Determining the corticospinal, intracortical and motor function responses to transcranial alternating current stimulation of the motor cortex in healthy adults: A systematic review and meta-analysis

Effects of Electrical Stimulation on Activation of Mirror Neuron Network in Healthy Adults during Motor Execution and Imitation: An fNIRS Study

BACKGROUND

Observation, execution, and imitation of target actions based on mirror neuron network (MNN) have become common physiotherapy strategies. Electrical stimulation (ES) is a common intervention to improve muscle strength and motor control in rehabilitation treatments. It is possible to enhance MNN's activation by combining motor execution (ME) and motor imitation (MI) with ES simultaneously. This study aims to reveal whether ES could impact cortical activation during ME and MI.

METHODS

We recruited healthy individuals and assigned them randomly to the control group (CG) or experiment group (EG). Participants in EG performed ME and MI tasks with ES, while participants in CG performed the same two tasks with sham ES. We utilized functional near-infrared spectroscopy (fNIRS) to detect brain activation of MNN during ME and MI with and without ES, a randomized block design experiment paradigm was designed. Descriptive analysis of oxy-hemoglobin (HbO) and deoxy-hemoglobin (HbR) were used to show the hemoglobin (Hb) concentration changes after different event onsets in both CG and EG, a linear mixed-effects model (LMM) of HbO data was employed to analyze the effect of ES on the activation of MNN.

RESULTS

A total of 102 healthy adults were recruited and 72 participants' data were analysed in the final report. The block averaged Hb data showed that HbO concentration increased and HbR concentration decreased in most MNN regions during ME and MI in both groups. The LMM results showed that ES can significantly improve the activation of inferior frontal gyrus, middle frontal gyrus, and precentral gyrus during MI, the supplementary motor area, inferior parietal lobule, and superior temporal gyri showed increased activation, but without statistical significance. Although the results did not reach statistical significance during ME, ES still showed positive effects on increased overall activations.

CONCLUSIONS

In this study, we present potential novel rehabilitation approaches that combines MNN strategies and low-frequency ES to enhance cortical activation. Our results revealed that ES has potential to increase activation of most MNN brain areas, providing evidence for related rehabilitative interventions and device development.

CLINICAL TRIAL REGISTRATION

This study was registered on the China Clinical Trial Registration Center (identifier: ChiCTR2200064082, registered 26, September 2022, https://www.chictr.org.cn/showproj.html?proj=178285).

The Modulatory Effects of Transcranial Alternating Current Stimulation on Brain Oscillatory Patterns in the Beta Band in Healthy Older Adults

Background: In the last few years, transcranial alternating current stimulation (tACS) has attracted attention as a promising approach to interact with ongoing oscillatory cortical activity and, consequently, to enhance cognitive and motor processes. While tACS findings are limited by high variability in young adults' responses, its effects on brain oscillations in older adults remain largely unexplored. In fact, the modulatory effects of tACS on cortical oscillations in healthy aging participants have not yet been investigated extensively, particularly during movement. This study aimed to examine the after-effects of 20 Hz and 70 Hz High-Definition tACS on beta oscillations both during rest and movement. Methods: We recorded resting state EEG signals and during a handgrip task in 15 healthy older participants. We applied 10 min of 20 Hz HD-tACS, 70 Hz HD-tACS or Sham stimulation for 10 min. We extracted resting-state beta power and movement-related beta desynchronization (MRBD) values to compare between stimulation frequencies and across time. Results: We found that 20 Hz HD-tACS induced a significant reduction in beta power for electrodes C3 and CP3, while 70 Hz did not have any significant effects. With regards to MRBD, 20 Hz HD-tACS led to more negative values, while 70 Hz HD-tACS resulted in more positive ones for electrodes C3 and FC3. Conclusions: These findings suggest that HD-tACS can modulate beta brain oscillations with frequency specificity. They also highlight the focal impact of HD-tACS, which elicits effects on the cortical region situated directly beneath the stimulation electrode.

Beta tACS of varying intensities differentially affect resting-state and movement-related M1-M1 connectivity

Due to the interconnected nature of the brain, changes in one region are likely to affect other structurally and functionally connected regions. Emerging evidence indicates that single-site transcranial alternating current stimulation (tACS) can modulate functional connectivity between stimulated and interconnected unstimulated brain regions. However, our understanding of the network response to tACS is incomplete. Here, we investigated the effect of beta tACS of different intensities on phase-based connectivity between the left and right primary motor cortices in 21 healthy young adults (13 female; mean age 24.30 ± 4.84 years). Participants underwent four sessions of 20 min of 20 Hz tACS of varying intensities (sham, 0.5 mA, 1.0 mA, or 1.5 mA) applied to the left primary motor cortex at rest. We recorded resting-state and event-related electroencephalography (EEG) before and after tACS, analyzing changes in sensorimotor beta (13-30 Hz) imaginary coherence (ImCoh), an index of functional connectivity. Event-related EEG captured movement-related beta activity as participants performed self-paced button presses using their right index finger. For resting-state connectivity, we observed intensity-dependent changes in beta ImCoh: sham and 0.5 mA stimulation resulted in an increase in beta ImCoh, while 1.0 mA and 1.5 mA stimulation decreased beta ImCoh. For event-related connectivity, 1.5 mA stimulation decreased broadband ImCoh (4-90 Hz) during movement execution. None of the other stimulation intensities significantly modulated event-related ImCoh during movement preparation, execution, or termination. Interestingly, changes in ImCoh during movement preparation following 1.0 mA and 1.5 mA stimulation were significantly associated with participants' pre-tACS peak beta frequency, suggesting that the alignment of stimulation frequency and peak beta frequency affected the extent of neuromodulation. Collectively, these results suggest that beta tACS applied to a single site influences connectivity within the motor network in a manner that depends on the intensity and frequency of stimulation. These findings have significant implications for both research and clinical applications.