Report on Single-Session Cross-Frequency Dual-Focus tACS Modulation of Visuomotor Network Activity in Healthy Young Adults and Stroke Patients

Academic Background and Research Significance

In neuroscience research, neural oscillations play a crucial role in regulating communication within and between brain regions. Long-distance phase synchronization provides the foundation for cross-regional communication and is important in modulating the flow of information processing to meet cognitive demands. For instance, the alpha band (8-13 Hz) has been found to be associated with improved performance in visual and tactile integration tasks, and high alpha band synchronization can reduce reaction times (Lobier, Palva & Palva, 2018). This phenomenon is particularly evident in the visual system, where dynamic coupling of alpha (8-13 Hz) and gamma (>40 Hz) oscillations, i.e., phase-amplitude coupling (PAC), plays a key role in promoting efficient neural processing (Canolty et al., 2006; van Kerkoerle et al., 2014). Therefore, cross-frequency (alpha-gamma) dual-focus transcranial alternating current stimulation (tACS) can intervene in the flow of information along the visuomotor pathway by synchronizing low-frequency and high-frequency signals.

However, although cross-frequency tACS has been shown to alter the flow of communication between brain regions, research on its specific impact on behavioral performance—such as performance in visual motion discrimination tasks—remains insufficient. The aim of this study is to verify whether single-session cross-frequency dual-focus tACS can modulate phase-amplitude coupling (PAC) between the V1 and MT regions of the visuomotor network and performance in direction discrimination tasks in healthy young adults and stroke patients.

Research Origin and Author Information

This study was collaboratively conducted by Michele Bevilacqua, Sarah Feroldi, Fabienne Windel, Pauline Menoud, Roberto F. Salamanca-Giron, Sarah B. Zandvliet, Lisa Fleury, Friedhelm C. Hummel, and Estelle Raffin, who are affiliated with the École Polytechnique Fédérale de Lausanne (EPFL) Neuro-X Institute, Clinique Romande de Réadaptation (EPFL Valais), and University of Milano-Bicocca School of Medicine and Surgery. The paper was published in the 2024 issue of the journal “Brain Stimulation”.

Research Process

Experimental Subjects and Methods

The study recruited 45 young healthy participants (aged 18-40, mean age 24.6, 55% female) and 16 stroke patients who had experienced visual damage (aged 34-74, mean age 59.93, 18.75% female). Healthy participants were randomly assigned to three groups to receive forward tACS (V1α-MTγ), backward tACS (MTα-V1γ), or sham treatment (sham tACS). The stroke patient group was divided into two groups in a crossover design to receive forward and backward tACS at different times.

During the experiments, participants performed direction discrimination and integration tasks (CDDI), which required them to observe random dot stimuli and report the direction of the dots. To ensure that the visual stimuli in the tests were located at the boundary of the blind spot and the healthy visual field, psychophysical mapping was conducted in advance for the patients.

Dual-Focus tACS Intervention

tACS was delivered using two custom-made central ring electrodes connected to two NeuroConn DC Plus stimulators, which worked synchronously to trigger gamma waves with alpha waves. Each tACS session lasted approximately 15 minutes for healthy participants and 25 minutes for stroke patients, with a current intensity of 3 mA and frequencies predefined for individual alpha frequency (8-12 Hz) and gamma frequency (30-45 Hz).

EEG Recording and Analysis

Resting-state and task-related EEG activities were recorded using a 64-channel system. Custom Matlab scripts and MNE-Python tools were used to preprocess EEG data, including filtering, resampling, and independent component analysis (ICA) for removing physiological artifacts.

Research Results

PAC Changes in Healthy Participants

Forward tACS successfully increased V1α-phase-MTγ-amplitude PAC early (100-150 ms post-stimulation), while backward tACS and sham treatments did not induce significant changes. Granger causality analysis revealed a significant increase of input from V1 to MT in the alpha/beta band in the forward tACS group.

PAC Changes in Stroke Patients

Similar to the results in the healthy group, forward tACS significantly increased V1α-phase-MTγ-amplitude PAC early but decreased it in the later period (250 and 350 ms). Backward tACS reduced PAC early (after 100 ms).

Changes in Direction Discrimination Task Performance

Although healthy participants showed significant improvement with task repetition (regardless of tACS conditions), there were no significant differences in the stroke patient group between time points and tACS conditions, indicating that patients require more training to observe behavioral improvement. The electrophysiological changes induced by tACS in this study did not translate into significant behavioral changes.

Conclusion and Significance

This study demonstrates that single-session cross-frequency dual-focus tACS can modulate inter-regional synchronization early in the brains of healthy individuals and stroke patients, but these changes do not translate into significant differences at the behavioral level. Greater efficiency may require prolonged training or precise adjustments of tACS timing and dosage to match endogenous oscillatory activity in the brain, thereby more effectively improving behavioral performance. This provides new insights and directions for optimizing tACS applications and researching inter-regional oscillatory dynamics in the future.

In terms of clinical applications, this study provides preliminary evidence for the combination of visual training with synchronous feedback electrical stimulation for rehabilitation, especially for potential applications in visual damage recovery and enhancement.