability (29).

Our tapping score measurement included a comprehensive evaluation of speed and accuracy like the score calculation of archery. The results indicated that TUS could shorten the reaction time and improve the reaction ability of the hand by direct stimulation of the brain. Ichijo et al. found that low-intensity TUS improved the ability of mice affected by stroke to walk on a wire rope and navigate a maze. Few previous studies have focused on the effects of TUS on human behavior (30).

The results of this study show that the MEP latency was shortened, and the amplitude of the MEP was increased after TUS stimulation, suggesting that low-intensity TUS could enhance the excitability of the motor cortex. When TMS is applied to the primary motor cortex, corticospinal neurons are activated, and an MEP is generated in the target muscle (31). MEP latency reflects the conduction capacity of the corticospinal tract, and MEP amplitude can be used to determine changes in cortical excitability. In agreement with our results, Gibson et al. used diagnostic TUS applied to the cortical area for 2 min and found that the MEP amplitude was increased after stimulation (10). Using self-made ultrasound equipment, Legon et al. also showed that focused TUS (0.5 MHz, 6.16 W/cm²) can regulate central excitability (11). However, in their case, the MEP amplitude decreased after stimulation, suggesting inhibition of the motor cortex. These result differences may be related to the specific parameter setting of TUS. In the present study, we used non-focused ultrasound, while Legon et al. employed transcranial-focused ultrasound; also, the intensity we used was lower than that used in Legon's study. Nevertheless, both results provide evidence that TUS can regulate the excitability of the human motor cortex.

In the present study, VAS was used as a tool to monitor the discomfort of subjects, reflecting their subjective feelings about the treatment. Because the low-intensity TUS we used is commercially available and does not have any thermal effects, we expected the participants to not experience any noticeable sensation during the treatment session, and indeed there were no significant differences in the VAS scores between the two conditions. Liu et al. suggested based on animal experiments that the heat generated by low-intensity TUS is extremely weak, far below the temperature threshold capable of causing biological effects (32). This means that the local temperature in the target area may remain almost unchanged, and there is no danger of thermal damage to normal tissues (33). As a non-invasive form of brain stimulation, low-intensity TUS does not involve the opening of the blood–brain barrier, nor does it produce morphological changes in the brain (7), and therefore, it can be considered safe. During the course of our study, a single participant experienced mild dizziness after TUS treatment, which disappeared spontaneously within 5 min.

Our study has several limitations. First, we focused exclusively on the effect of a single TUS stimulus, even though it has a statistically significant difference in MEP latency between the real and sham conditions after stimulation, the changes in mean value were small. Further research is needed to determine the potential effects of a different stimulus schedule on excitability. Second, we observed the regulatory effect of TUS on the brains of healthy adults, and it is not clear whether the same changes would also occur in pathological conditions. Third, this study was restricted to the effects of TUS on hand motor function and central regulation, but the relevance of this effect to behavioral training remains unclear. Finally, the assessment method employed in this study is relatively limited, and therefore, it would be necessary to conduct further research on the impact of TUS on neural networks through methods involving neuroimaging in the future.

In conclusion, low-intensity TUS can improve hand motor function in healthy adults by shortening the latency of MEP and enhancing the excitability of the motor cortex.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

The studies involving human participants were reviewed and approved by the Ethics Committee of Yue dong Hospital of the Third Affiliated Hospital of Sun Yat-sen University. The patients/participants provided their written informed consent to participate in this study.

Z-MT and F-BH: conceptualization. M-FZ and Z-MT: methodology. W-ZC: assessment. M-FZ: TUS operator and writing—original draft preparation. Z-YP: formal analysis and investigation. Z-MT: writing—review and editing and funding acquisition. All authors contributed to the study conception and design. All authors contributed to the article and approved the submitted version.

This work was supported by the National Natural Science Foundation of China (Grant Number: 81401872).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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